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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.

LM2738

SNVS556C –APRIL 2008–REVISED JANUARY 2016

LM2738 550-kHz/1.6-MHz 1.5-A Step-Down DC-DC Switching Regulator

1 Features

1• Space-Saving WSON and MSOP-PowerPAD™

Packages

• 3-V to 20-V Input Voltage Range

• 0.8-V to 18-V Output Voltage Range

• 1.5-A Output Current

• 550-kHz (LM2738Y) and 1.6-MHz (LM2738X)

Switching Frequencies

• 250-mΩNMOS Switch

• 400-nA Shutdown Current

• 0.8-V, 2% Internal Voltage Reference

• Internal Soft-Start

• Current-Mode, PWM Operation

• Thermal Shutdown

2 Applications

• Local Point of Load Regulation

• Core Power in HDDs

• Set-Top Boxes

• Battery Powered Devices

• USB Powered Devices

• DSL Modems

3 Description

The LM2738 regulator is a monolithic, high-

frequency, PWM step-down DC-DC converter in an 8-

pin WSON or 8-pin MSOP-PowerPAD package. It

provides all the active functions for local DC-DC

conversion with fast transient response and accurate

regulation in the smallest possible PCB area.

With a minimum of external components, the LM2738

is easy to use. The ability to drive 1.5-A loads with an

internal 250-mΩNMOS switch using state-of-the-art

0.5-µm BiCMOS technology results in the best power

density available. Switching frequency is internally set

to 550 kHz (LM2738Y) or 1.6 MHz (LM2738X),

allowing the use of extremely small surface-mount

inductors and chip capacitors. Even though the

operating frequencies are very high, efficiencies up to

90% are easy to achieve. External enable is included,

featuring an ultralow standby current of 400 nA. The

LM2738 utilizes current-mode control and internal

compensation to provide high-performance regulation

over a wide range of operating conditions. Additional

features include internal soft-start circuitry to reduce

in-rush current, cycle-by-cycle current limit, thermal

shutdown, and output over-voltage protection.

Device Information(1)

PART

NUMBER PACKAGE BODY SIZE (NOM)

LM2738 WSON (8) 3.00 mm × 3.00 mm

MSOP-PowerPAD (8) 3.00 mm × 3.00 mm

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

the end of the data sheet.

Typical Application Circuit Efficiency vs Load Current

VIN = 12 V, VOUT = 3.3 V

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Table of Contents

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

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

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

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

5 Pin Configuration and Functions......................... 3

6 Specifications......................................................... 4

6.1 Absolute Maximum Ratings ...................................... 4

6.2 ESD Ratings.............................................................. 4

6.3 Recommended Operating Conditions....................... 4

6.4 Thermal Information.................................................. 5

6.5 Electrical Characteristics........................................... 5

6.6 Typical Characteristics.............................................. 6

7 Detailed Description............................................ 10

7.1 Overview................................................................. 10

7.2 Functional Block Diagram....................................... 11

7.3 Feature Description................................................. 11

7.4 Device Functional Modes........................................ 14

8 Application and Implementation ........................ 15

8.1 Application Information............................................ 15

8.2 Typical Applications ................................................ 15

9 Power Supply Recommendations...................... 30

10 Layout................................................................... 30

10.1 Layout Guidelines ................................................. 30

10.2 Layout Example .................................................... 31

10.3 Thermal Considerations........................................ 31

11 Device and Documentation Support................. 33

11.1 Device Support...................................................... 33

11.2 Documentation Support ........................................ 33

11.3 Community Resources.......................................... 33

11.4 Trademarks........................................................... 33

11.5 Electrostatic Discharge Caution............................ 33

11.6 Glossary................................................................ 33

12 Mechanical, Packaging, and Orderable

Information........................................................... 33

4 Revision History

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

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

• Added Device Information table, ESD Ratings table, Thermal Information 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

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

• Changed layout of National Data Sheet to TI format ........................................................................................................... 29

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(1) I = Input, O = Output, and PWR = Power

5 Pin Configuration and Functions

NGQ Package

8-Pin WSON With Exposed Thermal Pad

Top View

DGN Package

8-Pin MSOP-PowerPAD

Top View

Pin Functions

PIN TYPE(1) DESCRIPTION

NO. NAME

1 BOOST I Boost voltage that drives the internal NMOS control switch. A bootstrap capacitor is

connected between the BOOST and SW pins.

2 VIN PWR Supply voltage for output power stage. Connect a bypass capacitor to this pin. Must tie pins

2 and 3 together at package.

3 VCC IInput supply voltage of the device. Connect a bypass capacitor to this pin. Must tie pins 2

and 3 together at the package.

4 EN I Enable control input. Logic high enables operation. Do not allow this pin to float or be greater

than VIN + 0.3 V.

5, 7 GND PWR Signal and power ground pins. Place the bottom resistor of the feedback network as close as

possible to these pins.

6 FB I Feedback pin. Connect FB to the external resistor divider to set output voltage.

8 SW O Output switch. Connects to the inductor, catch diode, and bootstrap capacitor.

DAP GND — Signal and power ground. Must be connected to GND on the PCB.

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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 or Aerospace specified devices are required, contact the Texas Instruments Sales Office or Distributors for availability and

specifications.

6 Specifications

6.1 Absolute Maximum Ratings

over operating free-air temperature range (unless otherwise noted) (1)(2)

MIN MAX UNIT

VIN, VCC –0.5 24 V

SW voltage –0.5 24 V

Boost voltage –0.5 30 V

Boost to SW voltage –0.5 6 V

FB voltage –0.5 3 V

EN voltage –0.5 VIN + 0.3 V

Junction temperature 150 °C

Soldering information Infrared and convection reflow (15 s) 220 °C

Wave soldering lead temperature (10 s) 260 °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.

(2) Human body model, 1.5 kΩin series with 100 pF.

6.2 ESD Ratings VALUE UNIT

V(ESD) Electrostatic discharge Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001(1)(2) ±2000 V

6.3 Recommended Operating Conditions

over operating free-air temperature range (unless otherwise noted) MIN MAX UNIT

VIN, VCC 3 20 V

SW voltage –0.5 20 V

Boost voltage –0.5 25.5 V

Boost to SW voltage 2.5 5.5 V

Junction temperature −40 125 °C

Thermal shutdown 165 °C

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

application report, SPRA953.

(2) Typical thermal shutdown occurs if the junction temperature exceeds 165°C. The maximum power dissipation is a function of TJ(MAX) ,

RθJA and TA. The maximum allowable power dissipation at any ambient temperature is PD= (TJ(MAX) – TA) / RθJA. All numbers apply for

packages soldered directly onto a 3 inches × 3 inches PC board with 2 oz. copper on 4 layers in still air in accordance to JEDEC

standards. Thermal resistance varies greatly with layout, copper thickness, number of layers in PCB, power distribution, number of

thermal vias, board size, ambient temperature, and air flow.

6.4 Thermal Information

THERMAL METRIC(1)

LM2738

UNITNGQ (WSON) DGN (MSOP

PowerPAD)

8 PINS 8 PINS

RθJA Junction-to-ambient thermal resistance (2) 45.9 50.3 °C/W

RθJC(top) Junction-to-case (top) thermal resistance 44.6 54.2 °C/W

RθJB Junction-to-board thermal resistance 13.2 31.4 °C/W

ψJT Junction-to-top characterization parameter 0.5 4.8 °C/W

ψJB Junction-to-board characterization parameter 13.2 31.2 °C/W

RθJC(bot) Junction-to-case (bottom) thermal resistance 5.8 4 °C/W

(1) Ensured to average outgoing quality level (AOQL).

(2) Typicals represent the most likely parametric norm.

6.5 Electrical Characteristics

All typical limits apply over TJ= 25°C, and all maximum and minimum limits apply over the full operating temperature range

(TJ= –40°C to +125°C). VIN = 12 V, VBOOST – VSW = 5 V unless otherwise specified. Data sheet minimum and maximum

specification limits are ensured by design, test, or statistical analysis.

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

VFB Feedback voltage 0.784 0.800 0.816 V

ΔVFB/ΔVIN Feedback voltage line regulation VIN = 3 V to 20 V 0.02 %/V

IFB Feedback input bias current Sink or source 0.1 100 nA

UVLO Undervoltage lockout VIN Rising 2.7 2.9 VUndervoltage lockout VIN Falling 2 2.3

UVLO hysteresis 0.4

FSW Switching frequency LM2738X 1.28 1.6 1.92 MHz

LM2738Y 0.364 0.55 0.676

DMAX Maximum duty cycle LM2738X , Load = 150 mA 92%

LM2738Y, Load = 150 mA 95%

DMIN Minimum duty cycle LM2738X 7.5%

LM2738Y 2%

RDS(ON) Switch ON resistance VBOOST – VSW = 3 V, Load = 400 mA 250 500 mΩ

ICL Switch current limit VBOOST – VSW = 3 V, VIN = 3 V 2 2.9 A

IQQuiescent current Switching 1.9 3 mA

Non-Switching 1.9 mA

Quiescent current (shutdown) VEN = 0 V 400 nA

IBOOST Boost pin current LM2738X (27% Duty Cycle) 4.5 mA

LM2738Y (27% Duty Cycle) 2.5

VEN_TH Shutdown threshold voltage VEN Falling 0.4 V

Enable threshold voltage VEN Rising 1.4

IEN Enable pin current Sink / Source 10 nA

ISW Switch leakage VIN = 20 V 100 nA

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6.6 Typical Characteristics

All curves taken at VIN = 12 V, VBOOST – VSW = 5 V, and TA= 25°C, unless specified otherwise.

VOUT = 5 V

Figure 1. Efficiency vs Load Current – X Version

VOUT = 5 V

Figure 2. Efficiency vs Load Current – Y Version

VOUT = 3.3 V

Figure 3. Efficiency vs Load Current – X Version

VOUT = 3.3 V

Figure 4. Efficiency vs Load Current – Y Version

VOUT = 1.5 V

Figure 5. Efficiency vs Load Current – X Version

VOUT = 1.5 V

Figure 6. Efficiency vs Load Current – Y Version

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Typical Characteristics (continued)

All curves taken at VIN = 12 V, VBOOST – VSW = 5 V, and TA= 25°C, unless specified otherwise.

Figure 7. Oscillator Frequency vs Temperature – X Version Figure 8. Oscillator Frequency vs Temperature – Y Version

VIN = 5 V

Figure 9. Current Limit vs Temperature Figure 10. IQNon-Switching vs Temperature

Figure 11. VFB vs Temperature Figure 12. RDSON vs Temperature

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Typical Characteristics (continued)

All curves taken at VIN = 12 V, VBOOST – VSW = 5 V, and TA= 25°C, unless specified otherwise.

VOUT = 1.5 V IOUT = 750 mA

Figure 13. Line Regulation – X Version

VOUT = 1.5 V IOUT = 750 mA

Figure 14. Line Regulation – Y Version

VOUT = 3.3 V IOUT = 750 mA

Figure 15. Line Regulation – X Version

VOUT = 3.3 V IOUT = 750 mA

Figure 16. Line Regulation – Y Version

VOUT = 1.5 V

Figure 17. Load Regulation – X Version

VOUT = 1.5 V

Figure 18. Load Regulation – Y Version

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Typical Characteristics (continued)

All curves taken at VIN = 12 V, VBOOST – VSW = 5 V, and TA= 25°C, unless specified otherwise.

VOUT = 3.3 V

Figure 19. Load Regulation – X Version

VOUT = 3.3 V

Figure 20. Load Regulation – Y Version

Figure 21. IQSwitching vs Temperature

VOUT = 3.3 V VIN = 12 V

Figure 22. Load Transient – X Version

VOUT = 3.3 V VIN = 12 V IOUT = 1.5 A

Figure 23. Startup – X Version (Resistive Load)

VOUT = 3.3 V VIN = 12 V IOUT = 1.5 A

Figure 24. In-Rush Current – X Version (Resistive Load)

VIN

TON

Inductor

Current

D = TON/TSW

VSW

TOFF

TSW

IPK

Voltage

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7 Detailed Description

7.1 Overview

The LM2738 is a constant frequency PWM buck regulator device that delivers a 1.5-A load current. The regulator

has a preset switching frequency of either 550 kHz (LM2738Y) or 1.6 MHz (LM2738X). These high frequencies

allow the LM2738 to operate with small surface-mount capacitors and inductors, resulting in DC-DC converters

that require a minimum amount of board space. The LM2738 is internally compensated, so it is simple to use and

requires few external components. The LM2738 uses current-mode control to regulate the output voltage.

The LM2738 supplies a regulated output voltage by switching the internal NMOS control switch at constant

frequency and variable duty cycle. A switching cycle begins at the falling edge of the reset pulse generated by

the internal oscillator. When this pulse goes low, the output control logic turns on the internal NMOS control

switch. During this on time, the SW pin voltage (VSW) swings up to approximately VIN, and the inductor current

(IL) increases with a linear slope. ILis measured by the current-sense amplifier, which generates an output

proportional to the switch current. The sense signal is summed with the regulator’s corrective ramp and

compared to the error amplifier’s output, which is proportional to the difference between the feedback voltage

and VREF. When the PWM comparator output goes high, the output switch turns off until the next switching cycle

begins. During the switch off-time, inductor current discharges through Schottky diode D1, which forces the SW

pin to swing below ground by the forward voltage (VD) of the catch diode. The regulator loop adjusts the duty

cycle (D) to maintain a constant output voltage. See Functional Block Diagram and Figure 25.

Figure 25. LM2738 Waveforms of SW Pin Voltage and Inductor Current

BOOST

Output

Control

Logic

Current

Limit

Thermal

Shutdown

Under

Voltage

Lockout

Corrective Ramp

Reset

Pulse

PWM

Comparator

Current-Sense Amplifier RSENSE

Internal

Regulator

and

Enable

Circuit

Oscillator

Driver 0.25:

Switch

Internal

Compensation

GND

Error Amplifier -

+VREF

0.8V

COUT

OFF

VBOOST

VSW

CBOOST

VOUT

CIN

VIN

ISENSE

-0.93V

OVP

Comparator

Error

Signal

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7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 Boost Function

Capacitor CBOOST and diode D2 in Figure 26 are used to generate a voltage VBOOST. VBOOST – VSW is the gate-

drive voltage to the internal NMOS control switch. To properly drive the internal NMOS switch during its on time,

VBOOST must be at least 2.5 V greater than VSW. TI recommends that VBOOST be greater than 2.5 V above VSW for

best efficiency. VBOOST – VSW must not exceed the maximum operating limit of 5.5 V. For best performance, see

Equation 1.

5.5 V > VBOOST – VSW > 2.5 V (1)

When the LM2738 starts up, internal circuitry from the BOOST pin supplies a maximum of 20 mA to CBOOST. This

current charges CBOOST to a voltage sufficient to turn the switch on. The BOOST pin continues to source current

to CBOOST until the voltage at the feedback pin is greater than 0.76 V.

There are various methods to derive VBOOST:

1. From the input voltage (3 V < VIN < 5.5 V)

2. From the output voltage (2.5 V < VOUT < 5.5 V)

3. From an external distributed voltage rail (2.5 V < VEXT < 5.5 V)

4. From a shunt or series Zener diode

As seen on the Functional Block Diagram, capacitor CBOOST and diode D2 supply the gate-drive voltage for the

NMOS switch. Capacitor CBOOST is charged via diode D2 by VIN. During a normal switching cycle, when the

internal NMOS control switch is off (TOFF) (refer to Figure 25), VBOOST equals VIN minus the forward voltage of D2

(VFD2), during which the current in the inductor (L) forward biases the Schottky diode D1 (VFD1). Therefore the

voltage stored across CBOOST is Equation 2:

VBOOST – VSW = VIN – VFD2 + VFD1 (2)

When the NMOS switch turns on (TON), the switch pin rises to Equation 3:

VSW = VIN – (RDSON × IL), (3)

forcing VBOOST to rise, thus reverse biasing D2. The voltage at VBOOST is then Equation 4:

VBOOST = 2 VIN – (RDSON × IL) – VFD2 + VFD1 (4)

which is approximately 2 VIN – 0.4 V for many applications. Thus the gate-drive voltage of the NMOS switch is

approximately VIN – 0.2 V.

LM2738

VIN BOOST

GND

CBOOST

VBOOST

VOUT

VIN

CIN

COUT

LM2738

BOOST

GND

COUT

CBOOST

VOUT

CIN

VIN

VBOOST

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Feature Description (continued)

An alternate method for charging CBOOST is to connect D2 to the output as shown in Figure 26. The output

voltage must be between 2.5 V and 5.5 V so that proper gate voltage is applied to the internal switch. In this

circuit, CBOOST provides a gate-drive voltage that is slightly less than VOUT.

Figure 26. VOUT Charges CBOOST

In applications where both VIN and VOUT are greater than 5.5 V, or less than 3 V, CBOOST cannot be charged

directly from these voltages. If VIN and VOUT are greater than 5.5 V, CBOOST can be charged from VIN or VOUT

minus a Zener voltage by placing a Zener diode D3 in series with D2, as shown in Figure 27. When using a

series Zener diode from the input, ensure that the regulation of the input supply does not create a voltage that

falls outside the recommended VBOOST voltage.

(VINMAX – VD3) < 5.5 V

(VINMIN – VD3) > 2.5 V

Figure 27. Zener Reduces Boost Voltage from VIN

An alternative method is to place the Zener diode D3 in a shunt configuration as shown in Figure 28. A small

350-mW to 500-mW 5.1-V Zener in a SOT-23 or SOD package can be used for this purpose. A small ceramic

capacitor such as a 6.3-V, 0.1-µF capacitor (C4) must be placed in parallel with the Zener diode. When the

internal NMOS switch turns on, a pulse of current is drawn to charge the internal NMOS gate capacitance. The

0.1-µF parallel shunt capacitor ensures that the VBOOST voltage is maintained during this time.

LM2738

VIN BOOST

GND

VBOOST

CBOOST

VOUT

COUT

VIN

CIN

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Feature Description (continued)

Figure 28. Boost Voltage Supplied from the Shunt Zener on VIN

Resistor R3 must be selected to provide enough RMS current to the Zener diode (D3) and to the BOOST pin. A

recommended choice for the Zener current (IZENER) is 1 mA. The current IBOOST into the BOOST pin supplies the

gate current of the NMOS control switch and varies typically according to the formula in Equation 5 for the X

version:

IBOOST = 0.56 × (D + 0.54) × (VZENER – VD2) mA (5)

IBOOST can be calculated for the Y version using Equation 6:

IBOOST = 0.22 × (D + 0.54) × (VZENER – VD2) µA

where

• D is the duty cycle

• VZENER and VD2 are in volts

• IBOOST is in milliamps

• VZENER is the voltage applied to the anode of the boost diode (D2)

• VD2 is the average forward voltage across D2 (6)

The formula for IBOOST in Equation 6 gives typical current. For the worst case IBOOST, increase the current by

40%. In that case, the worst case boost current is Equation 7:

IBOOST-MAX = 1.4 × IBOOST (7)

R3 is then given by Equation 8:

R3 = (VIN – VZENER) / (1.4 × IBOOST + IZENER) (8)

For example, using the X-version let VIN = 10 V, VZENER =5V,VD2 = 0.7 V, IZENER = 1 mA, and duty cycle

D = 50%. Then Equation 9 and Equation 10:

IBOOST = 0.56 × (0.5 + 0.54) × (5 – 0.7) mA = 2.5 mA (9)

R3 = (10 V – 5 V) / (1.4 × 2.5 mA + 1 mA) = 1.11 kΩ(10)

7.3.2 Soft-Start

This function forces VOUT to increase at a controlled rate during start-up. During soft-start, the error amplifier’s

reference voltage ramps from 0 V to its nominal value of 0.8 V in approximately 600 µs. This forces the regulator

output to ramp up in a more linear and controlled fashion, which helps reduce in-rush current.

7.3.3 Output Overvoltage Protection

The overvoltage comparator compares the FB pin voltage to a voltage that is 16% higher than the internal

reference VREF. Once the FB pin voltage goes 16% above the internal reference, the internal NMOS control

switch is turned off, which allows the output voltage to decrease toward regulation.

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Feature Description (continued)

7.3.4 Undervoltage Lockout

Undervoltage lockout (UVLO) prevents the LM2738 from operating until the input voltage exceeds 2.7 V (typical).

The UVLO threshold has approximately 400 mV of hysteresis, so the part operates until VIN drops below 2.3 V

(typical). Hysteresis prevents the part from turning off during power up if the VIN ramp-up is non-monotonic.

7.3.5 Current Limit

The LM2738 uses cycle-by-cycle current limiting to protect the output switch. During each switching cycle, a

current limit comparator detects if the output switch current exceeds 2.9 A (typical), and turns off the switch until

the next switching cycle begins.

7.3.6 Thermal Shutdown

Thermal shutdown limits total power dissipation by turning off the output switch when the device junction

temperature exceeds 165°C. After thermal shutdown occurs, the output switch doesn’t turn on until the junction

temperature drops to approximately 150°C.

7.4 Device Functional Modes

7.4.1 Enable Pin and Shutdown Mode

The LM2738 has a shutdown mode that is controlled by the enable pin (EN). When a logic low voltage is applied

to EN, the part is in shutdown mode, and its quiescent current drops to typically 400 nA. The voltage at this pin

must never exceed VIN + 0.3 V.

LM2738

VIN VIN

BOOST

GND

VOUT

C3 L1

OFF

C1 R3

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8 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 must

validate and test their design implementation to confirm system functionality.

8.1 Application Information

The LM2738 operates over a wide range of conditions, which is limited by the ON time of the device. Figure 29

shows the recommended operating area for the X version at the full load (1.5 A) and at 25°C ambient

temperature. The Y version of the LM2738 operates at a lower frequency, and therefore operates over the entire

range of operating voltages.

Figure 29. LM2738X – 1.6 MHz (25°C, Load = 1.5 A)

8.2 Typical Applications

8.2.1 LM2738X Circuit Example 1

Figure 30. LM2738X (1.6 MHz)

VBOOST Derived from VIN

5 V to 1.5 V/1.5 A

8.2.1.1 Design Requirements

The device must be able to operate at any voltage within the Recommended Operating Conditions. The load

current must be defined to properly size the inductor, input, and output capacitors. The inductor must be able to

support the full expected load current as well as the peak current generated from load transients and start-up.

IN OUT L

V V 2 i

L DT

-D

D = VO + VD

VIN + VD - VSW

D = VO

VIN

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Typical Applications (continued)

8.2.1.2 Detailed Design Procedure

Table 1. Bill of Materials for Figure 30

PART ID PART VALUE PART NUMBER MANUFACTURER

U1 1.5-A Buck Regulator LM2738X Texas Instruments

C1, Input Cap 10 µF, 6.3 V, X5R C3216X5ROJ106M TDK

C2, Output Cap 22 µF, 6.3 V, X5R C3216X5ROJ226M TDK

C3, Boost Cap 0.1 uF, 16 V, X7R C1005X7R1C104K TDK

D1, Catch Diode 0.34 VFSchottky 1.5 A, 30 V CRS08 Toshiba

D2, Boost Diode 1 VFat 100-mA Diode BAT54WS Diodes, Inc.

L1 2.2 µH, 1.9 A, MSS5131-222ML Coilcraft

R1 8.87 kΩ, 1% CRCW06038871F Vishay

R2 10.2 kΩ, 1% CRCW06031022F Vishay

R3 100 kΩ, 1% CRCW06031003F Vishay

8.2.1.2.1 Inductor Selection

The duty cycle (D) can be approximated quickly using the ratio of output voltage (VO) to input voltage (VIN), using

Equation 11:

(11)

The catch diode (D1) forward voltage drop and the voltage drop across the internal NMOS switch must be

included to calculate a more accurate duty cycle. Calculate D by using Equation 12:

(12)

VSW can be approximated by Equation 13:

VSW = IOUT × RDSON (13)

The diode forward drop (VD) can range from 0.3 V to 0.7 V depending on the quality of the diode. The lower the

VD, the higher the operating efficiency of the converter. The inductor value determines the output ripple current.

Lower inductor values decrease the size of the inductor, but increase the output ripple current. An increase in the

inductor value decreases the output ripple current.

One must ensure that the minimum current limit (2 A) is not exceeded, so the peak current in the inductor must

be calculated. The peak current (ILPK) in the inductor is calculated by Equation 14 and Equation 15:

ILPK = IOUT +ΔiL(14)

Figure 31. Inductor Current

(15)

In general in Equation 16,

ΔiL= 0.1 × (IOUT)→0.2 × (IOUT) (16)

RMS _IN OUT

I I D(1 D)= ´ -

RMS _IN OUT

2 i

I D I (1 D) 3

é ù

- +

ê ú

ë û

COND OUT DSON

OUT

P (I D) 1 R

3 I

æ ö

ç ÷

= ´ + ´ç ÷

ç ÷

è ø

LM2738

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Typical Applications (continued)

If ΔiL= 33.3% of 1.5 A, the peak current in the inductor is 2 A. The minimum specified current limit over all

operating conditions is 2 A. One can either reduce ΔiL, or make the engineering judgment that zero margin is

safe enough. The typical current limit is 2.9 A.

The LM2738 operates at frequencies allowing the use of ceramic output capacitors without compromising

transient response. Ceramic capacitors allow higher inductor ripple without significantly increasing output ripple.

See the Output Capacitor section for more details on calculating output voltage ripple. Now that the ripple current

is determined, the inductance is calculated by Equation 17:

where

• (17)

When selecting an inductor, make sure that it is capable of supporting the peak output current without saturating.

Inductor saturation results in a sudden reduction in inductance and prevents the regulator from operating

correctly. Because of the speed of the internal current limit, the peak current of the inductor need only be

specified for the required maximum output current. For example, if the designed maximum output current is 1 A

and the peak current is 1.25 A, the inductor must be specified with a saturation current limit of > 1.25 A. There is

no must specify the saturation or peak current of the inductor at the 2.9-A typical switch current limit. Because of

the operating frequency of the LM2738, ferrite based inductors are preferred to minimize core losses. This

presents little restriction because of the variety of ferrite-based inductors available. Lastly, inductors with lower

series resistance (RDCR) provide better operating efficiency. For recommended inductors see LM2738X Circuit

Example 1.

8.2.1.2.2 Input Capacitor

An input capacitor is necessary to ensure that VIN does not drop excessively during switching transients. The

primary specifications of the input capacitor are capacitance, voltage, RMS current rating, and equivalent series

inductance (ESL). The recommended input capacitance is 10 µF. The input voltage rating is specifically stated by

the capacitor manufacturer. Make sure to check any recommended deratings and also verify if there is any

significant change in capacitance at the operating input voltage and the operating temperature. The input

capacitor maximum RMS input current rating (IRMS-IN) must be greater than Equation 18:

(18)

Neglecting inductor ripple simplifies Equation 18 to Equation 19:

(19)

Equation 19 shows that maximum RMS capacitor current occurs when D = 0.5. Always calculate the RMS at the

point where the duty cycle D is closest to 0.5. The ESL of an input capacitor is usually determined by the

effective cross-sectional area of the current path. A large leaded capacitor has high ESL and a 0805 ceramic-

chip capacitor has very low ESL. At the operating frequencies of the LM2738, leaded capacitors may have an

ESL so large that the resulting impedance (2πfL) is higher than that required to provide stable operation. As a

result, surface-mount capacitors are strongly recommended.

Sanyo POSCAP, Tantalum or Niobium, Panasonic SP, and multilayer ceramic capacitors (MLCC) are all good

choices for both input and output capacitors and have very low ESL. For MLCCs, TI recommends using X7R or

X5R type capacitors due to their tolerance and temperature characteristics. Consult the capacitor manufacturer's

data sheets to see how rated capacitance varies over operating conditions.

8.2.1.2.3 Output Capacitor

The output capacitor is selected based upon the desired output ripple and transient response. The initial current

of a load transient is provided mainly by the output capacitor. The output ripple of the converter is Equation 20:

OUT D

IN D SW

V V

V V V

+ -

OUT

OUT LOSS

P P

h =

OUT

h =

REF

R1 1 R2

æ ö

= - ´

ç ÷

è ø

OUT L ESR

SW OUT

V I R

8 F C

æ ö

D = D +

ç ÷

´ ´

è ø

LM2738

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Typical Applications (continued)

(20)

When using MLCCs, the equivalent series resistance (ESR) is typically so low that the capacitive ripple may

dominate. When this occurs, the output ripple is approximately sinusoidal and 90° phase shifted from the

switching action. Given the availability and quality of MLCCs and the expected output voltage of designs using

the LM2738, there is really no must review any other capacitor technologies. Another benefit of ceramic

capacitors is the ability to bypass high-frequency noise. A certain amount of switching edge noise couples

through parasitic capacitances in the inductor to the output. A ceramic capacitor bypasses this noise while a

tantalum capacitor does not. Since the output capacitor is one of the two external components that control the

stability of the regulator control loop, most applications require a minimum of 22 µF of output capacitance.

Capacitance, in general, is often increased when operating at lower duty cycles. Refer to the Circuit Examples for

suggested output capacitances of common applications. Like the input capacitor, recommended multilayer

ceramic capacitors are X7R or X5R types.

8.2.1.2.4 Catch Diode

The catch diode (D1) conducts during the switch off time. A Schottky diode is recommended for its fast switching

times and low forward voltage drop. The catch diode must be chosen so that its current rating is greater than

Equation 21:

ID1 = IOUT × (1-D) (21)

The reverse breakdown rating of the diode must be at least the maximum input voltage plus appropriate margin.

To improve efficiency, choose a Schottky diode with a low forward-voltage drop.

8.2.1.2.5 Output Voltage

The output voltage is set using Equation 22 and Equation 23 where R2 is connected between the FB pin and

GND, and R1 is connected between VOand the FB pin. A good value for R2 is 10 kΩ. When designing a unity

gain converter (VO= 0.8 V), R1 must be between 0 Ωand 100 Ω, and R2 must not be loaded.

(22)

VREF = 0.80 V (23)

8.2.1.2.6 Calculating Efficiency and Junction Temperature

The complete LM2738 DC-DC converter efficiency can be calculated by Equation 24 or Equation 25:

(24)

or,

(25)

Calculations for determining the most significant power losses are shown in Equation 26. Other losses totaling

less than 2% are not discussed.

Power loss (PLOSS) is the sum of two basic types of losses in the converter: switching and conduction.

Conduction losses usually dominate at higher output loads, whereas switching losses remain relatively fixed and

dominate at lower output loads. The first step in determining the losses is to calculate the duty cycle (D):

(26)

VSW is the voltage drop across the internal NFET when it is on, and is equal to Equation 27:

VSW = IOUT × RDSON (27)

COND OUT DSON

OUT

P (I D) 1 R

3 I

æ ö

ç ÷

= ´ + ´ ç ÷

ç ÷

è ø

OUT D DCR

IN D DCR SW

V V V

V V V V

+ +

+ + -

LM2738

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Typical Applications (continued)

VDis the forward voltage drop across the Schottky catch diode. It can be obtained from the diode manufacturer's

data sheet Electrical Characteristics section. If the voltage drop across the inductor (VDCR) is accounted for, the

equation becomes Equation 28:

(28)

The conduction losses in the free-wheeling Schottky diode are calculated by Equation 29:

PDIODE = VD× IOUT × (1-D) (29)

Often this is the single most significant power loss in the circuit. Care must be taken to choose a Schottky diode

that has a low forward-voltage drop.

Another significant external power loss is the conduction loss in the output inductor. The equation can be

simplified to Equation 30:

PIND = IOUT2× RDCR (30)

The LM2738 conduction loss is mainly associated with the internal NFET switch in Equation 31:

(31)

If the inductor ripple current is fairly small, the conduction losses can be simplified to Equation 32:

PCOND = IOUT2× RDSON × D (32)

Switching losses are also associated with the internal NFET switch. They occur during the switch on and off

transition periods, where voltages and currents overlap resulting in power loss. The simplest means to determine

this loss is to empirically measure the rise and fall times (10% to 90%) of the switch at the switch node.

Switching Power Loss is calculated as follows in Equation 33,Equation 34, and Equation 35:

PSWR = 1/2(VIN × IOUT × FSW × TRISE) (33)

PSWF = 1/2(VIN × IOUT × FSW × TFALL) (34)

PSW = PSWR + PSWF (35)

Another loss is the power required for operation of the internal circuitry in Equation 36:

PQ= IQ× VIN (36)

IQis the quiescent operating current, and is typically around 1.9 mA for the 0.55-MHz frequency option.

Table 2 lists the power losses for a typical application, and in Equation 37,Equation 38, and Equation 39.

Table 2. Typical Configuration and Power Loss Calculation

PARAMETER VALUE POWER PARAMETER CALCULATED POWER

VIN 12 V — —

VOUT 3.3 V POUT 4.125 W

IOUT 1.25 A — —

VD0.34 V PDIODE 317 mW

FSW 550 kHz — —

IQ1.9 mA PQ22.8 mW

TRISE 8 nS PSWR 33 mW

TFALL 8 nS PSWF 33 mW

RDS(ON) 275 mΩPCOND 118 mW

INDDCR 70 mΩPIND 110 mW

D 0.275 PLOSS 634 mW

η86.7% PINTERNAL 207 mW

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ΣPCOND + PSW + PDIODE + PIND + PQ= PLOSS (37)

ΣPCOND + PSWF + PSWR + PQ= PINTERNAL (38)

PINTERNAL = 207 mW (39)

8.2.1.3 Application Curve

VOUT =5V

Figure 32. Efficiency vs Load Current – X Version

LM2738

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8.2.2 LM2738X Circuit Example 2

Figure 33. LM2738X (1.6 MHz)

VBOOST Derived from VOUT

12 V to 3.3 V / 1.5 A

8.2.2.1 Detailed Design Procedure

Table 3. Bill of Materials for Figure 33

PART ID PART VALUE PART NUMBER MANUFACTURER

U1 1.5-A Buck Regulator LM2738X Texas Instruments

C1, Input Cap 10 µF, 25 V, X7R C3225X7R1E106M TDK

C2, Output Cap 33 µF, 6.3 V, X5R C3216X5ROJ336M TDK

C3, Boost Cap 0.1 µF, 16 V, X7R C1005X7R1C104K TDK

D1, Catch Diode 0.34 VFSchottky 1.5 A, 30 V CRS08 Toshiba

D2, Boost Diode 1 VFat 100-mA Diode BAT54WS Diodes, Inc.

L1 5 µH, 2.9 A MSS7341- 502NL Coilcraft

R1 31.6 kΩ, 1% CRCW06033162F Vishay

R2 10 kΩ, 1% CRCW06031002F Vishay

R3 100 kΩ, 1% CRCW06031003F Vishay

8.2.2.2 Application Curve

VOUT = 3.3 V

Figure 34. Efficiency vs Load Current – X Version

LM2738

VIN VIN

BOOST

GND

VOUT

C3 L1

OFF

C1 R3

LM2738

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8.2.3 LM2738X Circuit Example 3

Figure 35. LM2738X (1.6 MHz)

VBOOST Derived from VSHUNT

18 V to 1.5 V / 1.5 A

8.2.3.1 Detailed Design Procedure

Table 4. Bill of Materials for Figure 35

PART ID PART VALUE PART NUMBER MANUFACTURER

U1 1.5-A Buck Regulator LM2738X Texas Instruments

C1, Input Cap 10 µF, 25 V, X7R C3225X7R1E106M TDK

C2, Output Cap 47 µF, 6.3 V, X5R C3216X5ROJ476M TDK

C3, Boost Cap 0.1 µF, 16 V, X7R C1005X7R1C104K TDK

C4, Shunt Cap 0.1 µF, 6.3 V, X5R C1005X5R0J104K TDK

D1, Catch Diode 0.34 VFSchottky 1.5 A, 30 V CRS08 Toshiba

D2, Boost Diode 1 VFat 100-mA Diode BAT54WS Diodes, Inc.

D3, Zener Diode 5.1-V 250-Mw SOT-23 BZX84C5V1 Vishay

L1 2.7 µH, 1.76 A VLCF5020T-2R7N1R7 TDK

R1 8.87 kΩ, 1% CRCW06038871F Vishay

R2 10.2 kΩ, 1% CRCW06031022F Vishay

R3 100 kΩ, 1% CRCW06031003F Vishay

R4 4.12 kΩ, 1% CRCW06034121F Vishay

LM2738

VIN VIN

BOOST

GND

VOUT

C3 L1

OFF

C1 R3

LM2738

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8.2.3.2 Application Curve

VOUT = 1.5 V

Figure 36. Efficiency vs Load Current – X Version

8.2.4 LM2738X Circuit Example 4

Figure 37. LM2738X (1.6 MHz)

VBOOST Derived from Series Zener Diode (VIN)

15 V to 1.5 V / 1.5 A

8.2.4.1 Detailed Design Procedure

Table 5. Bill of Materials for Figure 37

PART ID PART VALUE PART NUMBER MANUFACTURER

U1 1.5-A Buck Regulator LM2738X Texas Instruments

C1, Input Cap 10 µF, 25 V, X7R C3225X7R1E106M TDK

C2, Output Cap 47 µF, 6.3 V, X5R C3216X5ROJ476M TDK

C3, Boost Cap 0.1 µF, 16 V, X7R C1005X7R1C104K TDK

D1, Catch Diode 0.34 VFSchottky 1.5 A, 30 V CRS08 Toshiba

D2, Boost Diode 1 VFat 100-mA Diode BAT54WS Diodes, Inc.

D3, Zener Diode 11-V 350-Mw SOT-23 BZX84C11T Diodes, Inc.

L1 3.3 µH, 3.5 A MSS7341-332NL Coilcraft

R1 8.87 kΩ, 1% CRCW06038871F Vishay

R2 10.2 kΩ, 1% CRCW06031022F Vishay

R3 100 kΩ, 1% CRCW06031003F Vishay

LM2738

VIN VIN

BOOST

GND

VOUT

C3 L1

OFF

D2 D3

C1 R3

LM2738

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8.2.5 LM2738X Circuit Example 5

Figure 38. LM2738X (1.6 MHz)

VBOOST Derived from Series Zener Diode (VOUT)

15Vto9V/1.5A

8.2.5.1 Detailed Design Procedure

Table 6. Bill of Materials for Figure 38

PART ID PART VALUE PART NUMBER MANUFACTURER

U1 1.5-A Buck Regulator LM2738X Texas Instruments

C1, Input Cap 10 µF, 25 V, X7R C3225X7R1E106M TDK

C2, Output Cap 22 µF, 16 V, X5R C3216X5R1C226M TDK

C3, Boost Cap 0.1 µF, 16 V, X7R C1005X7R1C104K TDK

D1, Catch Diode 0.34 VFSchottky 1.5 A, 30 V CRS08 Toshiba

D2, Boost Diode 1 VFat 100-mA Diode BAT54WS Diodes, Inc.

D3, Zener Diode 4.3-V 350-mw SOT-23 BZX84C4V3 Diodes, Inc.

L1 6.2 µH, 2.5 A MSS7341-622NL Coilcraft

R1 102 kΩ, 1% CRCW06031023F Vishay

R2 10.2 kΩ, 1% CRCW06031022F Vishay

R3 100 kΩ, 1% CRCW06031003F Vishay

LM2738

VIN VIN

BOOST

GND

VOUT

C3 L1

OFF

C1 R3

LM2738

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8.2.6 LM2738Y Circuit Example 6

Figure 39. LM2738Y (550 kHz)

VBOOST Derived from VIN

5Vto1.5V/1.5A

8.2.6.1 Detailed Design Procedure

Table 7. Bill of Materials for Figure 39

PART ID PART VALUE PART NUMBER MANUFACTURER

U1 1.5-A Buck Regulator LM2738Y Texas Instruments

C1, Input Cap 10 µF, 6.3 V, X5R C3216X5ROJ106M TDK

C2, Output Cap 47 µF, 6.3 V, X5R C3216X5ROJ476M TDK

C3, Boost Cap 0.1 µF, 16 V, X7R C1005X7R1C104K TDK

D1, Catch Diode 0.34 VFSchottky 1.5 A, 30 V CRS08 Toshiba

D2, Boost Diode 1 VFat 100-mA Diode BAT54WS Diodes, Inc.

L1 6.2 µH, 2.5 A, MSS7341-622NL Coilcraft

R1 8.87 kΩ, 1% CRCW06038871F Vishay

R2 10.2 kΩ, 1% CRCW06031022F Vishay

R3 100 kΩ, 1% CRCW06031003F Vishay

8.2.6.2 Application Curve

VOUT = 1.5 V

Figure 40. Efficiency vs Load Current – Y Version

LM2738

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8.2.7 LM2738Y Circuit Example 7

Figure 41. LM2738Y (550 kHz)

VBOOST Derived from VOUT

12 V to 3.3 V / 1.5 A

8.2.7.1 Detailed Design Procedure

Table 8. Bill of Materials for Figure 41

PART ID PART VALUE PART NUMBER MANUFACTURER

U1 1.5-A Buck Regulator LM2738Y Texas Instruments

C1, Input Cap 10 µF, 25 V, X7R C3225X7R1E106M TDK

C2, Output Cap 47 µF, 6.3 V, X5R C3216X5ROJ476M TDK

C3, Boost Cap 0.1 µF, 16 V, X7R C1005X7R1C104K TDK

D1, Catch Diode 0.34 VFSchottky 1.5 A, 30 V CRS08 Toshiba

D2, Boost Diode 1 VFat 100-mA Diode BAT54WS Vishay

L1 12 µH, 1.7 A, MSS7341-123NL Coilcraft

R1 31.6 kΩ, 1% CRCW06033162F Vishay

R2 10 kΩ, 1% CRCW06031002F Vishay

R3 100 kΩ, 1% CRCW06031003F Vishay

8.2.7.2 Application Curve

VOUT = 3.3 V

Figure 42. Efficiency vs Load Current – Y Version

LM2738

VIN VIN

BOOST

GND

VOUT

C3 L1

OFF

C1 R3

LM2738

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8.2.8 LM2738Y Circuit Example 8

Figure 43. LM2738Y (550 kHz)

VBOOST Derived from VSHUNT

18 V to 1.5 V / 1.5 A

8.2.8.1 Detailed Design Procedure

Table 9. Bill of Materials for Figure 43

PART ID PART VALUE PART NUMBER MANUFACTURER

U1 1.5-A Buck Regulator LM2738Y Texas Instruments

C1, Input Cap 10 µF, 25 V, X7R C3225X7R1E106M TDK

C2, Output Cap (47 µF, 6.3 V, X5R) × 2 = 94 µF C3216X5ROJ476M TDK

C3, Boost Cap 0.1 µF, 16 V, X7R C1005X7R1C104K TDK

C4, Shunt Cap 0.1 µF, 6.3 V, X5R C1005X5R0J104K TDK

D1, Catch Diode 0.34 VFSchottky 1.5 A, 30 V CRS08 Toshiba

D2, Boost Diode 1 VFat 100-mA Diode BAT54WS Diodes, Inc.

D3, Zener Diode 5.1-V 250-Mw SOT-23 BZX84C5V1 Vishay

L1 8.7 µH, 2.2 A MSS7341-872NL Coilcraft

R1 8.87 kΩ, 1% CRCW06038871F Vishay

R2 10.2 kΩ, 1% CRCW06031022F Vishay

R3 100 kΩ, 1% CRCW06031003F Vishay

R4 4.12 kΩ, 1% CRCW06034121F Vishay

LM2738

VIN VIN

BOOST

GND

VOUT

C3 L1

OFF

C1 R3

LM2738

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8.2.8.2 Application Curve

VOUT = 1.5 V

Figure 44. Efficiency vs Load Current – Y Version

8.2.9 LM2738Y Circuit Example 9

Figure 45. LM2738Y (550 kHz)

VBOOST Derived from Series Zener Diode (VIN)

15 V to 1.5 V / 1.5 A

8.2.9.1 Detailed Design Procedure

Table 10. Bill of Materials for Figure 45

PART ID PART VALUE PART NUMBER MANUFACTURER

U1 1.5-A Buck Regulator LM2738Y Texas Instruments

C1, Input Cap 10 µF, 25 V, X7R C3225X7R1E106M TDK

C2, Output Cap (47 µF, 6.3 V, X5R) × 2 = 94 µF C3216X5ROJ476M TDK

C3, Boost Cap 0.1 µF, 16 V, X7R C1005X7R1C104K TDK

D1, Catch Diode 0.34 VFSchottky 1.5 A, 30 V CRS08 Toshiba

D2, Boost Diode 1 VFat 100-mA Diode BAT54WS Diodes, Inc.

D3, Zener Diode 11-V 350-Mw SOT-23 BZX84C11T Diodes, Inc.

L1 8.7 µH, 2.2 A MSS7341-872NL Coilcraft

R1 8.87 kΩ, 1% CRCW06038871F Vishay

R2 10.2 kΩ, 1% CRCW06031022F Vishay

R3 100 kΩ, 1% CRCW06031003F Vishay

LM2738

VIN VIN

BOOST

GND

VOUT

C3 L1

OFF

D2 D3

C1 R3

LM2738

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8.2.9.2 Application Curve

VOUT = 1.5 V

Figure 46. Efficiency vs Load Current – Y Version

8.2.10 LM2738Y Circuit Example 10

Figure 47. LM2738Y (550 kHz)

VBOOST Derived from Series Zener Diode (VOUT)

15Vto9V/1.5A

8.2.10.1 Detailed Design Procedure

Table 11. Bill of Materials for Figure 47

PART ID PART VALUE PART NUMBER MANUFACTURER

U1 1.5-A Buck Regulator LM2738Y Texas Instruments

C1, Input Cap 10 µF, 25 V, X7R C3225X7R1E106M TDK

C2, Output Cap 22 µF, 16 V, X5R C3216X5R1C226M TDK

C3, Boost Cap 0.1 µF, 16 V, X7R C1005X7R1C104K TDK

D1, Catch Diode 0.34 VFSchottky 1.5 A, 30 V CRS08 Toshiba

D2, Boost Diode 1 VFat 100-mA Diode BAT54WS Diodes, Inc.

D3, Zener Diode 4.3-V 350-mw SOT-23 BZX84C4V3 Diodes, Inc.

L1 15 µH, 2.1 A SLF7055T150M2R1-3PF TDK

R1 102 kΩ, 1% CRCW06031023F Vishay

R2 10.2 kΩ, 1% CRCW06031022F Vishay

R3 100 kΩ, 1% CRCW06031003F Vishay

LM2738

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Product Folder Links: LM2738

9 Power Supply Recommendations

The input voltage is rated as 3 V to 20 V. Care must be taken in certain circuit configurations, such as when

VBOOST is derived from VIN, where the requirement that VBOOST – VSW is less than 5.5 V must be observed. Also

for best efficiency, VBOOST must be at least 2.5 V above VSW. The voltage on the enable (EN) pin must not

exceed VIN by more than 0.3 V.

10 Layout

10.1 Layout Guidelines

When planning layout there are a few things to consider when trying to achieve a clean, regulated output. The

most important consideration is the close coupling of the GND connections of the input capacitor and the catch

diode D1. These ground ends must be close to one another and be connected to the GND plane with at least

two through-holes. Place these components as close as possible to the device. Next in importance is the location

of the GND connection of the output capacitor, which must be near the GND connections of CIN and D1. There

must be a continuous ground plane on the bottom layer of a two-layer board except under the switching node

island. The FB pin is a high-impedance node, and take care to make the FB trace short to avoid noise pickup

and inaccurate regulation. The feedback resistors must be placed as close to the device as possible, with the

GND of R1 placed as close to the GND of the device as possible. The VOUT trace to R2 must be routed away

from the inductor and any other traces that are switching. High AC currents flow through the VIN, SW, and VOUT

traces, so they must be as short and wide as possible. However, making the traces wide increases radiated

noise, so the designer must make this trade-off. Radiated noise can be decreased by choosing a shielded

inductor. The remaining components must also be placed as close to the device as possible. See AN-1229

SIMPLE SWITCHER®PCB Layout Guidelines (SNVA054) for further considerations, and the LM2738 demo

board as an example of a four-layer layout.

10.1.1 WSON Package

Figure 48. Internal WSON Connection

For certain high power applications, the PCB land may be modified to a dog-bone shape (see Figure 49). By

increasing the size of ground plane, and adding thermal vias, the RθJA for the application can be reduced.

J A

T T

Power

q =

Power

q =

LM2738

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10.2 Layout Example

Figure 49. 8-Lead WSON PCB Dog Bone Layout

10.3 Thermal Considerations

Heat in the LM2738 due to internal power dissipation is removed through conduction and/or convection.

Conduction: Heat transfer occurs through cross sectional areas of material. Depending on the material, the

transfer of heat can be considered to have poor to good thermal conductivity properties (insulator vs. conductor).

Heat Transfer goes as:

Silicon →package →lead frame →PCB

Convection: Heat transfer is by means of airflow. This could be from a fan or natural convection. Natural

convection occurs when air currents rise from the hot device to cooler air.

Thermal impedance is defined as Equation 40:

(40)

Thermal impedance from the silicon junction to the ambient air is defined as Equation 41:

(41)

The PCB size, weight of copper used to route traces and ground plane, and number of layers within the PCB can

greatly effect RθJA. The type and number of thermal vias can also make a large difference in the thermal

impedance. Thermal vias are necessary in most applications. They conduct heat from the surface of the PCB to

the ground plane. Four to six thermal vias must be placed under the exposed pad to the ground plane if the

WSON package is used.

Thermal impedance also depends on the thermal properties due to the application's operating conditions (VIN,

VO, IOand so forth), and the surrounding circuitry.

10.3.1 Silicon Junction Temperature Determination Methods

To accurately measure the silicon temperature for a given application, two methods can be used.

10.3.1.1 Method 1

The first method requires the user to know the thermal impedance of the silicon junction to top case temperature.

165 C 144 C

R 102 C/W

207 mW

° - °

q = = °

INTERNAL

165 T

° -

q =

J C

T T

Power

F =

LM2738

SNVS556C –APRIL 2008–REVISED JANUARY 2016

www.ti.com

Product Folder Links: LM2738

Thermal Considerations (continued)

To clarify:

RθJC is the thermal impedance from all six sides of a device package to silicon junction.

In this data sheet RΦJC is used, allowing the user to measure top case temperature with a small thermocouple

attached to the top case.

RΦJC is approximately 30°C/W for the 8-pin WSON package with the exposed pad. With the internal dissipation

from the efficiency calculation given previously, and the case temperature, RΦJC can be empirically measured on

the bench as Equation 42.

(42)

Therefore in Equation 43:

Tj= (RΦJC × PLOSS)+TC(43)

From the previous example, shows Equation 44 and Equation 45:

Tj= (RΦJC × PINTERNAL)+TC(44)

Tj= 30°C/W × 0.207 W + TC(45)

10.3.1.2 Method 2

The second method can give a very accurate silicon junction temperature.

The first step is to determine RθJA of the application. The LM2738 has overtemperature protection circuitry. When

the silicon temperature reaches 165°C, the device stops switching. The protection circuitry has a hysteresis of

about 15°C. Once the silicon temperature has decreased to approximately 150°C, the device starts to switch

again. Knowing this, the RθJA for any application can be characterized during the early stages of the design one

may calculate the RθJA by placing the PCB circuit into a thermal chamber. Raise the ambient temperature in the

given working application until the circuit enters thermal shutdown. If the SW pin is monitored, it is obvious when

the internal NFET stops switching, indicating a junction temperature of 165°C. Knowing the internal power

dissipation from the above equations, the junction temperature and the ambient temperature RθJA can be

determined with Equation 46.

(46)

Once RθJA is determined, the maximum ambient temperature allowed for a desired junction temperature can be

calculated.

An example of calculating RθJA for an application using the Texas Instruments LM2738 WSON demonstration

board is shown in Equation 48.

The four-layer PCB is constructed using FR4 with ½ oz copper traces. The copper ground plane is on the bottom

layer. The ground plane is accessed by two vias. The board measures 3 cm × 3 cm. It was placed in an oven

with no forced airflow. The ambient temperature was raised to 144°C, and at that temperature, the device went

into thermal shutdown.

From the previous example, Equation 47 and Equation 48 shows:

PINTERNAL = 207 mW (47)

(48)

If the junction temperature is kept below 125°C, then the ambient temperature cannot go above 109°C, seen in

Equation 49 and Equation 50.

Tj– (RθJA × PLOSS)=TA(49)

125°C – (102°C/W × 207 mW) = 104°C (50)

LM2738

www.ti.com

SNVS556C –APRIL 2008–REVISED JANUARY 2016

Product Folder Links: LM2738

11 Device and Documentation Support

11.1 Device Support

11.1.1 Third-Party Products Disclaimer

TI'S PUBLICATION OF INFORMATION REGARDING THIRD-PARTY PRODUCTS OR SERVICES DOES NOT

CONSTITUTE AN ENDORSEMENT REGARDING THE SUITABILITY OF SUCH PRODUCTS OR SERVICES

OR A WARRANTY, REPRESENTATION OR ENDORSEMENT OF SUCH PRODUCTS OR SERVICES, EITHER

ALONE OR IN COMBINATION WITH ANY TI PRODUCT OR SERVICE.

11.2 Documentation Support

11.2.1 Related Documentation

For related documentation see the following:

AN-1229 SIMPLE SWITCHER® PCB Layout Guidelines (SNVA054)

11.3 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.

11.4 Trademarks

PowerPAD, E2E are trademarks of Texas Instruments.

All other trademarks are the property of their respective owners.

11.5 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.

11.6 Glossary

SLYZ022 —TI Glossary.

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

12 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 8-Oct-2015

Addendum-Page 1

PACKAGING INFORMATION

Orderable Device Status

(1)

Package Type Package

Drawing Pins Package

Qty Eco Plan

(2)

Lead/Ball Finish

(6)

MSL Peak Temp

(3)

Op Temp (°C) Device Marking

(4/5)

Samples

LM2738XMY/NOPB ACTIVE MSOP-

PowerPAD DGN 8 1000 Green (RoHS

& no Sb/Br) CU SN Level-1-260C-UNLIM -40 to 125 STDB

LM2738XSD/NOPB ACTIVE WSON NGQ 8 1000 Green (RoHS

& no Sb/Br) CU SN Level-3-260C-168 HR -40 to 125 L237B

LM2738YMY/NOPB ACTIVE MSOP-

PowerPAD DGN 8 1000 Green (RoHS

& no Sb/Br) CU SN Level-1-260C-UNLIM -40 to 125 SJBB

LM2738YSD/NOPB ACTIVE WSON NGQ 8 1000 Green (RoHS

& no Sb/Br) CU SN Level-3-260C-168 HR -40 to 125 L174B

(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.

(2) Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check http://www.ti.com/productcontent for the latest availability

information and additional product content details.

TBD: The Pb-Free/Green conversion plan has not been defined.

Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that

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

Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between

the die and leadframe. The component is otherwise considered Pb-Free (RoHS compatible) as defined above.

Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame retardants (Br or Sb do not exceed 0.1% by weight

in homogeneous material)

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

value exceeds the maximum column width.

PACKAGE OPTION ADDENDUM

www.ti.com 8-Oct-2015

Addendum-Page 2

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

LM2738XMY/NOPB MSOP-

Power

PAD

DGN 8 1000 178.0 12.4 5.3 3.4 1.4 8.0 12.0 Q1

LM2738XSD/NOPB WSON NGQ 8 1000 178.0 12.4 3.3 3.3 1.0 8.0 12.0 Q1

LM2738YMY/NOPB MSOP-

Power

PAD

DGN 8 1000 178.0 12.4 5.3 3.4 1.4 8.0 12.0 Q1

LM2738YSD/NOPB WSON NGQ 8 1000 178.0 12.4 3.3 3.3 1.0 8.0 12.0 Q1

PACKAGE MATERIALS INFORMATION

www.ti.com 28-Oct-2016

Pack Materials-Page 1

*All dimensions are nominal

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

LM2738XMY/NOPB MSOP-PowerPAD DGN 8 1000 210.0 185.0 35.0

LM2738XSD/NOPB WSON NGQ 8 1000 210.0 185.0 35.0

LM2738YMY/NOPB MSOP-PowerPAD DGN 8 1000 210.0 185.0 35.0

LM2738YSD/NOPB WSON NGQ 8 1000 210.0 185.0 35.0

PACKAGE MATERIALS INFORMATION

www.ti.com 28-Oct-2016

Pack Materials-Page 2

MECHANICAL DATA

DGN0008A

www.ti.com

MUY08A (Rev A)

BOTTOM VIEW

www.ti.com

PACKAGE OUTLINE

8X 0.3

0.2

2 0.1

8X 0.5

0.3

1.5

1.6 0.1

6X 0.5

0.8

0.7

0.05

0.00

B3.1

2.9 A

3.1

2.9

(0.1) TYP

WSON - 0.8 mm max heightNGQ0008A

PLASTIC SMALL OUTLINE - NO LEAD

4214922/A 03/2018

PIN 1 INDEX AREA

SEATING PLANE

0.08 C

PIN 1 ID 0.1 C A B

0.05 C

THERMAL PAD

EXPOSED

SYMM

NOTES:

1. All linear dimensions are in millimeters. Any dimensions in parenthesis are for reference only. Dimensioning and tolerancing

per ASME Y14.5M.

2. This drawing is subject to change without notice.

3. The package thermal pad must be soldered to the printed circuit board for thermal and mechanical performance.

SCALE 4.000

www.ti.com

EXAMPLE BOARD LAYOUT

0.07 MIN

ALL AROUND

0.07 MAX

ALL AROUND

(1.6)

6X (0.5)

(2.8)

8X (0.25)

8X (0.6)

(2)

(R0.05) TYP ( 0.2) VIA

TYP

(0.75)

WSON - 0.8 mm max heightNGQ0008A

PLASTIC SMALL OUTLINE - NO LEAD

4214922/A 03/2018

SYMM

LAND PATTERN EXAMPLE

EXPOSED METAL SHOWN

SCALE:20X

NOTES: (continued)

4. This package is designed to be soldered to a thermal pad on the board. For more information, see Texas Instruments literature

number SLUA271 (www.ti.com/lit/slua271).

5. Vias are optional depending on application, refer to device data sheet. If any vias are implemented, refer to their locations shown

on this view. It is recommended that vias under paste be filled, plugged or tented.

SOLDER MASK

OPENING

SOLDER MASK

METAL UNDER

SOLDER MASK

DEFINED

EXPOSED METAL

METAL

SOLDER MASK

OPENING

SOLDER MASK DETAILS

NON SOLDER MASK

DEFINED

(PREFERRED)

EXPOSED METAL

www.ti.com

EXAMPLE STENCIL DESIGN

8X (0.25)

8X (0.6)

6X (0.5)

(1.79)

(1.47)

(2.8)

(R0.05) TYP

WSON - 0.8 mm max heightNGQ0008A

PLASTIC SMALL OUTLINE - NO LEAD

4214922/A 03/2018

NOTES: (continued)

6. Laser cutting apertures with trapezoidal walls and rounded corners may offer better paste release. IPC-7525 may have alternate

design recommendations.

SOLDER PASTE EXAMPLE

BASED ON 0.1 mm THICK STENCIL

EXPOSED PAD 9:

82% PRINTED SOLDER COVERAGE BY AREA UNDER PACKAGE

SCALE:20X

SYMM

METAL

TYP

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