LM3497MM/NOPB - NATIONAL SEMICONDUCTOR

LM34919B

LM34919B Ultra Small 40V, 600 mA Constant On-Time Buck Switching Regulator

Literature Number: SNVS623

LM34919B

May 26, 2010

Ultra Small 40V, 600 mA Constant On-Time Buck Switching

Regulator

General Description

The LM34919B Step Down Switching Regulator features all

of the functions needed to implement a low cost, efficient,

buck bias regulator capable of supplying 0.6A to the load. This

buck regulator contains an N-Channel Buck Switch, and is

available in a micro SMD package. The constant on-time

feedback regulation scheme requires no loop compensation,

results in fast load transient response, and simplifies circuit

implementation. The operating frequency remains constant

with line and load variations due to the inverse relationship

between the input voltage and the on-time. The valley current

limit results in a smooth transition from constant voltage to

constant current mode when current limit is detected, reduc-

ing the frequency and output voltage, without the use of

foldback. Additional features include: VCC under-voltage

lockout, thermal shutdown, gate drive under-voltage lockout,

and maximum duty cycle limiter.

Features

■AEC-Q100 Grade 1 qualified (-40°c to 125°c)

■Maximum switching frequency: 2.6 MHz

(VIN=14V,Vo=3.3V)

■Input Voltage Range: 6V to 40V

■Integrated N-Channel buck switch

■Integrated start-up regulator

■No loop compensation required

■Ultra-Fast transient response

■Operating frequency remains constant with load current

and input voltage

■Maximum Duty Cycle Limited During Start-Up

■Adjustable output voltage

■Valley Current Limit At 0.64A

■Precision internal reference

■Low bias current

■Highly efficient operation

■Thermal shutdown

Typical Applications

■Automotive Safety and Infotainment

■High Efficiency Point-Of-Load (POL) Regulator

■Non-Isolated Telecommunication Buck Regulator

■Secondary High Voltage Post Regulator

Package

■micro SMD (2mm x 2mm)

Basic Step Down Regulator

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LM34919B Ultra Small 40V, 600 mA Constant On-Time Buck Switching Regulator

Connection Diagrams

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Bump Side

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Top View

Ordering Information

Order Number Package Type

NSC

Package

Drawing

Junction

Temperature Range Supplied As Feature

LM34919BQTL 10-Bump micro SMD TLP10KAA

−40°C to + 125°C

250 Units on Tape

and Reel

AEC-Q100 Grade 1

qualitifed. Automotive

Grade Production

Flow. *

LM34919BQTLX 10–Bump micro SMD TLP10KAA 3000 Units on

Tape and Reel

LM34919BTL 10-Bump micro SMD TLP10KAA

−40°C to + 125°C

250 Units on Tape

and Reel

LM34919BTLX 10–Bump micro SMD TLP10KAA 3000 Units on

Tape and Reel

For detailed information on micro SMD packages, refer to the Application Note AN-1112.

*Automotive grade (Q) product incorporates enhanced manufacturing and support processes for the automotive market, includng

defect detection methodologies. Reliability qualification is compliant with the requirements and temperature grades defined in the

AEC-Q100 standard. Automotive grade products are identified with the letter Q. For more information go to http://www.national.com/

automotive.

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LM34919B

Pin Descriptions

Pin Number Name Description Application Information

A1 RON/SD On-time control and shutdown An external resistor from VIN to this pin sets the buck switch

on-time. Grounding this pin shuts down the regulator.

A2 RTN Circuit Ground Ground for all internal circuitry other than the current limit

detection.

A3 FB Feedback input from the regulated

output

Internally connected to the regulation and over-voltage

comparators. The regulation level is 2.5V.

B1 SGND Sense Ground Re-circulating current flows into this pin to the current sense

resistor.

B3 SS Softstart An internal current source charges an external capacitor to

2.5V, providing the softstart function.

C1 ISEN Current sense The re-circulating current flows through the internal sense

resistor, and out of this pin to the free-wheeling diode.

Current limit is nominally set at 0.64A.

C3 VCC Output from the startup regulator Nominally regulated at 7.0V. An external voltage (7V-14V)

can be applied to this pin to reduce internal dissipation. An

internal diode connects VCC to VIN.

D1 VIN Input supply voltage Nominal input range is 6.0V to 40V.

D2 SW Switching Node Internally connected to the buck switch source. Connect to

the inductor, free-wheeling diode, and bootstrap capacitor.

D3 BST Boost pin for bootstrap capacitor Connect a 0.022 µF capacitor from SW to this pin. The

capacitor is charged from VCC via an internal diode during

each off-time.

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LM34919B

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

VIN to RTN 44V

BST to RTN 52V

SW to RTN (Steady State) -1.5V to 44V

ESD Rating (Note 2)

Human Body Model 2kV

BST to VCC 44V

BST to SW 14V

VCC to RTN 14V

SGND to RTN -0.3V to +0.3V

SS, RON/SD to RTN -0.3V to 4V

FB to RTN -0.3 to 7V

Storage Temperature Range -65°C to +150°C

For soldering specs see:

www.national.com/ms/MS/MS-SOLDERING.pdf

JunctionTemperature 150°C

Operating Ratings (Note 1)

VIN 6.0V to 40V

Junction Temperature −40°C to + 125°C

Electrical Characteristics Specifications with standard type are for TJ = 25°C only; limits in boldface type apply

over the full Operating Junction Temperature (TJ) range. Minimum and Maximum limits are guaranteed through test, design, or

statistical correlation. Typical values represent the most likely parametric norm at TJ = 25°C, and are provided for reference

purposes only. Unless otherwise stated the following conditions apply: VIN = 12V, RON = 20kΩ. See (Note 4).

Symbol Parameter Conditions Min Typ Max Units

Start-Up Regulator, VCC

VCCReg VCC regulated output VIN = 12V 6.6 77.4 V

VIN =6V, ICC = 3 mA, 5.3 5.91

VIN-VCC dropout voltage ICC = 0 mA, non-switching

VCC = UVLOVCC + 250 mV

20 mV

VCC Output Impedance 0 mA ≤ ICC ≤ 5 mA, VIN = 6V 24

Ω

0 mA ≤ ICC ≤ 5 mA, VIN = 8V 12

VCC current limit (Note 3) VCC = 0V 15 mA

UVLOVCC VCC under-voltage lockout threshold

measured at VCC

VCC increasing 5.25 V

VCC decreasing 5.1 5.25 V

UVLOVCC hysteresis, at VCC 150 mV

VCC under-voltage lock-out threshold

measured at VIN

VIN increasing, ICC = 3 mA 5.25 5.6 V

VIN decreasing, ICC = 3 mA 5.1 5.4 V

UVLOVCC filter delay 100 mV overdrive 3 µs

IQIIN operating current Non-switching, FB = 3V, SW = Open 0.78 1.0 mA

ISD IIN shutdown current RON/SD = 0V, SW = Open 215 330 µA

Switch Characteristics

Rds(on) Buck Switch Rds(on) ITEST = 200 mA 0.5 1.0 Ω

UVLOGD Gate Drive UVLO VBST - VSW Increasing 2.65 3.6 4.40 V

VBST - VSW Decreasing 3.2

UVLOGD hysteresis 400 mV

Softstart Pin

VSS Pull-up voltage 2.5 V

Internal current source VSS = 1V 10.5 µA

Current Limit

ILIM Threshold Current out of ISEN 0.52 0.64 0.76 A

Resistance from ISEN to SGND 135 mΩ

Response time 50 ns

On Timer

tON - 1 On-time VIN = 12V, RON = 20kΩ127 170 213 ns

tON - 2 On-time VIN = 24V, RON = 20 kΩ 110 ns

tON - 3 On-time VIN = 6V, RON = 20 kΩ 335 ns

Shutdown threshold Voltage at RON/SD rising 0.4 0.74 1.2 V

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LM34919B

Symbol Parameter Conditions Min Typ Max Units

Threshold hysteresis Voltage at RON/SD 40 mV

Off Timer

tOFF Minimum Off-time VIN = 6V, ICC = 3mA 60 88 120 ns

VIN = 8V, ICC = 3mA 58 82 118

Regulation and Over-Voltage Comparators (FB Pin)

VREF FB regulation threshold SS pin = steady state 2.440 2.5 2.550 V

FB over-voltage threshold 2.9 V

FB bias current FB = 3V 1 nA

Thermal Shutdown

TSD Thermal shutdown temperature 175 °C

Thermal shutdown hysteresis 20 °C

Thermal Resistance

θJA Junction to Ambient

0 LFPM Air Flow

61 °C/W

Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the

device is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics.

Note 2: The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin.

Note 3: VCC provides self bias for the internal gate drive and control circuits. Device thermal limitations limit external loading

Note 4: Typical specifications represent the most likely parametric norm at 25°C operation.

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LM34919B

Typical Performance Characteristics

Efficiency at 2.1 MHz, 3.3V

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Efficiency at 250 kHz, 3.3V

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Efficiency at 2.1 MHz, 5V

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VCC vs. VIN

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VCC vs. ICC

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ICC vs. Externally Applied VCC

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LM34919B

ON-TIME vs. VIN and RON

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Voltage at the RON/SD Pin

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Operating Current into VIN

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Shutdown Current into VIN

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VCC UVLO at Vin vs. Temperature

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Gate Drive UVLO vs. Temperature

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LM34919B

VCC Voltage vs. Temperature

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VCC Output Impedance vs. Temperature

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VCC Current Limit vs. Temperature

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Reference Voltage vs. Temperature

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Soft-Start Current vs. Temperature

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On-Time vs. Temperature

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LM34919B

Minimum Off-Time vs. Temperature

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Current Limit Threshold vs. Temperature

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Operating & Shutdown Current vs. Temperature

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RON Pin Shutdown Threshold vs. Temperature

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LM34919B

Block Diagram

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LM34919B

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FIGURE 1. Start Up Sequence

Functional Description

The LM34919B Step Down Switching Regulator features all

the functions needed to implement a low cost, efficient buck

bias power converter capable of supplying at least 0.6A to the

load. This high voltage regulator contains an N-Channel buck

switch, is easy to implement, and is available in micro SMD

package. The regulator’s operation is based on a constant on-

time control scheme, where the on-time is determined by

VIN. This feature allows the operating frequency to remain

relatively constant with load and input voltage variations. The

feedback control requires no loop compensation resulting in

very fast load transient response. The valley current limit de-

tection circuit, internally set at 0.64A, holds the buck switch

off until the high current level subsides. This scheme protects

against excessively high current if the output is short-circuited

when VIN is high.

The LM34919B can be applied in numerous applications to

efficiently regulate down higher voltages. Additional features

include: Thermal shutdown, VCC under-voltage lockout, gate

drive under-voltage lockout, and maximum duty cycle limiter.

Control Circuit Overview

The LM34919B buck DC-DC regulator employs a control

scheme based on a comparator and a one-shot on-timer, with

the output voltage feedback (FB) compared to an internal ref-

erence (2.5V). If the FB voltage is below the reference the

buck switch is turned on for a time period determined by the

input voltage and a programming resistor (RON). Following the

on-time the switch remains off until the FB voltage falls below

the reference but not less than the minimum off-time. The

buck switch then turns on for another on-time period. Typi-

cally, during start-up, or when the load current increases

suddenly, the off-times are at the minimum. Once regulation

is established, the off-times are longer.

When in regulation, the LM34919B operates in continuous

conduction mode at heavy load currents and discontinuous

conduction mode at light load currents. In continuous con-

duction mode current always flows through the inductor, nev-

er reaching zero during the off-time. In this mode the

operating frequency remains relatively constant with load and

line variations. The minimum load current for continuous con-

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LM34919B

duction mode is one-half the inductor’s ripple current ampli-

tude. The operating frequency is approximately:

(1)

The buck switch duty cycle is approximately equal to:

(2)

In discontinuous conduction mode current through the induc-

tor ramps up from zero to a peak during the on-time, then

ramps back to zero before the end of the off-time. The next

on-time period starts when the voltage at FB falls below the

reference - until then the inductor current remains zero, and

the load current is supplied by the output capacitor. In this

mode the operating frequency is lower than in continuous

conduction mode, and varies with load current. Conversion

efficiency is maintained at light loads since the switching loss-

es decrease with the reduction in load and frequency. The

approximate discontinuous operating frequency can be cal-

culated as follows:

(3)

where RL = the load resistance.

The output voltage is set by two external resistors (R1, R2).

The regulated output voltage is calculated as follows:

VOUT = 2.5 x (R1 + R2) / R2

Output voltage regulation is based on ripple voltage at the

feedback input,normally obtained from the output voltage rip-

ple through the feedback resistors. The LM34919B requires

a minimum of 25 mV of ripple voltage at the FB pin. In cases

where the capacitor’s ESR is insufficient additional series re-

sistance may be required (R3).

Start-Up Regulator, VCC

The start-up regulator is integral to the LM34919B. The input

pin (VIN) can be connected directly to line voltage up to 40V,

with transient capability to 44V. The VCC output regulates at

7.0V, and is current limited at 15 mA. Upon power up, the

regulator sources current into the external capacitor at VCC

(C3). When the voltage on the VCC pin reaches the under-

voltage lockout threshold of 5.25V, the buck switch is enabled

and the Softstart pin is released to allow the Softstart capac-

itor (C6) to charge up.

The minimum input voltage is determined by the VCC UVLO

falling threshold (≊5.1V). When VCC falls below the falling

threshold the VCC UVLO activates to shut off the output. If

VCC is externally loaded, the minimum input voltage increas-

es.

To reduce power dissipation in the start-up regulator, an aux-

iliary voltage can be diode connected to the VCC pin. Setting

the auxiliary voltage to between 7V and 14V shuts off the in-

ternal regulator, reducing internal power dissipation. The sum

of the auxiliary voltage and the input voltage (VCC + VIN) can-

not exceed 52V. Internally, a diode connects VCC to VIN. See

Figure 2.

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FIGURE 2. Self Biased Configuration

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LM34919B

Regulation Comparator

The feedback voltage at FB is compared to the voltage at the

Softstart pin (2.5V). In normal operation (the output voltage is

regulated), an on-time period is initiated when the voltage at

FB falls below 2.5V. The buck switch stays on for the pro-

grammed on-time, causing the FB voltage to rise above 2.5V.

After the on-time period, the buck switch stays off until the FB

voltage falls below 2.5V. Input bias current at the FB pin is

less than 100 nA over temperature.

Over-Voltage Comparator

The voltage at FB is compared to an internal 2.9V reference.

If the voltage at FB rises above 2.9V the on-time pulse is im-

mediately terminated. This condition can occur if the input

voltage or the output load changes suddenly, or if the inductor

(L1) saturates. The buck switch remains off until the voltage

at FB falls below 2.5V.

ON-Time Timer, and Shutdown

The on-time is determined by the RON resistor and the input

voltage (VIN), and is calculated from:

(4)

The inverse relationship with VIN results in a nearly constant

frequency as VIN is varied. To set a specific continuous con-

duction mode switching frequency (FS), the RON resistor is

determined from the following:

(5)

In high frequency applicatons the minimum value for tON is

limited by the maximum duty cycle required for regulation and

the minimum off-time. The minimum off-time limits the maxi-

mum duty cycle achievable with a low voltage at VIN. At high

values of VIN, the minimum on-time is limited to ≊ 90 ns.

The LM34919B can be remotely shut down by taking the

RON/SD pin low. See Figure 3. In this mode the SS pin is

internally grounded, the on-timer is disabled, and bias cur-

rents are reduced. Releasing the RON/SD pin allows normal

operation to resume. The voltage at the RON/SD pin is be-

tween 1.4V and 5.0V, depending on VIN and the RON resistor.

30102713

FIGURE 3. Shutdown Implementation

Current Limit

Current limit detection occurs during the off-time by monitor-

ing the recirculating current through the free-wheeling diode

(D1). Referring to the Block Diagram, when the buck switch

is turned off the inductor current flows through the load, into

SGND, through the sense resistor, out of ISEN and through

D1. If that current exceeds 0.64A the current limit comparator

output switches to delay the start of the next on-time period.

The next on-time starts when the current out of ISEN is below

0.64A and the voltage at FB is below 2.5V. If the overload

condition persists causing the inductor current to exceed

0.64A during each on-time, that is detected at the beginning

of each off-time. The operating frequency is lower due to

longer-than-normal off-times.

Figure 4 illustrates the inductor current waveform. During nor-

mal operation the load current is Io, the average of the ripple

waveform. When the load resistance decreases the current

ratchets up until the lower peak reaches 0.64A. During the

Current Limited portion of Figure 4, the current ramps down

to 0.64A during each off-time, initiating the next on-time (as-

suming the voltage at FB is <2.5V). During each on-time the

current ramps up an amount equal to:

ΔI = (VIN - VOUT) x tON / L1

During this time the LM34919B is in a constant current mode,

with an average load current (IOCL) equal to 0.64A + ΔI/2.

Generally, in applications where the switching frequency is

higher than ≊300 kHz and uses a small value inductor, the

higher dl/dt of the inductor's ripple current results in an effec-

tively lower valley current limit threshold due to the response

time of the current limit detection circuit. However, since the

small value inductor results in a relatively high ripple current

amplitude (ΔI in Figure 4), the load current (IOCL) at current

limit is typically in excess of 640 mA.

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LM34919B

30102714

FIGURE 4. Inductor Current - Current Limit Operation

N - Channel Buck Switch and Driver

The LM34919B integrates an N-Channel buck switch and as-

sociated floating high voltage gate driver. The peak current

allowed through the buck switch is 1.5A, and the maximum

allowed average current is 1A. The gate driver circuit works

in conjunction with an external bootstrap capacitor and an in-

ternal high voltage diode. A 0.022 µF capacitor (C4) connect-

ed between BST and SW provides the voltage to the driver

during the on-time. During each off-time, the SW pin is at ap-

proximately -1V, and C4 charges from VCC through the inter-

nal diode. The minimum off-time forced by the LM34919B

ensures a minimum time each cycle to recharge the bootstrap

capacitor.

Softstart

The softstart feature allows the converter to gradually reach

a steady state operating point, thereby reducing start-up

stresses and current surges. Upon turn-on, after VCC reaches

the under-voltage threshold, an internal 10.5 µA current

source charges up the external capacitor at the SS pin to

2.5V. The ramping voltage at SS (and the non-inverting input

of the regulation comparator) ramps up the output voltage in

a controlled manner.

An internal switch grounds the SS pin if VCC is below the un-

der-voltage lockout threshold, or if the RON/SD pin is ground-

ed.

Thermal Shutdown

The LM34919B should be operated so the junction tempera-

ture does not exceed 125°C. If the junction temperature in-

creases, an internal Thermal Shutdown circuit, which acti-

vates (typically) at 175°C, takes the controller to a low power

reset state by disabling the buck switch. This feature helps

prevent catastrophic failures from accidental device over-

heating. When the junction temperature reduces below 155°

C (typical hysteresis = 20°C) normal operation resumes.

Applications Information

EXTERNAL COMPONENTS

The procedure for calculating the external components is il-

lustrated with the following design example. Referring to the

Block Diagram, the circuit is to be configured for the following

specifications:

- VOUT = 3.3V

- VIN = 6V to 24V

- Minimum load current = 200 mA

- Maximum load current = 600 mA

- Switching Frequency = 1.5 MHz

- Soft-start time = 5 ms

R1 and R2: These resistors set the output voltage. The ratio

of the feedback resistors is calculated from:

R1/R2 = (VOUT/2.5V) - 1

For this example, R1/R2 = 0.32. R1 and R2 should be chosen

from standard value resistors in the range of 1.0 kΩ - 10 kΩ

which satisfy the above ratio. For this example, 2.49kΩ is

chosen for R2 and 787Ω for R1.

RON: This resistor sets the on-time, and (by default) the

switching frequency. The switching frequncy must be less

than 1.53 MHz to ensure the minimum forced on-time does

not interfere with the circuit's proper operation at the maxi-

mum input voltage. The RON resistor is calculated from the

following equation, using the minimum input voltage.

Check that this value resistor does not set an on-time less

than 90 ns at maximum VIN.

A standard value 28 kΩ resistor is used, resulting in a nominal

frequency of 1.49 MHz. The minimum on-time is ≊129 ns at

Vin = 24V, and the maximum on-time is ≊424 ns at Vin = 6V.

Alternately, RON can be determined using Equation 4 if a spe-

cific on-time is required.

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LM34919B

L1: The main parameter affected by the inductor is the in-

ductor current ripple amplitude (IOR). The minimum load cur-

rent is used to determine the maximum allowable ripple in

order to maintain continuous conduction mode, where the

lower peak does not reach 0 mA. This is not a requirement of

the LM34919B, but serves as a guideline for selecting L1. For

this case the maximum ripple current is:

IOR(MAX) = 2 x IOUT(min) = 400 mA (6)

If the minimum load current is zero, use 20% of IOUT(max) for

IOUT(min) in equation 6. The ripple calculated in Equation 6 is

then used in the following equation:

(7)

A standard value 8.2 µH inductor is selected. The maximum

ripple amplitude, which occurs at maximum VIN, calculates to

325 mA p-p, and the peak current is 763 mA at maximum load

current. Ensure the selected inductor is rated for this peak

current.

C2 and R3: Since the LM34919B requires a minimum of 25

mVp-p ripple at the FB pin for proper operation, the required

ripple at VOUT is increased by R1 and R2. This necessary rip-

ple is created by the inductor ripple current flowing through

R3, and to a lesser extent by C2 and its ESR. The minimum

inductor ripple current is calculated using equation 7, rear-

ranged to solve for IOR at minimum VIN.

The minimum value for R3 is equal to:

A standard value 0.27Ω resistor is used for R3 to allow for

tolerances. C2 should generally be no smaller than 3.3 µF,

although that is dependent on the frequency and the desired

output characteristics. C2 should be a low ESR good quality

ceramic capacitor. Experimentation is usually necessary to

determine the minimum value for C2, as the nature of the load

may require a larger value. A load which creates significant

transients requires a larger value for C2 than a non-varying

load.

C1 and C5: C1’s purpose is to supply most of the switch cur-

rent during the on-time, and limit the voltage ripple at VIN, on

the assumption that the voltage source feeding VIN has an

output impedance greater than zero.

At maximum load current, when the buck switch turns on, the

current into VIN suddenly increases to the lower peak of the

inductor’s ripple current, ramps up to the upper peak, then

drops to zero at turn-off. The average current during the on-

time is the load current. For a worst case calculation, C1 must

supply this average load current during the maximum on-time,

without letting the voltage at VIN drop more than 0.5V. The

minimum value for C1 is calculated from:

where tON is the maximum on-time, and ΔV is the allowable

ripple voltage (0.5V). C5’s purpose is to minimize transients

and ringing due to long lead inductance leading to the VIN pin.

A low ESR, 0.1 µF ceramic chip capacitor must be located

close to the VIN and RTN pins.

C3: The capacitor at the VCC pin provides noise filtering and

stability for the Vcc regulator. C3 should be no smaller than

0.1 µF, and should be a good quality, low ESR, ceramic ca-

pacitor. C3’s value, and the VCC current limit, determine a

portion of the turn-on-time (t1 in Figure 1).

C4: The recommended value for C4 is 0.022 µF. A high quality

ceramic capacitor with low ESR is recommended as C4 sup-

plies a surge current to charge the buck switch gate at each

turn-on. A low ESR also helps ensure a complete recharge

during each off-time.

C6: The capacitor at the SS pin determines the softstart time,

i.e. the time for the output voltage, to reach its final value (t2

in Figure 1). The capacitor value is determined from the fol-

lowing:

D1: A Schottky diode is recommended. Ultra-fast recovery

diodes are not recommended as the high speed transitions at

the SW pin may inadvertently affect the IC’s operation through

external or internal EMI. The diode should be rated for the

maximum input voltage, the maximum load current, and the

peak current which occurs when the current limit and maxi-

mum ripple current are reached simultaneously. The diode’s

average power dissipation is calculated from:

PD1 = VF x IOUT x (1-D)

where VF is the diode’s forward voltage drop, and D is the on-

time duty cycle.

FINAL CIRCUIT

The final circuit is shown in Figure 5, and its performance is

shown in Figure 6 and Figure 7. Current limit measured ap-

proximately 780 mA at 6V, and 812 mA at 24V.

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LM34919B

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FIGURE 5. Example Circuit

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FIGURE 6. Efficiency (Circuit of Figure 5)

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FIGURE 7. Frequency vs. VIN (Circuit of Figure 5)

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LM34919B

LOW OUTPUT RIPPLE CONFIGURATIONS

For applications where lower ripple at VOUT is required, the

following options can be used to reduce or nearly eliminate

the ripple.

a) Reduced ripple configuration: In Figure 8, Cff is added

across R1 to AC-couple the ripple at VOUT directly to the FB

pin. This allows the ripple at VOUT to be reduced to a minimum

of 25 mVp-p by reducing R3, since the ripple at VOUT is not

attenuated by the feedback resistors. The minimum value for

Cff is determined from:

where tON(max) is the maximum on-time, which occurs at VIN

(min). The next larger standard value capacitor should be used

for Cff. R1 and R2 should each be towards the upper end of

the 2 kΩ to 10 kΩ range.

30102725

FIGURE 8. Reduced Ripple Configuration

b) Minimum ripple configuration: The circuit of Figure 9

provides minimum ripple at VOUT, determined primarily by

C2’s characteristics and the inductor’s ripple current since R3

is removed. RA and CA are chosen to generate a sawtooth

waveform at their junction, and that voltage is AC-coupled to

the FB pin via CB. To determine the values for RA, CA and

CB, use the following procedure:

Calculate VA = VOUT - (VSW x (1 - (VOUT/VIN(min))))

where VSW is the absolute value of the voltage at the SW pin

during the off-time (typically 1V). VA is the DC voltage at the

RA/CA junction, and is used in the next equation.

where tON is the maximum on-time (at minimum input volt-

age), and ΔV is the desired ripple amplitude at the RA/CA

junction, typically 50 mV. RA and CA are then chosen from

standard value components to satisfy the above product. Typ-

ically CA is 3000 pF to 5000 pF, and RA is 10 kΩ to 300 kΩ.

CB is then chosen large compared to CA, typically 0.1 µF. R1

and R2 should each be towards the upper end of the 2 kΩ to

10 kΩ range.

30102727

FIGURE 9. Minimum Output Ripple Using Ripple Injection

c) Alternate minimum ripple configuration: The circuit in

Figure 10 is the same as that in Figure 5, except the output

voltage is taken from the junction of R3 and C2. The ripple at

VOUT is determined by the inductor’s ripple current and C2’s

characteristics. However, R3 slightly degrades the load reg-

ulation. This circuit may be suitable if the load current is fairly

constant.

30102728

FIGURE 10. Alternate Minimum Output Ripple

Configuration

Minimum Load Current

The LM34919B requires a minimum load current of 1 mA. If

the load current falls below that level, the bootstrap capacitor

(C4) may discharge during the long off-time, and the circuit

will either shutdown, or cycle on and off at a low frequency. If

the load current is expected to drop below 1 mA in the appli-

cation, R1 and R2 should be chosen low enough in value so

they provide the minimum required current at nominal VOUT.

PC BOARD LAYOUT

Refer to application note AN-1112 for PC board guidelines for

the Micro SMD package.

The LM34919B regulation, over-voltage, and current limit

comparators are very fast, and respond to short duration

noise pulses. Layout considerations are therefore critical for

optimum performance. The layout must be as neat and com-

pact as possible, and all of the components must be as close

as possible to their associated pins. The two major current

loops have currents which switch very fast, and so the loops

should be as small as possible to minimize conducted and

radiated EMI. The first loop is that formed by C1, through the

VIN to SW pins, L1, C2, and back to C1.The second current

loop is formed by D1, L1, C2 and the SGND and ISEN pins.

The power dissipation within the LM34919B can be approxi-

mated by determining the total conversion loss (PIN - POUT),

and then subtracting the power losses in the free-wheeling

diode and the inductor. The power loss in the diode is ap-

proximately:

17 www.national.com

LM34919B

PD1 = Iout x VF x (1-D)

where Iout is the load current, VF is the diode’s forward volt-

age drop, and D is the on-time duty cycle. The power loss in

the inductor is approximately:

PL1 = Iout2 x RL x 1.1

where RL is the inductor’s DC resistance, and the 1.1 factor

is an approximation for the AC losses. If it is expected that the

internal dissipation of the LM34919B will produce excessive

junction temperatures during normal operation, good use of

the PC board’s ground plane can help to dissipate heat. Ad-

ditionally the use of wide PC board traces, where possible,

can help conduct heat away from the IC. Judicious positioning

of the PC board within the end product, along with the use of

any available air flow (forced or natural convection) can help

reduce the junction temperatures.

www.national.com 18

LM34919B

Physical Dimensions inches (millimeters) unless otherwise noted

Note: X1 = 1.753 mm, ±0.030 mm

X2 = 1.987 mm, ±0.030 mm

X3 = 0.60 mm, ±0.075 mm

10 Bump micro SMD Package

NS Package Number TLP10KAA

19 www.national.com

LM34919B

Notes

LM34919B Ultra Small 40V, 600 mA Constant On-Time Buck Switching Regulator

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