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An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,

intellectual property matters and other important disclaimers. PRODUCTION DATA.

LM2599

SNVS123D –APRIL 1998–REVISED MAY 2016

LM2599 SIMPLE SWITCHER® Power Converter 150-kHz 3-A Step-Down Voltage Regulator,

With Features

1 Features

1• 3.3-V, 5-V, 12-V, and Adjustable Output Versions

• Adjustable Version Output Voltage Range: 1.2 V

to 37 V ±4% Maximum Over Line and Load

Conditions

• 3-A Output Current

• Available in 7-Pin TO-220 and TO-263 (Surface-

Mount) Package

• Input Voltage Range Up to 40 V

• 150-kHz Fixed-Frequency Internal Oscillator

• Shutdown and Soft-Start

• Out-of-Regulation Error Flag

• Error Output Delay

• Low Power Standby Mode, IQ, Typically 80 μA

• High Efficiency

• Uses Readily Available Standard Inductors

• Thermal Shutdown and Current-Limit Protection

2 Applications

• Simple High-Efficiency Step-Down (Buck)

Regulator

• Efficient Preregulator for Linear Regulators

• On-Card Switching Regulators

• Positive to Negative Converter

3 Description

The LM2599 series of regulators are monolithic

integrated circuits that provide all the active functions

for a step-down (buck) switching regulator, capable of

driving a 3-A load with excellent line and load

regulation. These devices are available in fixed output

voltages of 3.3 V, 5 V, 12 V, and an adjustable output

version.

The LM2598 is a member of the LM259x family, with

additional supervisory and performance features

added.

Requiring a minimum number of external

components, these regulators are simple to use and

include internal frequency compensation, improved

line and load specifications, fixed-frequency oscillator,

Shutdown and Soft-start, error flag delay, and error

flag output.

Device Information(1)

PART NUMBER PACKAGE BODY SIZE (NOM)

LM2599 TO-220 (7) 14.986 mm × 10.16 mm

TO-263 (7) 10.10 mm × 8.89 mm

(1) For all available packages, see the orderable addendum at

the end of the data sheet.

Typical Application

(fixed output voltage versions)

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Table of Contents

1 Features.................................................................. 1

2 Applications ........................................................... 1

3 Description............................................................. 1

4 Revision History..................................................... 2

5 Description (continued)......................................... 3

6 Pin Configuration and Functions......................... 3

7 Specifications......................................................... 4

7.1 Absolute Maximum Ratings ..................................... 4

7.2 ESD Ratings.............................................................. 4

7.3 Recommended Operating Conditions....................... 4

7.4 Thermal Information.................................................. 4

7.5 Electrical Characteristics – 3.3-V Version................. 5

7.6 Electrical Characteristics – 5-V Version.................... 5

7.7 Electrical Characteristics – 12-V Version.................. 5

7.8 Electrical Characteristics – Adjustable Voltage

Version....................................................................... 5

7.9 Electrical Characteristics – All Output Voltage

Versions..................................................................... 6

7.10 Typical Characteristics............................................ 7

8 Detailed Description............................................ 10

8.1 Overview................................................................. 10

8.2 Functional Block Diagram....................................... 10

8.3 Feature Description................................................. 10

8.4 Device Functional Modes........................................ 15

9 Application and Implementation ........................ 16

9.1 Application Information............................................ 16

9.2 Typical Applications ................................................ 25

10 Power Supply Recommendations ..................... 34

11 Layout................................................................... 34

11.1 Layout Guidelines ................................................. 34

11.2 Layout Examples................................................... 34

11.3 Thermal Considerations........................................ 35

12 Device and Documentation Support................. 38

12.1 Community Resources.......................................... 38

12.2 Trademarks........................................................... 38

12.3 Electrostatic Discharge Caution............................ 38

12.4 Glossary................................................................ 38

13 Mechanical, Packaging, and Orderable

Information........................................................... 38

4 Revision History

NOTE: Page numbers for previous revisions may differ from page numbers in the current version.

Changes from Revision C (April 2013) to Revision D Page

• Added ESD Ratings table, Feature Description section, Device Functional Modes,Application and Implementation

section, Power Supply Recommendations section, Layout section, Device and Documentation Support section, and

Mechanical, Packaging, and Orderable Information section.................................................................................................. 1

• Removed all references to design software Switchers Made Simple ................................................................................... 1

Changes from Revision B (April 2013) to Revision C Page

• Changed layout of National Semiconductor Data Sheet to TI format .................................................................................. 35

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5 Description (continued)

The LM2599 series operates at a switching frequency of 150 kHz, thus allowing smaller sized filter components

than what would be needed with lower frequency switching regulators. Available in a standard 7-pin TO-220

package with several different lead bend options, and a 7-pin TO-263 surface-mount package.

A standard series of inductors (both through-hole and surface-mount types) are available from several different

manufacturers optimized for use with the LM2599 series. This feature greatly simplifies the design of switch-

mode power supplies.

Other features include a ±4% tolerance on output voltage under all conditions of input voltage and output load

conditions, and ±15% on the oscillator frequency. External shutdown is included, featuring typically 80-μA

standby current. Self-protection features include a two stage current limit for the output switch and an

overtemperature shutdown for complete protection under fault conditions.

6 Pin Configuration and Functions

NDZ Package

7-Pin TO-220

Top View KTW Package

7-Pin TO-263

Top View

(1) If any of the above three features (Shutdown/Soft-start, Error Flag, or Delay) are not used, the respective pins must be left open.

Pin Functions(1)

PIN I/O DESCRIPTION

NO. NAME

1 +VIN IThis is the positive input supply for the IC switching regulator. A suitable input bypass

capacitor must be present at this pin to minimize voltage transients and to supply the

switching currents needed by the regulator.

2 Output O Internal switch. The voltage at this pin switches between approximately (+VIN −VSAT) and

approximately −0.5 V, with a duty cycle of VOUT/VIN. To minimize coupling to sensitive

circuitry, the PCB copper area connected to this pin must be kept to a minimum.

3 Error Flag O

Open-collector output that provides a low signal (flag transistor ON) when the regulated

output voltage drops more than 5% from the nominal output voltage. On start up, Error Flag

is low until VOUT reaches 95% of the nominal output voltage and a delay time determined by

the Delay pin capacitor. This signal can be used as a reset to a microprocessor on power-

up.

4 Ground — Circuit ground.

5 Delay O At power-up, this pin can be used to provide a time delay between the time the regulated

output voltage reaches 95% of the nominal output voltage, and the time the error flag output

goes high.

6 Feedback I Senses the regulated output voltage to complete the feedback loop.

7 Shutdown/Soft-start I This dual function pin provides the following features: (a) Allows the switching regulator

circuit to be shut down using logic level signals thus dropping the total input supply current

to approximately 80 μA. (b) Adding a capacitor to this pin provides a soft-start feature which

minimizes start-up current and provides a controlled ramp up of the output voltage.

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SNVS123D –APRIL 1998–REVISED MAY 2016

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(1) Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings

only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended

Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.

(2) If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/Distributors for availability and

specifications.

(3) Voltage internally clamped. If clamp voltage is exceeded, limit current to a maximum of 1 mA.

7 Specifications

7.1 Absolute Maximum Ratings (1)(2)

MIN MAX UNIT

Maximum supply voltage, VIN 45 V

SD/SS pin input voltage(3) 6 V

Delay pin voltage(3) 1.5 V

Flag pin voltage –0.3 45 V

Feedback pin voltage –0.3 25 V

Output voltage to ground (steady-state) –1 V

Power dissipation Internally limited

Lead temperature KTW package Vapor phase (60 s) 215 °CInfrared (10 s) 245

NDZ package (soldering, 10 s) 260

Maximum junction temperature 150 °C

Storage temperature, Tstg −65 150 °C

(1) JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.

7.2 ESD Ratings VALUE UNIT

V(ESD) Electrostatic

discharge Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001(1) ±2000 V

7.3 Recommended Operating Conditions MIN MAX UNIT

Supply voltage 4.5 40 V

Temperature –40 125 °C

(1) For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application

report, SPRA953.

(2) The package thermal impedance is calculated in accordance to JESD 51-7.

(3) Thermal resistances were simulated on a 4-layer, JEDEC board.

(4) Junction to ambient thermal resistance (no external heat sink) for the package mounted TO-220 package mounted vertically, with the

leads soldered to a PCB with 1-oz copper area of approximately 1 in2.

(5) Junction to ambient thermal resistance with the TO-263 package tab soldered to a single-sided PCB with 0.5 in2of 1-oz copper area.

(6) Junction to ambient thermal resistance with the TO-263 package tab soldered to a single-sided PCB with 2.5 in2of 1-oz copper area.

(7) Junction to ambient thermal resistance with the TO-263 package tab soldered to a double-sided PCB with 3 in2of 1-oz copper area on

the LM2599S side of the board, and approximately 16 in2of copper on the other side of the PCB.

7.4 Thermal Information

THERMAL METRIC(1)

LM2599

UNITKTW (TO-263) NDZ (TO-220)

7 PINS 7 PINS

RθJA Junction-to-ambient thermal resistance(2)(3)

See(4) — 50

°C/W

See(5) 50 —

See(6) 30 —

See(7) 20 —

RθJC(top) Junction-to-case (top) thermal resistance 2 2 °C/W

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(1) All room temperature limits are 100% production tested. All limits at temperature extremes are specified via correlation using standard

Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).

(2) Typical numbers are at 25°C and represent the most likely norm.

(3) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.

When the LM2599 is used as shown in Figure 43, system performance is shown in the test conditions column.

7.5 Electrical Characteristics – 3.3-V Version

Specifications are for TJ= 25°C (unless otherwise noted).

PARAMETER TEST CONDITIONS MIN(1) TYP(2) MAX (1) UNIT

SYSTEM PARAMETERS(3) (see Figure 43 for test circuit)

VOUT Output voltage 4.75 V ≤VIN ≤40 V,

0.2 A ≤ILOAD ≤3 A TJ= 25°C 3.168 3.3 3.432 V

–40°C ≤TJ≤125°C 3.135 3.465

ηEfficiency VIN = 12 V, ILOAD = 3 A 73%

(1) All room temperature limits are 100% production tested. All limits at temperature extremes are specified via correlation using standard

Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).

(2) Typical numbers are at 25°C and represent the most likely norm.

(3) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.

When the LM2599 is used as shown in Figure 43, system performance is shown in the test conditions column.

7.6 Electrical Characteristics – 5-V Version

Specifications are for TJ= 25°C (unless otherwise noted).

PARAMETER TEST CONDITIONS MIN(1) TYP(2) MAX(1) UNIT

SYSTEM PARAMETERS(3) (see Figure 43 for test circuit)

VOUT Output voltage 7 V ≤VIN ≤40 V,

0.2 A ≤ILOAD ≤3 A TJ= 25°C 4.8 5 5.2 V

–40°C ≤TJ≤125°C 4.75 5.25

ηEfficiency VIN = 12 V, ILOAD = 3 A 80%

(1) All room temperature limits are 100% production tested. All limits at temperature extremes are specified via correlation using standard

Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).

(2) Typical numbers are at 25°C and represent the most likely norm.

(3) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.

When the LM2599 is used as shown in Figure 43, system performance is shown in the test conditions column.

7.7 Electrical Characteristics – 12-V Version

Specifications are for TJ= 25°C (unless otherwise noted).

PARAMETER TEST CONDITIONS MIN(1) TYP(2) MAX(1) UNIT

SYSTEM PARAMETERS(3) (see Figure 43 for test circuit)

VOUT Output voltage 15 V ≤VIN ≤40 V,

0.2 A ≤ILOAD ≤3 A TJ= 25°C 11.52 12 12.48 V

–40°C ≤TJ≤125°C 11.4 12.6

ηEfficiency VIN = 25 V, ILOAD = 3 A 90%

(1) All room temperature limits are 100% production tested. All limits at temperature extremes are specified via correlation using standard

Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).

(2) Typical numbers are at 25°C and represent the most likely norm.

(3) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.

When the LM2599 is used as shown in Figure 43, system performance is shown in the test conditions column.

7.8 Electrical Characteristics – Adjustable Voltage Version

Specifications are for TJ= 25°C (unless otherwise noted).

PARAMETER TEST CONDITIONS MIN(1) TYP(2) MAX(1) UNIT

SYSTEM PARAMETERS(3) (see Figure 43 for test circuit)

VFB Feedback voltage 4.5 V ≤VIN ≤40 V, 0.2 A ≤ILOAD ≤3 A 1.23 V

VOUT programmed for 3 V,

circuit of Figure 43 TJ= 25°C 1.193 1.267

–40°C ≤TJ≤125°C 1.18 1.28

ηEfficiency VIN = 12 V, VOUT = 3 V, ILOAD = 3 A 73%

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(1) All room temperature limits are 100% production tested. All limits at temperature extremes are specified via correlation using standard

Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).

(2) Typical numbers are at 25°C and represent the most likely norm.

(3) The switching frequency is reduced when the second stage current limit is activated. The amount of reduction is determined by the

severity of current overload.

(4) No diode, inductor or capacitor connected to output pin.

(5) Feedback pin removed from output and connected to 0 V to force the output transistor switch ON.

(6) Feedback pin removed from output and connected to 12 V for the 3.3-V, 5-V, and the adjustable versions, and 15 V for the 12-V

version, to force the output transistor switch OFF.

7.9 Electrical Characteristics – All Output Voltage Versions

Specifications are for TJ= 25°C, ILOAD = 500 mA, VIN = 12 V for the 3.3-V, 5-V, and Adjustable version, and VIN = 24 V for the

12-V version (unless otherwise noted).

PARAMETER TEST CONDITIONS MIN(1) TYP(2) MAX(1) UNIT

DEVICE PARAMETERS

IbFeedback bias current Adjustable voltage

version only, VFB = 1.3

TJ= 25°C 10 50 nA

–40°C ≤TJ≤125°C 100

fOOscillator frequency(3) TJ= 25°C 127 150 173 kHz

–40°C ≤TJ≤125°C 110 173

VSAT Saturation voltage IOUT = 3 A(4)(5) TJ= 25°C 1.16 1.4 V

–40°C ≤TJ≤125°C 1.5

DC Max duty cycle (ON)(5) 100%

Min duty cycle (OFF)(6) 0%

ICL Current limit Peak current(4) (5) TJ= 25°C 3.6 4.5 6.9 A

–40°C ≤TJ≤125°C 3.4 7.5

ILOutput leakage current Output = 0 V, VIN = 40 V(4) (6) 50 μA

Output = −1 V 2 30 mA

IQOperating quiescent current SD/SS pin open(6) 5 10 mA

ISTBY Current standby quiescent SD/SS pin = 0 V, VIN =

40 V TJ= 25°C 80 200 μA

–40°C ≤TJ≤125°C 250 μA

SHUTDOWN/SOFT-START CONTROL – See Figure 43

VSD Shutdown threshold voltage 1.3 VLow (Shutdown mode), –40°C ≤TJ≤125°C 0.6

High (Soft-start mode), –40°C ≤TJ≤125°C 2

VSS Soft-start voltage VOUT = 20% of nominal output voltage 2 V

VOUT = 100% of nominal output voltage 3

ISD Shutdown current VSHUTDOWN = 0.5 V 5 10 μA

ISS Soft-start current VSoft-start = 2.5 V 1.6 5 μA

FLAG/DELAY CONTROL – See Figure 43

Regulator dropout detector

threshold voltage Low (flag ON) 92% 96% 98%

VFSAT Voltage flag output saturation ISINK = 3 mA 0.3 V

VDELAY = 0.5 V TJ= 25°C 0.7 V

–40°C ≤TJ≤125°C 1

IFLFlag output leakage current VFLAG = 40 V 0.3 μA

Voltage delay pin threshold 1.25 V

Low (flag ON) 1.21 V

High (flag OFF) and VOUT regulated 1.29

Delay pin source current VDELAY = 0.5 V 3 6 μA

Delay pin saturation Low (flag ON) TJ= 25°C 55 350 mV

–40°C ≤TJ≤125°C 400

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7.10 Typical Characteristics

See Figure 43 for test circuit

Figure 1. Normalized Output Voltage Figure 2. Line Regulation

Figure 3. Efficiency Figure 4. Switch Saturation Voltage

Figure 5. Switch Current Limit Figure 6. Dropout Voltage

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Typical Characteristics (continued)

See Figure 43 for test circuit

Figure 7. Operating Quiescent Current Figure 8. Shutdown Quiescent Current

Figure 9. Minimum Operating Supply Voltage Figure 10. Feedback Pin Bias Current

Figure 11. Flag Saturation Voltage Figure 12. Switching Frequency

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Typical Characteristics (continued)

See Figure 43 for test circuit

Figure 13. Soft-Start Figure 14. Shutdown/Soft-Start Current

Figure 15. Daisy Pin Current Figure 16. Soft-Start Response

Figure 17. Shutdown/Soft-Start Threshold Voltage

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8 Detailed Description

8.1 Overview

The LM2599 SIMPLE SWITCHER®regulator is an easy-to-use, nonsynchronous, step-down DC-DC converter

with a wide input voltage range up to 40 V. The regulator is capable of delivering up to 3-A DC load current with

excellent line and load regulation. These devices are available in fixed output voltages of 3.3 V, 5 V, 12 V, and

an adjustable output version. The family requires few external components, and the pin arrangement was

designed for simple, optimum PCB layout.

8.2 Functional Block Diagram

8.3 Feature Description

8.3.1 Shutdown/Soft-Start

The circuit shown in Figure 20 is a standard buck regulator with 20-VIN, 12-VOUT, 1-A load using a 0.068-μF soft-

start capacitor. Figure 18 and Figure 19 show the effects of soft-start on the output voltage, the input current,

with, and without a soft-start capacitor. The reduced input current required at startup is very evident when

comparing the two photos. The soft-start feature reduces the start-up current from 2.6 A down to 650 mA, and

delays and slows down the output voltage rise time.

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Feature Description (continued)

Figure 18. Output Voltage and Input Current

at Start-Up With Soft-Start Figure 19. Output Voltage and Input Current

at Start-Up Without Soft-Start

This reduction in start-up current is useful in situations where the input power source is limited in the amount of

current it can deliver. In some applications soft-start can be used to replace undervoltage lockout or delayed

start-up functions.

If a very slow output voltage ramp is desired, the soft-start capacitor can be made much larger. Many seconds or

even minutes are possible.

If only the shutdown feature is needed, the soft-start capacitor can be eliminated.

Figure 20. Typical Circuit Using Shutdown/Soft-Start and Error Flag Features

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Feature Description (continued)

Figure 21. Inverting −5-V Regulator With Shutdown and Soft-Start

8.3.2 Inverting Regulator

The circuit in Figure 21 converts a positive input voltage to a negative output voltage with a common ground. The

circuit operates by bootstrapping the ground pin of the regulator to the negative output voltage, then grounding

the feedback pin, the regulator senses the inverted output voltage and regulates it.

This example uses the LM2599-5 to generate a −5-V output, but other output voltages are possible by selecting

other output voltage versions, including the adjustable version. Because this regulator topology can produce an

output voltage that is either greater than or less than the input voltage, the maximum output current greatly

depends on both the input and output voltage. The curve shown in Figure 22 provides a guide as to the amount

of output load current possible for the different input and output voltage conditions.

The maximum voltage appearing across the regulator is the absolute sum of the input and output voltage, and

this must be limited to a maximum of 40 V. In this example, when converting 20 V to −5 V, the regulator would

see 25 V between the input pin and ground pin. The LM2599 has a maximum input voltage rating of 40 V.

Figure 22. Maximum Load Current for Inverting Regulator Circuit

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Feature Description (continued)

An additional diode is required in this regulator configuration. Diode D1 is used to isolate input voltage ripple or

noise from coupling through the CIN capacitor to the output, under light or no load conditions. Also, this diode

isolation changes the topology to closely resemble a buck configuration, thus providing good closed-loop stability.

TI recommends a Schottky diode for low input voltages, (because of its lower voltage drop) but for higher input

voltages, a IN5400 diode could be used.

Because of differences in the operation of the inverting regulator, the standard design procedure is not used to

select the inductor value. In the majority of designs, a 33-μH, 3.5-A inductor is the best choice. Capacitor

selection can also be narrowed down to just a few values. Using the values shown in Figure 21 provides good

results in the majority of inverting designs.

This type of inverting regulator can require relatively large amounts of input current when starting up, even with

light loads. Input currents as high as the LM2599 current limit (approximately 4.5 A) are needed for 2 ms or

more, until the output reaches its nominal output voltage. The actual time depends on the output voltage and the

size of the output capacitor. Input power sources that are current limited or sources that can not deliver these

currents without getting loaded down, may not work correctly. Because of the relatively high startup currents

required by the inverting topology, the soft-start feature shown in Figure 21 is recommended.

Also shown in Figure 21 are several shutdown methods for the inverting configuration. With the inverting

configuration, some level shifting is required, because the ground pin of the regulator is no longer at ground, but

is now at the negative output voltage. The shutdown methods shown accept ground referenced shutdown

signals.

8.3.3 Undervoltage Lockout

Some applications require the regulator to remain off until the input voltage reaches a predetermined voltage.

Figure 23 contains a undervoltage lockout circuit for a buck configuration, while Figure 24 and Figure 25 are for

the inverting types (only the circuitry pertaining to the undervoltage lockout is shown). Figure 23 uses a Zener

diode to establish the threshold voltage when the switcher begins operating. When the input voltage is less than

the Zener voltage, resistors R1 and R2 hold the shutdown/soft-start pin low, keeping the regulator in the

shutdown mode. As the input voltage exceeds the Zener voltage, the Zener conducts, pulling the shutdown/soft-

start pin high, allowing the regulator to begin switching. The threshold voltage for the undervoltage lockout

feature is approximately 1.5 V greater than the Zener voltage.

Figure 23. Undervoltage Lockout for a Buck Regulator

Figure 24 and Figure 25 apply the same feature to an inverting circuit. Figure 24 features a constant threshold

voltage for turn on and turn off (Zener voltage plus approximately one volt). If hysteresis is needed, the circuit in

Figure 25 has a turn ON voltage which is different than the turn OFF voltage. The amount of hysteresis is

approximately equal to the value of the output voltage. Because the SD/SS pin has an internal 7-V Zener clamp,

R2 is needed to limit the current into this pin to approximately 1 mA when Q1 is on.

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Feature Description (continued)

Figure 24. Undervoltage Lockout Without Hysteresis for an Inverting Regulator

Figure 25. Undervoltage Lockout With Hysteresis for an Inverting Regulator

8.3.4 Negative Voltage Charge Pump

Occasionally a low-current negative voltage is needed for biasing parts of a circuit. A simple method of

generating a negative voltage using a charge pump technique and the switching waveform present at the OUT

pin, is shown in Figure 26. This unregulated negative voltage is approximately equal to the positive input voltage

(minus a few volts), and can supply up to a 600 mA of output current. There is a requirement however, that there

be a minimum load of 1.2 A on the regulated positive output for the charge pump to work correctly. Also, resistor

R1 is required to limit the charging current of C1 to some value less than the LM2599 current limit (typically

4.5 A).

This method of generating a negative output voltage without an additional inductor can be used with other

members of the SIMPLE SWITCHER Family, using either the buck or boost topology.

Figure 26. Charge Pump for Generating a Low-Current, Negative-Output Voltage

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8.4 Device Functional Modes

8.4.1 Discontinuous Mode Operation

The selection guide chooses inductor values suitable for continuous mode operation, but for low current

applications or high input voltages, a discontinuous mode design may be a better choice. The design would use

an inductor that would be physically smaller, and would need only one half to one third the inductance value

needed for a continuous mode design. The peak switch and inductor currents is higher in a discontinuous design,

but at these low load currents (1 A and below), the maximum switch current is still less than the switch current

limit.

Discontinuous operation can have voltage waveforms that are considerably different than a continuous design.

The output pin (switch) waveform can have some damped sinusoidal ringing present (see Figure 44). This

ringing is normal for discontinuous operation, and is not caused by feedback loop instabilities. In discontinuous

operation, there is a period of time where neither the switch nor the diode are conducting, and the inductor

current has dropped to zero. During this time, a small amount of energy can circulate between the inductor and

the switch and diode parasitic capacitance causing this characteristic ringing. Normally this ringing is not a

problem, unless the amplitude becomes great enough to exceed the input voltage, and even then, there is very

little energy present to cause damage.

Different inductor types or core materials produce different amounts of this characteristic ringing. Ferrite core

inductors have very little core loss and therefore produce the most ringing. The higher core loss of powdered iron

inductors produce less ringing. If desired, a series RC could be placed in parallel with the inductor to dampen the

ringing.

Figure 27. Post Ripple Filter Waveform

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9 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

9.1 Application Information

9.1.1 Soft-Start Capacitor (CSS)

The capacitor on this pin provides the regulator with a soft-start feature (slow start-up). When the DC input

voltage is first applied to the regulator, or when the Shutdown/Soft-start pin is allowed to go high, a constant

current (approximately 5 μA begins charging this capacitor). As the capacitor voltage rises, the regulator goes

through four operating regions (see the bottom curve in Figure 28):

1. Regulator in Shutdown: When the SD/SS pin voltage is between 0 V and 1.3 V, the regulator is in shutdown,

the output voltage is zero, and the IC quiescent current is approximately 85 μA.

2. Regulator ON, but the output voltage is zero: With the SD/SS pin voltage between approximately 1.3 V and

1.8 V, the internal regulator circuitry is operating, the quiescent current rises to approximately 5 mA, but the

output voltage is still zero. Also, as the 1.3-V threshold is exceeded, the soft-start capacitor charging current

decreases from 5 μA down to approximately 1.6 μA. This decreases the slope of capacitor voltage ramp.

3. Soft-start Region: When the SD/SS pin voltage is between 1.8 V and 2.8 V (at 25°C), the regulator is in a

soft-start condition. The switch (Pin 2) duty cycle initially starts out very low, with narrow pulses and gradually

get wider as the capacitor SD/SS pin ramps up towards 2.8 V. As the duty cycle increases, the output

voltage also increases at a controlled ramp up (see the center curve in Figure 28) The input supply current

requirement also starts out at a low level for the narrow pulses and ramp up in a controlled manner. This is a

very useful feature in some switcher topologies that require large start-up currents (such as the inverting

configuration) which can load down the input power supply.

NOTE

The lower curve shown in Figure 28 shows the soft-start region from 0% to 100%. This is

not the duty cycle percentage, but the output voltage percentage. Also, the soft-start

voltage range has a negative temperature coefficient associated with it. See the soft-start

section in Electrical Characteristics – All Output Voltage Versions.

4. Normal operation: Above 2.8 V, the circuit operates as a standard Pulse Width Modulated switching

regulator. The capacitor continues to charge up until it reaches the internal clamp voltage of approximately 7

V. If this pin is driven from a voltage source, the current must be limited to about 1 mA.

If the part is operated with an input voltage at or below the internal soft-start clamp voltage of approximately 7 V,

the voltage on the SD/SS pin tracks the input voltage and can be disturbed by a step in the voltage. To maintain

proper function under these conditions, TI strongly recommends clamping the SD/SS pin externally between the

3-V maximum soft-start threshold and the 4.5-V minimum input voltage. Figure 30 is an example of an external

3.7-V (approximately) clamp that prevents a line-step related glitch but does not interfere with the soft-start

behavior of the device.

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Figure 28. Soft-Start, Delay, Error, Output

Figure 29. Timing Diagram for 5-V Output

CSS

SD/SS

VIN

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Figure 30. External 3.7-V Soft-Start Clamp

9.1.2 Delay Capacitor (CDELAY)

This capacitor provides delay for the error flag output (see Figure 28 and Figure 29). A capacitor on this pin

provides a time delay between the time the regulated output voltage (when it is increasing in value) reaches 95%

of the nominal output voltage, and the time the error flag output goes high. A 3-μA constant current from the

delay pin charges the delay capacitor resulting in a voltage ramp. When this voltage reaches a threshold of

approximately 1.3 V, the open-collector error flag output (or power OK) goes high. This signal can be used to

indicate that the regulated output has reached the correct voltage and has stabilized.

If, for any reason, the regulated output voltage drops by 5% or more, the error output flag (Pin 3) immediately

goes low (internal transistor turns on). The delay capacitor provides very little delay if the regulated output is

dropping out of regulation. The delay time for an output that is decreasing is approximately a 1000 times less

than the delay for the rising output. For a 0.1-μF delay capacitor, the delay time would be approximately 50 ms

when the output is rising and passes through the 95% threshold, but the delay for the output dropping would only

be approximately 50 μs.

9.1.2.1 RPULLUP

The error flag output (or power OK) is the collector of a NPN transistor, with the emitter internally grounded. To

use the error flag, a pullup resistor to a positive voltage is needed. The error flag transistor is rated up to a

maximum of 45 V and can sink approximately 3 mA. If the error flag is not used, it can be left open.

9.1.3 Feedforward Capacitor (CFF) for Adjustable Output Voltage Version Only

A feedforward capacitor, CFF, is used when the output voltage is greater than 10 V or when COUT has a very low

ESR. This capacitor adds lead compensation to the feedback loop and increases the phase margin for better

loop stability.

If the output ripple is large (> 5% of the nominal output voltage), this ripple can be coupled to the feedback pin

through the feedforward capacitor and cause the error comparator to trigger the error flag. In this situation,

adding a resistor, RFF, in series with the feedforward capacitor, approximately 3 times R1, attenuates the ripple

voltage at the feedback pin.

9.1.4 Input Capacitor (CIN)

A low-ESR aluminum or tantalum bypass capacitor is required between the input pin and ground pin. The

capacitor must be placed near the regulator using short leads. This capacitor prevents large voltage transients

from appearing at the input, and provides the instantaneous current needed each time the switch turns on.

The important parameters for the Input capacitor are the voltage rating and the RMS current rating. Because of

the relatively high RMS currents flowing in a input capacitor of the buck regulator, this capacitor must be chosen

for its RMS current rating rather than its capacitance or voltage ratings, although the capacitance value and

voltage rating are directly related to the RMS current rating.

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The RMS current rating of a capacitor could be viewed as a power rating of the capacitor. The RMS current

flowing through the capacitors internal ESR produces power which causes the internal temperature of the

capacitor to rise. The RMS current rating of a capacitor is determined by the amount of current required to raise

the internal temperature approximately 10°C above an ambient temperature of 105°C. The ability of the capacitor

to dissipate this heat to the surrounding air determines the amount of current the capacitor can safely sustain.

Capacitors that are physically large and have a large surface area typically has a higher RMS current ratings. For

a given capacitor value, a higher voltage electrolytic capacitor is physically larger than a lower voltage capacitor,

and thus be able to dissipate more heat to the surrounding air, and therefore has a higher RMS current rating.

Figure 31. RMS Current Ratings for Low-ESR Electrolytic

Capacitors (Typical)

Figure 32. Capacitor ESR vs Capacitor Voltage Rating

(Typical Low-ESR Electrolytic Capacitor)

The consequences of operating an electrolytic capacitor above the RMS current rating is a shortened operating

life. The higher temperature speeds up the evaporation of the capacitor's electrolyte, resulting in eventual failure.

Selecting an input capacitor requires consulting the manufacturers data sheet for maximum allowable RMS ripple

current. For a maximum ambient temperature of 40°C, a general guideline would be to select a capacitor with a

ripple current rating of approximately 50% of the DC load current. For ambient temperatures up to 70°C, a

current rating of 75% of the DC load current would be a good choice for a conservative design. The capacitor

voltage rating must be at least 1.25 times greater than the maximum input voltage, and often a much higher

voltage capacitor is needed to satisfy the RMS current requirements.

Figure 31 shows the relationship between an electrolytic capacitor value, its voltage rating, and the RMS current

it is rated for. These curves were obtained from the Nichicon PL series of low-ESR, high-reliability electrolytic

capacitors designed for switching regulator applications. Other capacitor manufacturers offer similar types of

capacitors, but always check the capacitor data sheet.

Standard electrolytic capacitors typically have much higher ESR numbers, lower RMS current ratings and

typically have a shorter operating lifetime.

Because of their small size and excellent performance, surface-mount solid tantalum capacitors are often used

for input bypassing, but several precautions must be observed. A small percentage of solid tantalum capacitors

can short if the inrush current rating is exceeded. This can happen at turnon when the input voltage is suddenly

applied, and of course, higher input voltages produce higher inrush currents. Several capacitor manufacturers do

a 100% surge current testing on their products to minimize this potential problem. If high turnon currents are

expected, it may be necessary to limit this current by adding either some resistance or inductance before the

tantalum capacitor, or select a higher voltage capacitor. As with aluminum electrolytic capacitors, the RMS ripple

current rating must be sized to the load current.

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9.1.5 Output Capacitor (COUT)

An output capacitor is required to filter the output and provide regulator loop stability. Low impedance or low-ESR

Electrolytic or solid tantalum capacitors designed for switching regulator applications must be used. When

selecting an output capacitor, the important capacitor parameters are; the 100-kHz Equivalent Series Resistance

(ESR), the RMS ripple current rating, voltage rating, and capacitance value. For the output capacitor, the ESR

value is the most important parameter.

The output capacitor requires an ESR value that has an upper and lower limit. For low output ripple voltage, a

low ESR value is needed. This value is determined by the maximum allowable output ripple voltage, typically 1%

to 2% of the output voltage. But if the ESR of the selected capacitor' is extremely low, there is a possibility of an

unstable feedback loop, resulting in an oscillation at the output. Using the capacitors listed in the tables, or

similar types, provides design solutions under all conditions.

If very low output ripple voltage (less than 15 mV) is required, see Output Voltage Ripple and Transients for a

post ripple filter.

An aluminum electrolytic capacitor's ESR value is related to the capacitance value and its voltage rating. In most

cases, higher voltage electrolytic capacitors have lower ESR values (see Figure 32). Often, capacitors with much

higher voltage ratings may be needed to provide the low ESR values required for low output ripple voltage.

The output capacitor for many different switcher designs often can be satisfied with only three or four different

capacitor values and several different voltage ratings. See the quick design component selection tables in

Table 3 and Table 6 for typical capacitor values, voltage ratings, and manufacturer capacitor types.

Electrolytic capacitors are not recommended for temperatures below −25°C. The ESR rises dramatically at cold

temperatures and typically rises 3X at −25°C and as much as 10X at −40°C (see Figure 33).

Solid tantalum capacitors have a much better ESR specification for cold temperatures and are recommended for

temperatures below −25°C.

9.1.6 Catch Diode

Buck regulators require a diode to provide a return path for the inductor current when the switch turns off. This

must be a fast diode and must be placed close to the LM2599 using short leads and short printed circuit traces.

Because of their very fast switching speed and low forward voltage drop, Schottky diodes provide the best

performance, especially in low output voltage applications (5 V and lower). Ultra-fast recovery, or high-efficiency

rectifiers are also a good choice, but some types with an abrupt turnoff characteristic may cause instability or

EMI problems. Ultra-fast recovery diodes typically have reverse recovery times of 50 ns or less. Rectifiers such

as the IN5400 series are much too slow and must not be used.

Figure 33. Capacitor ESR Change vs Temperature

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Application Information (continued)

9.1.7 Inductor Selection

All switching regulators have two basic modes of operation; continuous and discontinuous. The difference

between the two types relates to the inductor current, whether it is flowing continuously, or if it drops to zero for a

period of time in the normal switching cycle. Each mode has distinctively different operating characteristics,

which can affect the regulators performance and requirements. Most switcher designs operates in the

discontinuous mode when the load current is low.

The LM2599 (or any of the SIMPLE SWITCHER family) can be used for both continuous or discontinuous modes

of operation.

In many cases the preferred mode of operation is the continuous mode. This mode offers greater output power,

lower peak switch, inductor and diode currents, and can have lower output ripple voltage. However, the

continuous mode requires larger inductor values to keep the inductor current flowing continuously, especially at

low output load currents and/or high input voltages.

To simplify the inductor selection process, an inductor selection guide (nomograph) was designed (see Figure 35

through Figure 38). This guide assumes that the regulator is operating in the continuous mode, and selects an

inductor that allows a peak-to-peak inductor ripple current to be a certain percentage of the maximum design

load current. This peak-to-peak inductor ripple current percentage is not fixed, but is allowed to change as

different design load currents are selected (see Figure 34).

Figure 34. ΔIIND, Peak-to-Peak Inductor Ripple Current

(as a Percentage of the Load Current) vs Load Current

By allowing the percentage of inductor ripple current to increase for low load currents, the inductor value and size

can be kept relatively low.

When operating in the continuous mode, the inductor current waveform ranges from a triangular to a sawtooth

type of waveform (depending on the input voltage), with the average value of this current waveform equal to the

DC output load current.

Inductors are available in different styles such as pot core, toroid, E-core, bobbin core, and so forth, as well as

different core materials, such as ferrites and powdered iron. The least expensive, the bobbin, rod or stick core,

consists of wire wound on a ferrite bobbin. This type of construction makes for an inexpensive inductor, but

because the magnetic flux is not completely contained within the core, it generates more Electro-Magnetic

Interference (EMl). This magnetic flux can induce voltages into nearby printed-circuit traces, thus causing

problems with both the switching regulator operation and nearby sensitive circuitry, and can give incorrect scope

readings because of induced voltages in the scope probe (see Open Core Inductors).

When multiple switching regulators are placed on the same PCB, open core magnetics can cause interference

between two or more of the regulator circuits, especially at high currents. A torroid or E-core inductor (closed

magnetic structure) must be used in these situations.

The inductors listed in the selection chart include ferrite E-core construction for Schott, ferrite bobbin core for

Renco and Coilcraft, and powdered iron toroid for Pulse Engineering.

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Exceeding an inductor's maximum current rating may cause the inductor to overheat because of the copper wire

losses, or the core may saturate. If the inductor begins to saturate, the inductance decreases rapidly and the

inductor begins to look mainly resistive (the DC resistance of the winding). This can cause the switch current to

rise very rapidly and force the switch into a cycle-by-cycle current limit, thus reducing the DC output load current.

This can also result in overheating of the inductor and/or the LM2599. Different inductor types have different

saturation characteristics, so keep this in mind when selecting an inductor.

The inductor manufacturer's data sheets include current and energy limits to avoid inductor saturation.

For continuous mode operation, see the inductor selection graphs in Figure 35 through Figure 38.

Figure 35. 3.3-V Version Figure 36. 5-V Version

Figure 37. 12-V Version Figure 38. Adjustable Voltage Version

Table 1. Inductor Manufacturers Part Numbers

INDUCTANCE

(μH) CURRENT

(A)

SCHOTTKY RENCO PULSE ENGINEERING COILCRAFT

THROUGH-

HOLE SURFACE-

MOUNT THROUGH-HOLE SURFACE-

MOUNT THROUGH-

HOLE SURFACE-

MOUNT SURFACE-

MOUNT

L15 22 0.99 67148350 67148460 RL-1284-22-43 RL1500-22 PE-53815 PE-53815-S DO3308-223

L21 68 0.99 67144070 67144450 RL-5471-5 RL1500-68 PE-53821 PE-53821-S DO3316-683

L22 47 1.17 67144080 67144460 RL-5471-6 — PE-53822 PE-53822-S DO3316-473

L23 33 1.40 67144090 67144470 RL-5471-7 — PE-53823 PE-53823-S DO3316-333

L24 22 1.70 67148370 67148480 RL-1283-22-43 — PE-53824 PE-53825-S DO3316-223

L25 15 2.1 67148380 67148490 RL-1283-15-43 — PE-53825 PE-53824-S DO3316-153

L26 330 0.80 67144100 67144480 RL-5471-1 — PE-53826 PE-53826-S DOS022P-334

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Application Information (continued)

Table 1. Inductor Manufacturers Part Numbers (continued)

INDUCTANCE

(μH) CURRENT

(A)

SCHOTTKY RENCO PULSE ENGINEERING COILCRAFT

THROUGH-

HOLE SURFACE-

MOUNT THROUGH-HOLE SURFACE-

MOUNT THROUGH-

HOLE SURFACE-

MOUNT SURFACE-

MOUNT

L27 220 1.00 67144110 67144490 RL-5471-2 — PE-53827 PE-53827-S DOS022P-224

L28 150 1.20 67144120 67144500 RL-5471-3 — PE-53828 PE-53828-S DOS022P-154

L29 100 1.47 67144130 67144510 RL-5471-4 — PE-53829 PE-53829-S DOS022P-104

L30 68 1.78 67144140 67144520 RL-5471-5 — PE-53830 PE-53830-S DOS022P-683

L31 47 2.2 67144150 67144530 RL-5471-6 — PE-53831 PE-53831-S DOS022P-473

L32 33 2.5 67144160 67144540 RL-5471-7 — PE-53932 PE-53932-S DOS022P-333

L33 22 3.1 67148390 67148500 RL-1283-22-43 — PE-53933 PE-53933-S DOS022P-223

L34 15 3.4 67148400 67148790 RL-1283-15-43 — PE-53934 PE-53934-S DOS022P-153

L35 220 1.70 67144170 — RL-5473-1 — PE-53935 PE-53935-S —

L36 150 2.1 67144180 — RL-5473-4 — PE-54036 PE-54036-S —

L37 100 2.5 67144190 — RL-5472-1 — PE-54037 PE-54037-S —

L38 68 3.1 67144200 — RL-5472-2 — PE-54038 PE-54038-S —

L39 47 3.5 67144210 — RL-5472-3 — PE-54039 PE-54039-S —

L40 33 3.5 67144220 67148290 RL-5472-4 — PE-54040 PE-54040-S —

L41 22 3.5 67144230 67148300 RL-5472-5 — PE-54041 PE-54041-S —

L42 150 2.7 67148410 — RL-5473-4 — PE-54042 PE-54042-S —

L43 100 3.4 67144240 — RL-5473-2 — PE-54043 — —

L44 68 3.4 67144250 — RL-5473-3 — PE-54044 — —

9.1.8 Output Voltage Ripple and Transients

The output voltage of a switching power supply operating in the continuous mode contains a sawtooth ripple

voltage at the switcher frequency, and may also contain short voltage spikes at the peaks of the sawtooth

waveform.

The output ripple voltage is a function of the inductor sawtooth ripple current and the ESR of the output

capacitor. A typical output ripple voltage can range from approximately 0.5% to 3% of the output voltage. To

obtain low ripple voltage, the ESR of the output capacitor must be low; however, exercise caution when using

extremely low ESR capacitors because they can affect the loop stability, resulting in oscillation problems. If very

low output ripple voltage is needed (less than 20 mV), TI recommends a post ripple filter (see Figure 40). The

inductance required is typically between 1 μH and 5 μH, with low DC resistance, to maintain good load

regulation. A low ESR output filter capacitor is also required to assure good dynamic load response and ripple

reduction. The ESR of this capacitor may be as low as desired, because it is out of the regulator feedback loop.

Figure 27 shows a typical output ripple voltage, with and without a post ripple filter.

When observing output ripple with a scope, it is essential that a short, low inductance scope probe ground

connection be used. Most scope probe manufacturers provide a special probe terminator which is soldered onto

the regulator board, preferably at the output capacitor. This provides a very short scope ground, thus eliminating

the problems associated with the 3-inch ground lead normally provided with the probe, and provides a much

cleaner and more accurate picture of the ripple voltage waveform.

The voltage spikes are caused by the fast switching action of the output switch, the diode, and the parasitic

inductance of the output filter capacitor, and its associated wiring. To minimize these voltage spikes, the output

capacitor must be designed for switching regulator applications, and the lead lengths must be kept very short.

Wiring inductance, stray capacitance, as well as the scope probe used to evaluate these transients, all contribute

to the amplitude of these spikes.

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Figure 39. Peak-to-Peak Inductor Ripple Current vs Load Current

When a switching regulator is operating in the continuous mode, the inductor current waveform ranges from a

triangular to a sawtooth type of waveform (depending on the input voltage). For a given input and output voltage,

the peak-to-peak amplitude of this inductor current waveform remains constant. As the load current increases or

decreases, the entire sawtooth current waveform also rises and falls. The average value (or the center) of this

current waveform is equal to the DC load current.

If the load current drops to a low enough level, the bottom of the sawtooth current waveform reaches zero, and

the switcher smoothly changes from a continuous to a discontinuous mode of operation. Most switcher designs

(regardless how large the inductor value is) is forced to run discontinuous if the output is lightly loaded. This is a

perfectly acceptable mode of operation.

In a switching regulator design, knowing the value of the peak-to-peak inductor ripple current (ΔIIND) can be

useful for determining a number of other circuit parameters. Parameters such as, peak inductor or peak switch

current, minimum load current before the circuit becomes discontinuous, output ripple voltage and output

capacitor ESR can all be calculated from the peak-to-peak ΔIIND. When the inductor nomographs shown in

Figure 35 through Figure 38 are used to select an inductor value, the peak-to-peak inductor ripple current can

immediately be determined. Figure 39 shows the range of (ΔIIND) that can be expected for different load currents.

Figure 39 also shows how the peak-to-peak inductor ripple current (ΔIIND) changes from the lower border to the

upper border (for a given load current) within an inductance region. The upper border represents a higher input

voltage, while the lower border represents a lower input voltage (see Inductor Selection).

These curves are only correct for continuous mode operation, and only if the inductor selection guides are used

to select the inductor value

Consider the following example:

VOUT = 5 V, maximum load current of 2.5 A

VIN = 12 V, nominal, varying between 10 V and 16 V.

The selection guide in Figure 36 shows that the vertical line for a 2.5-A load current and the horizontal line for the

12-V input voltage intersect approximately midway between the upper and lower borders of the 33-μH inductance

region. A 33-μH inductor allows a peak-to-peak inductor current (ΔIIND) to flow at a percentage of the maximum

load current. In Figure 39, follow the 2.5-A line approximately midway into the inductance region, and read the

peak-to-peak inductor ripple current (ΔIIND) on the left hand axis (approximately 620 mAp-p).

As the input voltage increases to 16 V, it approaches the upper border of the inductance region, and the inductor

ripple current increases. Figure 39 shows that for a load current of 2.5 A, the peak-to-peak inductor ripple current

(ΔIIND) is 620 mA with 12 VIN, and can range from 740 mA at the upper border (16 VIN) to 500 mA at the lower

border (10 VIN).

Once the ΔIIND value is known, the following formulas can be used to calculate additional information about the

switching regulator circuit.

1. Peak Inductor or peak switch current

2. Minimum load current before the circuit becomes discontinuous

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3. Output Ripple Voltage = (ΔIIND) × (ESR of COUT = 0.62 A × 0.1 Ω= 62 mVp-p

9.1.9 Open Core Inductors

Another possible source of increased output ripple voltage or unstable operation is from an open core inductor.

Ferrite bobbin or stick inductors have magnetic lines of flux flowing through the air from one end of the bobbin to

the other end. These magnetic lines of flux induces a voltage into any wire or PCB copper trace that comes

within the inductor's magnetic field. The strength of the magnetic field, the orientation and location of the PC

copper trace to the magnetic field, and the distance between the copper trace and the inductor, determine the

amount of voltage generated in the copper trace. Another way of looking at this inductive coupling is to consider

the PCB copper trace as one turn of a transformer (secondary) with the inductor winding as the primary. Many

millivolts can be generated in a copper trace placed near an open core inductor which can cause stability

problems or high output ripple voltage problems.

If unstable operation is seen, and an open core inductor is used, it is possible that the location of the inductor

with respect to other PC traces may be the problem. To determine if this is the problem, temporarily raise the

inductor away from the board by several inches and then check circuit operation. If the circuit now operates

correctly, then the magnetic flux from the open core inductor is causing the problem. Substituting a closed core

inductor such as a torroid or E-core corrects the problem, or re-arranging the PC layout may be necessary.

Magnetic flux cutting the IC device ground trace, feedback trace, or the positive or negative traces of the output

capacitor must be minimized.

Sometimes, placing a trace directly beneath a bobbin inductor provides good results, provided it is exactly in the

center of the inductor (because the induced voltages cancel themselves out). However, problems could arise if

the trace is off center. If flux problems are present, even the direction of the inductor winding can make a

difference in some circuits.

This discussion on open core inductors is not to frighten users, but to alert them on what kind of problems to

watch out for. Open core bobbin or stick inductors are an inexpensive, simple way of making a compact, efficient

inductor, and they are used by the millions in many different applications.

9.2 Typical Applications

9.2.1 Fixed Output Voltage Version

Component values shown are for VIN = 15 V,

VOUT = 5 V, ILOAD = 3 A.

CIN = –70-μF, –50-V, aluminum electrolytic Nichicon PL Series

COUT = 220-μF, 25-V aluminum electrolytic Nichicon PL Series

D1 = 5-A, 40-V Schottky Rectifier, 1N5825

L1 = 68, L38

Typical Values

CSS = 0.1 μF

CDELAY = 0.1 μF

RPullup = 47k

Figure 40. Typical Application for Fixed Output Voltage Versions

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Typical Applications (continued)

9.2.1.1 Design Requirements

Table 2 lists the design parameters of this example.

Table 2. Design Parameters

PARAMETER EXAMPLE VALUE

Regulated output voltage (3.3 V, 5 V or 12 V), VOUT 5 V

Maximum DC input voltage, VIN(max) 12 V

Maximum load current, ILOAD(max) 3 A

9.2.1.2 Detailed Design Procedure

9.2.1.2.1 Inductor Selection (L1)

Select the correct inductor value selection guide from Figure 35,Figure 36,orFigure 37 (output voltages of 3.3

V, 5 V, or 12 V respectively). Use the inductor selection guide for the 5-V version shown in Figure 36.

From the inductor value selection guide, identify the inductance region intersected by the maximum input voltage

line and the maximum load current line. Each region is identified by an inductance value and an inductor code

(LXX). From the inductor value selection guide shown in Figure 36, the inductance region intersected by the 12-V

horizontal line and the 3-A vertical line is 33 μH, and the inductor code is L40.

Select an appropriate inductor from the four manufacturer's part numbers listed in Table 1. The inductance value

required is 33 μH. See row L30 of Table 3 and choose an inductor part number from any of the manufactures (in

most instance, both through-hole and surface-mount inductors are available).

9.2.1.2.2 Output Capacitor Selection (COUT)

In the majority of applications, low ESR (Equivalent Series Resistance) electrolytic capacitors between 82 μF and

820 μF and low ESR solid tantalum capacitors between 10 μF and 470 μF provide the best results. This

capacitor must be placed close to the IC using short capacitor leads and short copper traces. Do not use

capacitors larger than 820 μF. For additional information, see Output Capacitor (COUT).

To simplify the capacitor selection procedure, see Table 3. This table contains different input voltages, output

voltages, and load currents, and lists various inductors and output capacitors that provides the best design

solutions.

From the quick design component selection table shown in Table 3, locate the 5-V output voltage section. In the

load current column, choose the load current line that is closest to the current needed in your application, for this

example, use the 3-A line. In the maximum input voltage column, select the line that covers the input voltage

needed in your application, in this example, use the 15-V line. Continuing on this line are recommended

inductors and capacitors that provides the best overall performance.

The capacitor list contains both through-hole electrolytic and surface-mount tantalum capacitors from four

different capacitor manufacturers. TI recommends that both the manufacturers and the manufacturer's series that

are listed in the table be used. In this example, aluminum electrolytic capacitors from several different

manufacturers are available with the range of ESR numbers needed:

• 330-μF, 35-V Panasonic HFQ series

• 330-μF, 35-V Nichicon PL series

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Table 3. LM2599 Fixed Voltage Quick Design Component Selection Table

CONDITIONS INDUCTOR OUTPUT CAPACITOR

THROUGH-HOLE ELECTROLYTIC SURFACE-MOUNT TANTALUM

OUTPUT

VOLTAGE

(V)

LOAD

CURRENT

(A)

MAX INPUT

VOLTAGE (V) INDUCTANCE

(μH) INDUCTOR

(#) PANASONIC HFQ

SERIES (μF/V) NICHICON PL

SERIES (μF/V) AVX TPS

SERIES (μF/V) SPRAGUE 595D

SERIES (μF/V)

3.3

5 22 L41 470/25 560/16 330/6.3 390/6.3

7 22 L41 560/35 560/35 330/6.3 390/6.3

10 22 L41 680/35 680/35 330/6.3 390/6.3

40 33 L40 560/35 470/35 330/6.3 390/6.3

6 22 L33 470/25 470/35 330/6.3 390/6.3

10 33 L32 330/35 330/35 330/6.3 390/6.3

40 47 L39 330/35 270/50 220/10 330/10

8 22 L41 470/25 560/16 220/10 330/10

10 22 L41 560/25 560/25 220/10 330/10

15 33 L40 330/35 330/35 220/10 330/10

40 47 L39 330/35 270/35 220/10 330/10

9 22 L33 470/25 560/16 220/10 330/10

20 68 L38 180/35 180/35 100/10 270/10

40 68 L38 180/35 180/35 100/10 270/10

15 22 L41 470/25 470/25 100/16 180/16

18 33 L40 330/25 330/25 100/16 180/16

30 68 L44 180/25 180/25 100/16 120/20

40 68 L44 180/35 180/35 100/16 120/20

15 33 L32 330/25 330/25 100/16 180/16

20 68 L38 180/25 180/25 100/16 120/20

40 150 L42 82/25 82/25 68/20 68/25

The capacitor voltage rating for electrolytic capacitors must be at least 1.5 times greater than the output voltage,

and often much higher voltage ratings are needed to satisfy the low ESR requirements for low output ripple

voltage.

For a 5-V output, a capacitor voltage rating at least 7.5 V or more is needed. But even a low ESR, switching

grade, 220-μF 10-V aluminum electrolytic capacitor would exhibit approximately 225 mΩof ESR (see the curve

in Figure 32 for the ESR vs voltage rating). This amount of ESR would result in relatively high output ripple

voltage. To reduce the ripple to 1% of the output voltage, or less, a capacitor with a higher value or with a higher

voltage rating (lower ESR) must be selected. A 16-V or 25-V capacitor reduces the ripple voltage by

approximately half.

9.2.1.2.3 Catch Diode Selection (D1)

The catch diode current rating must be at least 1.3 times greater than the maximum load current. Also, if the

power supply design must withstand a continuous output short, the diode must have a current rating equal to the

maximum current limit of the LM2599. The most stressful condition for this diode is an overload or shorted output

condition. See Table 4. In this example, a 5-A, 20-V, 1N5823 Schottky diode provides the best performance, and

is not overstressed even for a shorted output.

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Table 4. Diode Selection Table

3-A DIODES 4-A TO 6-A DIODES

SURFACE-MOUNT THROUGH-HOLE SURFACE-MOUNT THROUGH-HOLE

SCHOTTKY ULTRA FAST

RECOVERY SCHOTTKY ULTRA FAST

RECOVERY

20 V

All of these diodes

are rated to at least

50 V

1N5820

All of these diodes

are rate to at least

50 V

All of these diodes

are rated to at least

50 V

SR502

All of these diodes

are rated to at least

50 V

SK32 SR302 1N5823

MBR320 SB520

30 V

30WQ03 1N5821

SK33 MBR330 50WQ03 SR503

31DQ03 1N5824

40 V

1N5822 SB530

SK34 SR304 50WQ04 SR504

MBRS340 MBR340 1N5825

30WQ04 MURS320 31DQ04 MUR320 MURS620 SB540 MUR620

50 V

SK35 30WF10 SR305 50WF10 HER601

MBRS360 MBR350 50WQ05 SB550

30WQ05 31DQ05 50SQ080

The reverse voltage rating of the diode must be at least 1.25 times the maximum input voltage.

This diode must be fast (short reverse recovery time) and must be placed close to the LM2599 using short leads

and short printed-circuit traces. Because of their fast switching speed and low forward voltage drop, Schottky

diodes provide the best performance and efficiency, and must be the first choice, especially in low output voltage

applications. Ultra-fast recovery, or high-efficiency rectifiers also provide good results. Ultra-fast recovery diodes

typically have reverse recovery times of 50 ns or less. Rectifiers such as the IN5400 series are much too slow

and must not be used.

9.2.1.2.4 Input Capacitor (CIN)

A low-ESR aluminum or tantalum bypass capacitor is needed between the input pin and ground to prevent large

voltage transients from appearing at the input. In addition, the RMS current rating of the input capacitor must be

selected to be at least ½ the DC load current. The capacitor manufacturers data sheet must be checked to

assure that this current rating is not exceeded. The curve shown in Figure 31 shows typical RMS current ratings

for several different aluminum electrolytic capacitor values.

This capacitor must be placed close to the IC using short leads and the voltage rating must be approximately 1.5

times the maximum input voltage.

If solid tantalum input capacitors are used, TI recommends that they be surge current tested by the

manufacturer.

Use caution when using ceramic capacitors for input bypassing, because it may cause severe ringing at the VIN

pin.

The important parameters for the Input capacitor are the input voltage rating and the RMS current rating. With a

nominal input voltage of 12 V, an aluminum electrolytic capacitor with a voltage rating greater than 18 V (1.5 ×

VIN) would be needed. The next higher capacitor voltage rating is 25 V.

The RMS current rating requirement for the input capacitor in a buck regulator is approximately ½ the DC load

current. In this example, with a 3-A load, a capacitor with a RMS current rating of at least 1.5 A is needed. The

curves shown in Figure 31 can be used to select an appropriate input capacitor. From the curves, locate the 35-V

line and note which capacitor values have RMS current ratings greater than 1.5 A. A 680-μF, 35-V capacitor

could be used.

For a through-hole design, a 680-μF, 35-V electrolytic capacitor (Panasonic HFQ series or Nichicon PL series or

equivalent) would be adequate. other types or other manufacturers capacitors can be used provided the RMS

ripple current ratings are adequate.

For surface-mount designs, solid tantalum capacitors are recommended. The TPS series available from AVX,

and the 593D series from Sprague are both surge current tested.For additional information, see Input Capacitor

(CIN).

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9.2.1.3 Application Curves

Horizontal time base: 2 μs/div,

VIN = 20 V, VOUT = 5 V, ILOAD = 2 A

L = 32 μH, COUT = 220 μF, COUT, ESR = 50 mΩ

A: Output pin voltage, 10 V/div.

B: Inductor current, 1 A/div.

C: Output ripple voltage, 50 mV/div.

Figure 41. Continuous Mode Switching Waveforms

Horizontal time base: 50 μs/div,

VIN = 20 V, VOUT = 5 V, ILOAD = 500 mA to 2 A,

L = 32 μH, COUT = 220 μF, COUT ESR = 50 mΩ

A: Output Voltage, 100 mV/div. (AC)

B: 500-mA to 2-A load pulse

Figure 42. Load Transient Response for Continuous Mode

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9.2.2 Adjustable Output Voltage Version

where VREF = 1.23 V

Select R1to be approximately 1 kΩ, use a 1% resistor for best stability.

Component values shown are for VIN = 20 V,

VOUT = 10 V, ILOAD = 3 A.

CIN = 470-μF, 35-V, aluminum electrolytic Nichicon PL Series

COUT = 220-μF, 35-V, aluminum electrolytic Nichicon PL Series

D1 = 5-A, 30-V Schottky Rectifier, 1N5824

L1 = 68 μH, L38

R1= 1 kΩ, 1%

R2= 7.15k, 1%

CFF = 3.3 nF

RFF = 3 kΩ

Typical Values

CSS = 0.1 μF

CDELAY = 0.1 μF

RPULLUP = 4.7k

Figure 43. Typical Application for Adjustable Output Voltage Versions

9.2.2.1 Design Requirements

Table 5 lists the design parameters of this example.

Table 5. Design Parameters

PARAMETER EXAMPLE VALUE

Regulated output voltage, VOUT 20 V

Maximum DC Input voltage, VIN(max) 28 V

Maximum load current, ILOAD(max) 3 A

Switching frequency, F Fixed at a nominal 150 kHz

9.2.2.2 Detailed Design Procedure

9.2.2.2.1 Programming Output Voltage

Select R1and R2.

Use Equation 1 to select the appropriate resistor values.

(1)

Select a value for R1between 240 Ωand 1.5 kΩusing Equation 2. The lower resistor values minimize noise

pickup in the sensitive feedback pin (for the lowest temperature coefficient and the best stability with time, use

1% metal film resistors).

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(2)

Select R1to be 1 kΩ, 1%. Solve for R2with Equation 3.

(3)

R2= 1k (16.26 −1) = 15.26k, closest 1% value is 15.4 kΩ.

R2= 15.4 kΩ.

9.2.2.2.2 Inductor Selection (L1)

1. Calculate the inductor Volt × microsecond constant E × T (V × μs) from Equation 4.

where

• VSAT = internal switch saturation voltage = 1.16 V

• VD= diode forward voltage drop = 0.5 V (4)

Calculate the inductor Volt × microsecond constant (E × T) with Equation 5.

(5)

2. Use the E × T value from the previous formula and match it with the E × T number on the vertical axis of the

Inductor Value Selection Guide shown in Figure 38.

E × T = 34.2 (V × μs)

3. On the horizontal axis, select the maximum load current.

ILOAD(max) = 3 A

4. Identify the inductance region intersected by the E × T value and the Maximum Load Current value. Each

region is identified by an inductance value and an inductor code (LXX). From the inductor value selection

guide shown in Figure 38, the inductance region intersected by the 34 (V × μs) horizontal line and the 3-A

vertical line is 47 μH, and the inductor code is L39.

5. Select an appropriate inductor from the four manufacturer's part numbers listed in Table 1. From the table in

Table 1, locate line L39, and select an inductor part number from the list of manufacturers part numbers.

9.2.2.2.3 Output Capacitor Selection (COUT)

In the majority of applications, low-ESR electrolytic or solid tantalum capacitors between 82 μF and 820 μF

provide the best results. This capacitor must be placed close to the IC using short capacitor leads and short

copper traces. Do not use capacitors larger than 820 μF. For additional information, see Output Capacitor (COUT).

To simplify the capacitor selection procedure, see Table 6. This table contains different output voltages, and lists

various output capacitors that provides the best design solutions.

From the quick design table shown in Table 6, locate the output voltage column. From that column, locate the

output voltage closest to the output voltage in your application. In this example, select the 24-V line. Under the

Output Capacitor (COUT)section, select a capacitor from the list of through-hole electrolytic or surface-mount

tantalum types from four different capacitor manufacturers. TI recommends that both the manufacturers and the

manufacturers series that are listed in the table be used.

In this example, through-hole aluminum electrolytic capacitors from several different manufacturers are available.

• 220-V, 35-A Panasonic HFQ series

• 150-V, 35-A Nichicon PL series

The capacitor voltage rating must be at least 1.5 times greater than the output voltage, and often much higher

voltage ratings are needed to satisfy the low ESR requirements needed for low output ripple voltage.

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For a 20-V output, a capacitor rating of at least 30 V or more is needed. In this example, either a 35-V or 50-V

capacitor would work. A 50-V rating was chosen because it has a lower ESR which provides a lower output

ripple voltage.

Other manufacturers or other types of capacitors may also be used, provided the capacitor specifications

(especially the 100-kHz ESR) closely match the types listed in the table. See the capacitor manufacturers data

sheet for this information.

9.2.2.2.4 Feedforward Capacitor (CFF)

For output voltages greater than approximately 10 V, an additional capacitor is required. The compensation

capacitor is typically between 100 pF and 33 nF, and is wired in parallel with the output voltage setting resistor,

R2. It provides additional stability for high output voltages, low input-output voltages, and/or very low ESR output

capacitors, such as solid tantalum capacitors.

(6)

This capacitor type can be ceramic, plastic, silver mica, and so forth (because of the unstable characteristics of

ceramic capacitors made with Z5U material, they are not recommended).

Table 6 contains feedforward capacitor values for various output voltages. In this example, a 560-pF capacitor is

needed.

Table 6. Output Capacitor and Feedforward Capacitor Selection Table

OUTPUT

VOLTAGE

(V)

THROUGH-HOLE OUTPUT CAPACITOR SURFACE-MOUNT OUTPUT CAPACITOR

PANASONIC HFQ

SERIES (μF/V) NICHICON PL

SERIES (μF/V) FEEDFORWARD

CAPACITOR AVX TPS

SERIES (μF/V) SPRAGUE 595D

SERIES (μF/V) FEEDFORWARD

CAPACITOR

2820/35 820/35 33 nF 330/6.3 470/4 33 nF

4560/35 470/35 10 nF 330/6.3 390/6.3 10 nF

6470/25 470/25 3.3 nF 220/10 330/10 3.3 nF

9330/25 330/25 1.5 nF 100/16 180/16 1.5 nF

1 2 330/25 330/25 1 nF 100/16 180/16 1 nF

1 5 220/35 220/35 680 pF 68/20 120/20 680 pF

2 4 220/35 150/35 560 pF 33/25 33/25 220 pF

2 8 100/50 100/50 390 pF 10/35 15/50 220 pF

9.2.2.2.5 Catch Diode Selection (D1)

The catch diode current rating must be at least 1.3 times greater than the maximum load current. Also, if the

power supply design must withstand a continuous output short, the diode must have a current rating equal to the

maximum current limit of the LM2599. The most stressful condition for this diode is an overload or shorted output

condition (see Table 4). Schottky diodes provide the best performance, and in this example a 3-A, 40-V, 1N5825

Schottky diode would be a good choice. The 3-A diode rating is more than adequate and is not overstressed

even for a shorted output.

The reverse voltage rating of the diode must be at least 1.25 times the maximum input voltage.

This diode must be fast (short reverse recovery time) and must be placed close to the LM2599 using short leads

and short printed-circuit traces. Because of their fast switching speed and low forward voltage drop, Schottky

diodes provide the best performance and efficiency, and must be the first choice, especially in low output voltage

applications. Ultra-fast recovery, or high-efficiency rectifiers are also a good choice, but some types with an

abrupt turn-off characteristic may cause instability or EMl problems. Ultra-fast recovery diodes typically have

reverse recovery times of 50 ns or less. Rectifiers such as the 1N4001 series are much too slow and must not be

used.

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9.2.2.2.6 Input Capacitor (CIN)

A low-ESR aluminum or tantalum bypass capacitor is needed between the input pin and ground to prevent large

voltage transients from appearing at the input. In addition, the RMS current rating of the input capacitor must be

selected to be at least ½ the DC load current. The capacitor manufacturers data sheet must be checked to

assure that this current rating is not exceeded. The curve shown in Figure 31 shows typical RMS current ratings

for several different aluminum electrolytic capacitor values.

This capacitor must be placed close to the IC using short leads and the voltage rating must be approximately 1.5

times the maximum input voltage.

If solid tantalum input capacitors are used, TI recommends that they be surge current tested by the

manufacturer.

Use caution when using a high dielectric constant ceramic capacitor for input bypassing, because it may cause

severe ringing at the VIN pin.

The important parameters for the Input capacitor are the input voltage rating and the RMS current rating. With a

nominal input voltage of 28 V, an aluminum electrolytic aluminum electrolytic capacitor with a voltage rating

greater than 42 V (1.5 × VIN) would be needed. Because the the next higher capacitor voltage rating is 50 V, a

50-V capacitor must be used. The capacitor voltage rating of (1.5 × VIN) is a conservative guideline, and can be

modified somewhat if desired.

The RMS current rating requirement for the input capacitor of a buck regulator is approximately ½ the DC load

current. In this example, with a 3-A load, a capacitor with a RMS current rating of at least 1.5 A is needed.

The curves shown in Figure 31 can be used to select an appropriate input capacitor. From the curves, locate the

50-V line and note which capacitor values have RMS current ratings greater than 1.5 A. Either a 470-μF or 680-

μF, 50-V capacitor could be used.

For a through-hole design, a 680-μF/50V electrolytic capacitor (Panasonic HFQ series or Nichicon PL series or

equivalent) would be adequate. Other types or other manufacturers capacitors can be used provided the RMS

ripple current ratings are adequate.

For surface-mount designs, solid tantalum capacitors can be used, but caution must be exercised with regard to

the capacitor sure current rating. The TPS series available from AVX, and the 593D series from Sprague are

both surge current tested. For additional information, see Input Capacitor (CIN).

9.2.2.3 Application Curves

Horizontal time base: 2 μs/div

VIN = 20 V, VOUT = 5 V, ILOAD = 500 mA,

L = 10 μH, COUT = 330 μF, COUT ESR = 45 mΩ

A: Output pin voltage, 10 V/div.

B: Inductor current, 0.5 A/div.

C: Output ripple voltage, 100 mV/div.

Figure 44. Discontinuous Mode Switching Waveforms

Horizontal Time Base: 200 μs/div

VIN = 20 V, VOUT = 5 V, ILOAD = 500 mA to 2 A

L = 10 μH, COUT = 330 μF, COUT ESR = 45 mΩ

A: Output voltage, 100 mV/div. (AC)

B: 500-mA to 2-A load pulse

Figure 45. Load Transient Response

for Discontinuous Mode

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10 Power Supply Recommendations

The LM2599 is designed to operate from an input voltage supply up to 40 V. This input supply must be well

regulated and able to withstand maximum input current and maintain a stable voltage.

11 Layout

11.1 Layout Guidelines

As in any switching regulator, layout is very important. Rapidly switching currents associated with wiring

inductance can generate voltage transients which can cause problems. For minimal inductance and ground

loops, the wires indicated by heavy lines must be wide printed-circuit traces and must be kept as short as

possible. For best results, external components must be placed as close to the switcher lC as possible using

ground plane construction or single point grounding.

If open core inductors are used, take special care to select the location and positioning of this type of inductor.

Allowing the inductor flux to intersect sensitive feedback, lC groundpath and COUT wiring can cause problems.

When using the adjustable version, take special care to select the location of the feedback resistors and the

associated wiring. Physically place both resistors near the IC, and route the wiring away from the inductor,

especially an open core type of inductor (see Open Core Inductors for more information).

11.2 Layout Examples

CIN = 470-μF, 50-V, aluminum electrolytic Panasonic HFQ Series

COUT = 330-μF, 35-V, aluminum electrolytic Panasonic HFQ Series

D1 = 5-A, 40-V Schottky rectifier, 1N5825

L1 = 47-μH, L39, Renco through hole

RPULL UP = 10k

CDELAY = 0.1 μF

CSD/SS = 0.1 μF

Thermalloy heat sink #7020

Figure 46. Typical Through-Hole PCB Layout, Fixed Output (1x Size), Double-Sided

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Layout Examples (continued)

CIN = 470-μF, 50-V, aluminum electrolytic Panasonic, HFQ Series

COUT = 220-μF, 35-V, aluminum electrolytic Panasonic, HFQ Series

D1 = 5-A, 40-V Schottky Rectifier, 1N5825

L1 = 47-μH, L39, Renco, through-hole

R1= 1 kΩ, 1%

R2= Use formula in Detailed Design Procedure

CFF = See Figure 35

RFF = See Feedforward Capacitor (CFF) for Adjustable Output Voltage Version Only

RPULL UP = 10k

CDELAY = 0.1 μF

CSD/SS= 0.1 μF

Thermalloy heat sink #7020

Figure 47. Typical Through-Hole PCB Layout, Adjustable Output (1x Size), Double-Sided

11.3 Thermal Considerations

The LM2599 is available in two packages, a 7-pin TO-220 and a 7-pin surface-mount TO-263.

The TO-220 package needs a heat sink under most conditions. The size of the heat sink depends on the input

voltage, the output voltage, the load current and the ambient temperature. The curves in Figure 48 show the

LM2599T junction temperature rises above ambient temperature for a 3-A load and different input and output

voltages. The data for these curves was taken with the LM2599T (TO-220 package) operating as a buck

switching regulator in an ambient temperature of 25°C (still air). These temperature rise numbers are all

approximate and there are many factors that can affect these temperatures. Higher ambient temperatures require

more heat sinking.

The TO-263 surface-mount package tab is designed to be soldered to the copper on a printed-circuit board. The

copper and the board are the heat sink for this package and the other heat producing components, such as the

catch diode and inductor. The PCB copper area that the package is soldered to must be at least 0.4 in2, and

ideally must have 2 or more square inches of 2-oz. (0.0028 in) copper. Additional copper area improves the

thermal characteristics, but with copper areas greater than approximately 6 in2, only small improvements in heat

dissipation are realized. If further thermal improvements are needed, double-sided, multilayer PCBs with large

copper areas or airflow are recommended.

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Thermal Considerations (continued)

The curves shown in Figure 49 show the LM2599S (TO-263 package) junction temperature rise above ambient

temperature with a 2-A load for various input and output voltages. This data was taken with the circuit operating

as a buck switching regulator with all components mounted on a PCB to simulate the junction temperature under

actual operating conditions. This curve can be used for a quick check for the approximate junction temperature

for various conditions, but be aware that there are many factors that can affect the junction temperature. When

load currents higher than 2 A are used, double-sided or multilayer PCBs with large copper areas or airflow might

be needed, especially for high ambient temperatures and high output voltages.

CIRCUIT DATA FOR TEMPERATURE RISE CURVE TO-220 PACKAGE (NDZ)

Capacitors Through-hole electrolytic

Inductor Through-hole Renco

Diode Through-hole, 5-A, 40-V Schottky

PCB 3-square inch, single-sided, 2-oz copper (0.0028″)

Figure 48. Junction Temperature Rise, TO-220

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Thermal Considerations (continued)

CIRCUIT DATA FOR TEMPERATURE RISE CURVE TO-263 PACKAGE (KTW)

Capacitors Surface-mount, tantalum molded Dsize

Inductor Surface-mount, Pulse engineering, 68 μH

Diode Surface-mount, 5-A, 40-V, Schottky

PCB 9-square inch, single-sided, 2-oz copper (0.0028″)

Figure 49. Junction Temperature Rise, TO-263

For the best thermal performance, wide copper traces and generous amounts of printed-circuit board copper

must be used in the board layout. (One exception to this is the output (switch) pin, which must not have large

areas of copper). Large areas of copper provide the best transfer of heat (lower thermal resistance) to the

surrounding air, and moving air lowers the thermal resistance even further.

Package thermal resistance and junction temperature rise numbers are all approximate, and there are many

factors that affects these numbers. Some of these factors include board size, shape, thickness, position, location,

and even board temperature. Other factors are, trace width, total printed-circuit copper area, copper thickness,

single- or double-sided, multilayer board and the amount of solder on the board. The effectiveness of the PCB to

dissipate heat also depends on the size, quantity and spacing of other components on the board, as well as

whether the surrounding air is still or moving. Furthermore, some of these components such as the catch diode

adds heat to the PCB and the heat can vary as the input voltage changes. For the inductor, depending on the

physical size, type of core material and the DC resistance, it could either act as a heat sink taking heat away

from the board, or it could add heat to the board.

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12 Device and Documentation Support

12.1 Community Resources

The following links connect to TI community resources. Linked contents are provided "AS IS" by the respective

contributors. They do not constitute TI specifications and do not necessarily reflect TI's views; see TI's Terms of

Use.

TI E2E™ Online Community TI's Engineer-to-Engineer (E2E) Community. Created to foster collaboration

among engineers. At e2e.ti.com, you can ask questions, share knowledge, explore ideas and help

solve problems with fellow engineers.

Design Support TI's Design Support Quickly find helpful E2E forums along with design support tools and

contact information for technical support.

12.2 Trademarks

E2E is a trademark of Texas Instruments.

SIMPLE SWITCHER is a registered trademark of Texas Instruments.

All other trademarks are the property of their respective owners.

12.3 Electrostatic Discharge Caution

These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam

during storage or handling to prevent electrostatic damage to the MOS gates.

12.4 Glossary

SLYZ022 —TI Glossary.

This glossary lists and explains terms, acronyms, and definitions.

13 Mechanical, Packaging, and Orderable Information

The following pages include mechanical, packaging, and orderable information. This information is the most

current data available for the designated devices. This data is subject to change without notice and revision of

this document. For browser-based versions of this data sheet, refer to the left-hand navigation.

PACKAGE OPTION ADDENDUM

www.ti.com 11-Jan-2021

Addendum-Page 1

PACKAGING INFORMATION

Orderable Device Status

(1)

Package Type Package

Drawing Pins Package

Qty Eco Plan

(2)

Lead finish/

Ball material

(6)

MSL Peak Temp

(3)

Op Temp (°C) Device Marking

(4/5)

Samples

LM2599S-12/NOPB ACTIVE DDPAK/

TO-263 KTW 7 45 RoHS-Exempt

& Green SN Level-3-245C-168 HR -40 to 125 LM2599S

-12 P+

LM2599S-3.3/NOPB ACTIVE DDPAK/

TO-263 KTW 7 45 RoHS-Exempt

& Green SN Level-3-245C-168 HR -40 to 125 LM2599S

-3.3 P+

LM2599S-5.0 NRND DDPAK/

TO-263 KTW 7 45 Non-RoHS

& Green Call TI Call TI -40 to 125 LM2599S

-5.0 P+

LM2599S-5.0/NOPB ACTIVE DDPAK/

TO-263 KTW 7 45 RoHS-Exempt

& Green SN Level-3-245C-168 HR -40 to 125 LM2599S

-5.0 P+

LM2599S-ADJ/NOPB ACTIVE DDPAK/

TO-263 KTW 7 45 RoHS-Exempt

& Green SN Level-3-245C-168 HR -40 to 125 LM2599S

-ADJ P+

LM2599SX-12/NOPB ACTIVE DDPAK/

TO-263 KTW 7 500 RoHS-Exempt

& Green SN Level-3-245C-168 HR -40 to 125 LM2599S

-12 P+

LM2599SX-3.3/NOPB ACTIVE DDPAK/

TO-263 KTW 7 500 RoHS-Exempt

& Green SN Level-3-245C-168 HR -40 to 125 LM2599S

-3.3 P+

LM2599SX-5.0/NOPB ACTIVE DDPAK/

TO-263 KTW 7 500 RoHS-Exempt

& Green SN Level-3-245C-168 HR -40 to 125 LM2599S

-5.0 P+

LM2599SX-ADJ/NOPB ACTIVE DDPAK/

TO-263 KTW 7 500 RoHS-Exempt

& Green SN Level-3-245C-168 HR -40 to 125 LM2599S

-ADJ P+

LM2599T-12/NOPB ACTIVE TO-220 NDZ 7 45 RoHS & Green SN Level-1-NA-UNLIM -40 to 125 LM2599T

-12 P+

LM2599T-5.0/NOPB ACTIVE TO-220 NDZ 7 45 RoHS & Green SN Level-1-NA-UNLIM -40 to 125 LM2599T

-5.0 P+

LM2599T-ADJ NRND TO-220 NDZ 7 45 Non-RoHS

& Green Call TI Call TI -40 to 125 LM2599T

-ADJ P+

LM2599T-ADJ/NOPB ACTIVE TO-220 NDZ 7 45 RoHS & Green SN Level-1-NA-UNLIM -40 to 125 LM2599T

-ADJ P+

(1) The marketing status values are defined as follows:

ACTIVE: Product device recommended for new designs.

LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.

NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.

PREVIEW: Device has been announced but is not in production. Samples may or may not be available.

OBSOLETE: TI has discontinued the production of the device.

PACKAGE OPTION ADDENDUM

www.ti.com 11-Jan-2021

Addendum-Page 2

(2) RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance

do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may

reference these types of products as "Pb-Free".

RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.

Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of <=1000ppm threshold. Antimony trioxide based

flame retardants must also meet the <=1000ppm threshold requirement.

(3) MSL, Peak Temp. - The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature.

(4) There may be additional marking, which relates to the logo, the lot trace code information, or the environmental category on the device.

(5) Multiple Device Markings will be inside parentheses. Only one Device Marking contained in parentheses and separated by a "~" will appear on a device. If a line is indented then it is a continuation

of the previous line and the two combined represent the entire Device Marking for that device.

(6) Lead finish/Ball material - Orderable Devices may have multiple material finish options. Finish options are separated by a vertical ruled line. Lead finish/Ball material values may wrap to two

lines if the finish value exceeds the maximum column width.

Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information

provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and

continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals.

TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release.

In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis.

TAPE AND REEL INFORMATION

*All dimensions are nominal

Device Package

Type Package

Drawing Pins SPQ Reel

Diameter

(mm)

Reel

Width

W1 (mm)

(mm) B0

(mm) K0

(mm) P1

(mm) W

(mm) Pin1

Quadrant

LM2599SX-12/NOPB DDPAK/

TO-263 KTW 7 500 330.0 24.4 10.75 14.85 5.0 16.0 24.0 Q2

LM2599SX-3.3/NOPB DDPAK/

TO-263 KTW 7 500 330.0 24.4 10.75 14.85 5.0 16.0 24.0 Q2

LM2599SX-5.0/NOPB DDPAK/

TO-263 KTW 7 500 330.0 24.4 10.75 14.85 5.0 16.0 24.0 Q2

LM2599SX-ADJ/NOPB DDPAK/

TO-263 KTW 7 500 330.0 24.4 10.75 14.85 5.0 16.0 24.0 Q2

PACKAGE MATERIALS INFORMATION

www.ti.com 15-Feb-2016

Pack Materials-Page 1

*All dimensions are nominal

Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm)

LM2599SX-12/NOPB DDPAK/TO-263 KTW 7 500 367.0 367.0 45.0

LM2599SX-3.3/NOPB DDPAK/TO-263 KTW 7 500 367.0 367.0 45.0

LM2599SX-5.0/NOPB DDPAK/TO-263 KTW 7 500 367.0 367.0 45.0

LM2599SX-ADJ/NOPB DDPAK/TO-263 KTW 7 500 367.0 367.0 45.0

PACKAGE MATERIALS INFORMATION

www.ti.com 15-Feb-2016

Pack Materials-Page 2

MECHANICAL DATA

NDZ0007B

www.ti.com

TA07B (Rev E)

MECHANICAL DATA

KTW0007B

www.ti.com

BOTTOM SIDE OF PACKAGE

TS7B (Rev E)

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