LM3537TMX/NOPB - NATIONAL SEMICONDUCTOR

LM3430

LM3430 Boost Controller for LED Backlighting

Literature Number: SNVS472A

January 2007

LM3430

Boost Controller for LED Backlighting

General Description

The LM3430 is a high voltage low-side N-channel MOSFET

controller. Ideal for use in a boost regulator, the LM3430 can

power the LED backlight in LCD panels, such as in notebook

PCs. It contains all of the features needed to implement single

ended primary topologies. Output voltage regulation is based

on current-mode control, which eases the design of loop com-

pensation while providing inherent input voltage feed-forward.

The LM3430 includes a start-up regulator that operates over

a wide input range of 6V to 40V. The PWM controller is de-

signed for high speed capability including an oscillator fre-

quency range up to 2 MHz and total propagation delays less

than 100 ns. Additional features include an error amplifier,

precision reference, line under-voltage lockout, cycle-by-cy-

cle current limit, slope compensation, soft-start, external syn-

chronization capability and thermal shutdown. The LM3430 is

available in the LLP-12 package.

Features

■Internal 40V Startup Regulator

■1A Peak MOSFET Gate Driver

■VIN Range 6V to 40V

■Duty Cycle Limit in Excess of 90%

■Programmable UVLO with Hysteresis

■Cycle-by-Cycle Current Limit

■External Synchronizable (AC-coupled)

■Single Resistor Oscillator Frequency Set

■Slope Compensation

■Adjustable Soft-start

■LLP-12 (3mm x 3mm)

Applications

■LED Backlight Driver (companion to LM3432)

■Boost Converter

■SEPIC Converter

Typical Application

20209601

LM3430 Boost Controller for LED Backlighting

Connection Diagram

20209602

12-Lead LLP Package

NS Package Number SDF12A

Ordering Information

Part Number NS Package Drawing Supplied As

LM3430SD SDF12A 1000 Units on Tape and Reel

LM3430SDX SDF12A 4500 Units on Tape and Reel

Pin Descriptions

Pin(s) Name Description Application Information

1 VDHC Proprietary control input from LM3432 This pin accepts a control signal from LM3432 to adjust the output

voltage in real time.

2 VIN Source input voltage Input to the start-up regulator. Operates from 6V to 40V.

3 FB Feedback pin

Inverting input to the internal voltage error amplifier. The non-

inverting input of the error amplifier connects to a 1.25V

reference.

4 COMP Error amplifier output and PWM

comparator input

The control loop compensation components connect between

this pin and the FB pin.

5 VCC Output of the internal, high voltage linear

regulator.

This pin should be bypassed to the GND pin with a ceramic

capacitor.

6 OUT Output of gate driver Connect this pin to the gate of the external MOSFET. The gate

driver has a 1A peak current capability.

7 GND System ground

8 UVLO Input Under-Voltage Lock-out

Set the start-up and shutdown levels by connecting this pin to the

input voltage through a resistor divider. A 20 µA current source

provides hysteresis.

9 CS Current Sense input Input for the switch current sensing used for current mode control

and for current limiting.

10 RT/SYNC Oscillator frequency adjust pin and

synchronization input

An external resistor connected from this pin to GND sets the

oscillator frequency. This pin can also accept an ac-coupled input

for synchronization from an external clock.

11 SS Soft-start pin

An external capacitor placed from this pin to ground will be

charged by a 10 µA current source, creating a ramp voltage to

control the regulator start-up.

12 NC No-connect Leave this pin open-circuit.

DAP EP Exposed Pad Thermal connection pad, connect to GND.

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LM3430

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 GND -0.3V to 45V

VCC to GND -0.3V to 16V

RT/SYNC to GND -0.3V to 5.5V

OUT to GND -1.5V for < 100 ns

All other pins to GND -0.3V to 7V

Power Dissipation Internally Limited

Junction Temperature 150°C

Storage Temperature -65°C to +150°C

Soldering Information

Vapor Phase (60 sec.) 215°C

Infrared (15 sec.) 220°C

ESD Rating

Human Body Model (Note 2) 2 kV

Operating Ranges (Note 4)

Supply Voltage 6V to 40V

External Volatge at VCC 7.5V to 14V

Junction Temperature Range -40°C to +125°C

Electrical Characteristics Limits in standard type are for TJ = 25°C only; limits in boldface type apply over the

junction temperature (TJ) range of -40°C to +125°C. 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.

VIN = 18V and RT = 27.4 kΩ unless otherwise indicated. (Note 3)

Symbol Parameter Conditions Min Typ Max Units

SYSTEM PARAMETERS

VFB FB Pin Voltage -40°C ≤ TJ ≤ 125°C 1.225 1.250 1.275 V

START-UP REGULATOR

VCC

VCC Regulation 9V ≤ VIN ≤ 40V, ICC = 1 mA 6.6 77.4

VCC Regulation 6V ≤ VIN < 9V, VCC Pin Open

Circuit

ICC Supply Current OUT Pin Capacitance = 0

VCC = 10V

3.5 4mA

ICC-LIM VCC Current Limit VCC = 0V, (Note 4, 6) 15 35 mA

VIN - VCC Dropout Voltage Across Bypass Switch ICC = 0 mA, fSW < 200 kHz

6V ≤ VIN ≤ 8.5V

200 mV

VBYP-HI Bypass Switch Turn-off Threshold VIN increasing 8.7 V

VBYP-HYS Bypass Switch Threshold Hysteresis VIN Decreasing 260 mV

ZVCC

VCC Pin Output Impedance

0 mA ≤ ICC ≤ 5 mA

VIN = 6.0V 58

Ω

VIN = 8.0V 53

VIN = 18.0V 1.1

VCC-HI VCC Pin UVLO Rising Threshold 5 V

VCC-HYS VCC Pin UVLO Falling Hysteresis 300 mV

IVIN Start-up Regulator Leakage VIN = 40V 150 500 µA

IIN-SD Shutdown Current VUVLO = 0V, VCC = Open Circuit 350 450 µA

ERROR AMPLIFIER

GBW Gain Bandwidth 4 MHz

ADC DC Gain 75 dB

ICOMP COMP Pin Current Sink Capability VFB = 1.5V

VCOMP = 1V

517 mA

UVLO

VSD Shutdown Threshold 1.22 1.25 1.28 V

ISD-HYS

Shutdown

Hysteresis Current Source

16 20 24 µA

CURRENT LIMIT

tLIM-DLY Delay from ILIM to Output CS steps from 0V to 0.6V

OUT transitions to 90% of VCC

30 ns

VCS Current Limit Threshold Voltage 0.45 0.5 0.55 V

tBLK Leading Edge Blanking Time 65 ns

RCS CS Pin Sink Impedance Blanking active 40 75 Ω

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LM3430

Symbol Parameter Conditions Min Typ Max Units

SOFT-START

ISS Soft-start Current Source 710 13 µA

VSS-OFF Soft-start to COMP Offset 0.35 0.55 0.75 V

OSCILLATOR

fSW

RT to GND = 84.5 kΩ(Note 5) 170 200 230 kHz

RT to GND = 27.4 kΩ(Note 5) 525 600 675 kHz

RT to GND = 16.2 kΩ(Note 5) 865 990 1115 kHz

VSYNC-HI Synchronization Rising Threshold 3.8 V

PWM COMPARATOR

tCOMP-DLY Delay from COMP to OUT Transition VCOMP = 2V

CS stepped from 0V to 0.4V

25 ns

DMIN Minimum Duty Cycle VCOMP = 0V 0%

DMAX Maximum Duty Cycle 90 95 %

APWM COMP to PWM Comparator Gain 0.33 V/V

VCOMP-OC COMP Pin Open Circuit Voltage VFB = 0V 4.3 5.2 6.1 V

ICOMP-SC COMP Pin Short Circuit Current VCOMP = 0V, VFB = 1.5V 0.6 1.1 1.5 mA

SLOPE COMPENSATION

VSLOPE Slope Compensation Amplitude 80 105 130 mV

MOSFET DRIVER

VSAT-HI Output High Saturation Voltage (VCC –

VOUT)

IOUT = 50 mA 0.25 0.75 V

VSAT-LO Output Low Saturation Voltage (VOUT) IOUT = 100 mA 0.25 0.75 V

tRISE OUT Pin Rise Time OUT Pin load = 1 nF 18 ns

tFALL OUT Pin Fall Time OUT Pin load = 1 nF 15 ns

THERMAL CHARACTERISTICS

TSD Thermal Shutdown Threshold 165 °C

TSD-HYS Thermal Shutdown Hysteresis 25 °C

θJA Junction to Ambient Thermal Resistance SDF-12A Package 122 °C/W

Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. The Recommended Operating Limits define the conditions within

which 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 100 pF capacitor discharged through a 1.5kΩ resistor into each pin.

Note 3: Min and Max limits are 100% production tested at 25°C. Limits over the operating temperature range are guaranteed through correlation using Statistical

Quality Control (SQC) methods. Limits are used to calculate National’s Average Outgoing Quality Level (AOQL).

Note 4: Device thermal limitations may limit usable range.

Note 5: Specification applies to the oscillator frequency.

Note 6: VCC provides bias for the internal gate drive and control circuits.

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LM3430

Typical Performance Characteristics

Efficiency, VO = 40V

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VFB vs. Temp (VIN = 18V)

20209603

VFB vs. VIN (TA = 25°C)

20209604

VCC vs. VIN (TA = 25°C)

20209605

Max Duty Cycle vs. fSW (TA = 25°C)

20209606

fSW vs. Temperature (RT = 16.2 kΩ)

20209607

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LM3430

RT vs. fSW (TA = 25°C)

20209608

SS vs. Temperature

20209609

OUT Pin tR vs. Gate Capacitance

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OUT Pin tF vs. Gate Capacitance

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LM3430

Block Diagram

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LM3430

Example Circuit: LM3430 and LM3432

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FIGURE 1. LM3430 with LM3432

Application Parameters for Various LED Configurations

Input Voltage 8V to 21V 8V to 21V 8V to 18V

Maximum Output

Voltage

33V 50V 33V

LED Configuration 8 LEDs x 6 Strings, 20 mA per

string

12 LEDs x 6 Strings, 20 mA per string 8 LEDs x 6 Strings, 20 mA per string

Switching Frequency 1 MHz 1 MHz 2 MHz

L1 22 µH 22 µH 4.7 µH

Co 10 µF, 50V 20 µF, 100V 10 µF, 50V

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LM3430

Cin 10 µF, 50V 10 µF, 50V 10 µF, 50V

Cinx 100 nF 100 nF 100 nF

Css 1 nF 1 nF 1 nF

Rt 16.5 kΩ16.5 kΩ8.25 kΩ

Rfb1 4.64 kΩ3.01 kΩ16.5 kΩ

Rfb2 118 kΩ118 kΩ422 kΩ

Ruv1 10 kΩ10 kΩ10 kΩ

Ruv2 49.9 kΩ49.9 kΩ49.9 kΩ

Control Loop Compensation

C1 12 pF 12 pF 39 pF

R1 75 kΩ118 kΩ150 kΩ

C2 220 nF 47 nF 4.7 nF

VDHC

Rdhc 30 kΩ9.09 kΩ60.4 kΩ

Slope Compensation

Rs1 4.02 kΩ4.02 kΩ4.02 kΩ

Rs2 301Ω 301Ω 301Ω

Current Sensing

Rsns 0.2Ω (1W) 0.2Ω (1W) Two 1Ω (1W) in parallel

Csns 1 nF 1 nF 1 nF

Applications Information

OVERVIEW

The LM3430 is a low-side N-channel MOSFET controller that

contains all of the features needed to implement single ended

power converter topologies. The LM3430 includes a high-

voltage startup regulator that operates over a wide input

range of 6V to 40V. The PWM controller is designed for high

speed capability including an oscillator frequency range up to

2 MHz and total propagation delays less than 100 ns. Addi-

tional features include an error amplifier, precision reference,

input under-voltage lockout, cycle-by-cycle current limit, slope

compensation, soft-start, oscillator sync capability and ther-

mal shutdown.

The LM3430 is designed for current-mode control power con-

verters that require a single drive output, such as boost and

SEPIC topologies. The LM3430 provides all of the advan-

tages of current-mode control including input voltage feed-

forward, cycle-by-cycle current limiting and simplified loop

compensation.

HIGH VOLTAGE START-UP REGULATOR

The LM3430 contains an internal high-voltage startup regu-

lator that allows the VIN pin to be connected directly to line

voltages as high as 40V. The regulator output is internally

current limited to 35 mA (typical). When power is applied, the

regulator is enabled and sources current into an external ca-

pacitor, CF, connected to the VCC pin. The recommended

capacitance range for CF is 0.1 µF to 100 µF. When the volt-

age on the VCC pin reaches the rising threshold of 5V, the

controller output is enabled. The controller will remain en-

abled until VCC falls below 4.7V. In applications using a

transformer, an auxiliary winding can be connected through

a diode to the VCC pin. This winding should raise the VCC

pin voltage to above 7.5V to shut off the internal startup reg-

ulator. Powering VCC from an auxiliary winding improves

conversion efficiency while reducing the power dissipated in

the controller. The capacitance of CF must be high enough

that the it maintains the VCC voltage greater than the VCC

UVLO falling threshold (4.7V) during the initial start-up. Dur-

ing a fault condition when the converter auxiliary winding is

inactive, external current draw on the VCC line should be lim-

ited such that the power dissipated in the start-up regulator

does not exceed the maximum power dissipation capability of

the controller.

An external start-up or other bias rail can be used instead of

the internal start-up regulator by connecting the VCC and the

VIN pins together and feeding the external bias voltage (7.5V

to 14V) to the two pins.

INPUT UNDER-VOLTAGE DETECTOR

The LM3430 contains an input Under Voltage Lock Out (UV-

LO) circuit. UVLO is programmed by connecting the UVLO

pin to the center point of an external voltage divider from VIN

to GND. The resistor divider must be designed such that the

voltage at the UVLO pin is greater than 1.25V when VIN is in

the desired operating range. If the under voltage threshold is

not met, all functions of the controller are disabled and the

controller remains in a low power standby state. UVLO hys-

teresis is accomplished with an internal 20 µA current source

that is switched on or off into the impedance of the set-point

divider. When the UVLO threshold is exceeded, the current

source is activated to instantly raise the voltage at the UVLO

pin. When the UVLO pin voltage falls below the 1.25V thresh-

old the current source is turned off, causing the voltage at the

UVLO pin to fall. The UVLO pin can also be used to implement

a remote enable / disable function. If an external transistor

pulls the UVLO pin below the 1.25V threshold, the converter

will be disabled. This external shutdown method is shown in

Figure 2.

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LM3430

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FIGURE 2. Enable/Disable Using UVLO

ERROR AMPLIFIER

An internal high gain error amplifier is provided within the

LM3430. The amplifier’s non-inverting input is internally set to

a fixed reference voltage of 1.25V. The inverting input is con-

nected to the FB pin. In non-isolated applications such as the

boost converter the output voltage, VO, is connected to the FB

pin through a resistor divider. The control loop compensation

components are connected between the COMP and FB pins.

For most isolated applications the error amplifier function is

implemented on the secondary side of the converter and the

internal error amplifier is not used. The internal error amplifier

is configured as an open drain output and can be disabled by

connecting the FB pin to ground. An internal 5 kΩ pull-up re-

sistor between a 5V reference and COMP can be used as the

pull-up for an opto-coupler in isolated applications.

CURRENT SENSING AND CURRENT LIMITING

The LM3430 provides a cycle-by-cycle over current protec-

tion function. Current limit is accomplished by an internal

current sense comparator. If the voltage at the current sense

comparator input exceeds 0.5V, the MOSFET gate drive will

be immediately terminated. A small RC filter, located near the

controller, is recommended to filter noise from the current

sense signal. The CS input has an internal MOSFET which

discharges the CS pin capacitance at the conclusion of every

cycle. The discharge device remains on an additional 65 ns

after the beginning of the new cycle to attenuate leading edge

ringing on the current sense signal.

The LM3430 current sense and PWM comparators are very

fast, and may respond to short duration noise pulses. Layout

considerations are critical for the current sense filter and

sense resistor. The capacitor associated with the CS filter

must be located very close to the device and connected di-

rectly to the pins of the controller (CS and GND). If a current

sense transformer is used, both leads of the transformer sec-

ondary should be routed to the sense resistor and the current

sense filter network. The current sense resistor can be locat-

ed between the source of the primary power MOSFET and

power ground, but it must be a low inductance type. When

designing with a current sense resistor all of the noise sensi-

tive low-power ground connections should be connected to-

gether locally to the controller and a single connection should

be made to the high current power ground (sense resistor

ground point).

OSCILLATOR, SHUTDOWN AND SYNC

A single external resistor, RT, connected between the RT/

SYNC and GND pins sets the LM3430 oscillator frequency.

To set the switching frequency, fSW, RT can be calculated

from:

The LM3430 can also be synchronized to an external clock.

The external clock must have a higher frequency than the free

running oscillator frequency set by the RT resistor. The clock

signal should be capacitively coupled into the RT/SYNC pin

with a 100 pF capacitor, shown in Figure 3. A peak voltage

level greater than 3.8V at the RT/SYNC pin is required for

detection of the sync pulse. The sync pulse width should be

set between 15 ns to 150 ns by the external components. The

RT resistor is always required, whether the oscillator is free

running or externally synchronized. The voltage at the RT/

SYNC pin is internally regulated to 2V, and the typical delay

from a logic high at the RT/SYNC pin to the rise of the OUT

pin voltage is 120 ns. RT should be located very close to the

device and connected directly to the pins of the controller (RT/

SYNC and GND).

20209653

FIGURE 3. Sync Operation

PWM COMPARATOR AND SLOPE COMPENSATION

The PWM comparator compares the current ramp signal with

the error voltage derived from the error amplifier output. The

error amplifier output voltage at the COMP pin is offset by

1.4V and then further attenuated by a 3:1 resistor divider. The

PWM comparator polarity is such that 0V on the COMP pin

will result in a zero duty cycle at the controller output. For duty

cycles greater than 50%, current mode control circuits can

experience sub-harmonic oscillation. By adding an additional

fixed-slope voltage ramp signal (slope compensation) this os-

cillation can be avoided. The LM3430 generates the slope

compensation with a 45 µAP-P sawtooth-waveform current

source generated by the clock. (See Figure 4) This current

flows through an internal 2 kΩ resistor to create a minimum

compensation ramp voltage of 100 mV (typical). The ampli-

tude of the compensation ramp increases when external re-

sistance is added for filtering the current sense (RS1) or in the

position RS2. As shown in Figure 4 and the block diagram, the

sensed current slope and the compensation slope add to-

gether to create the signal used for current limiting and for the

control loop itself.

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LM3430

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FIGURE 4. Slope Compensation

In addition to preventing sub-harmonic oscillation, increasing

the amplitude of the compensation ramp voltage decreases

the voltage across RSNS required to trip the current limit com-

parator. This technique can be used to lower the value of

RSNS and reduce power dissipation. Care must be taken not

to add too much slope compensation, however. Reducing

RSNS causes the control loop gain to increase, and too large

of a compensation ramp can overwhelm the sensed current

signal. This imbalance causes the system to act more like a

voltage-mode regulator with a low frequency double pole that

is more difficult to compensate.

SOFT-START

The soft-start feature allows the power converter output to

gradually reach the initial steady state output voltage, thereby

reducing start-up stresses and current surges. At power on,

after the VCC and input under-voltage lockout thresholds are

satisfied, an internal 10 µA current source charges an external

capacitor connected to the SS pin. The capacitor voltage will

ramp up slowly and will limit the COMP pin voltage and the

switch current.

MOSFET GATE DRIVER

The LM3430 provides an internal gate driver through the OUT

pin that can source and sink a peak current of 1A to control

external, ground-referenced MOSFETs.

DYNAMIC HEADROOM CONTROL

The VDHC pin of the LM3430 can be used in conjunction with

the LM3432 to provide on-the-fly adjustments to the output

voltage for maximum efficiency when driving an array of

LEDs. When this feature is not being used the VDHC pin

should be left open circuit.

THERMAL SHUTDOWN

Internal thermal shutdown circuitry is provided to protect the

LM3430 in the event that the maximum junction temperature

is exceeded. When activated, typically at 165°C, the controller

is forced into a low power standby state, disabling the output

driver and the VCC regulator. After the temperature is re-

duced (typical hysteresis is 25°C) the VCC regulator will be

re-enabled and the LM3430 will perform a soft-start.

Design Considerations

The most common circuit controlled by the LM3430 is a non-

isolated boost regulator. The boost regulator steps up the

input voltage and has a duty ratio D of:

(VD is the forward voltage drop of the output diode)

The following is a design procedure for selecting all the com-

ponents for the boost converter portion of the Example Circuit

of Figure 1. This circuit operates in continuous conduction

mode (CCM), where inductor current stays above 0A at all

times, and delivers an output voltage of 33.0V ±2% at an out-

put current of 180 mA. The load is a white LED-based LCD

monitor backlight, formed by six parallel strings of seven white

LEDs each with a multi-channel linear current regulator to sink

30 mA through each string. The forward voltage of each LED

varies from 3.0V to 4.2V over process and temperature and

the current regulator requires approximately 4V of headroom.

The required output voltage is therefore 7 x 4.2V + 4V ≊ 33V.

The input voltage will come from a three-to-four-cell stack of

lithium ion batteries (VIN = 9.0V to 16.8V) or a poorly regulated

AC-DC adapter that supplies 14.0V to 20.9V. The diode drop

VD will be 0.5V, typical of a Schottky diode.

SWITCHING FREQUENCY

The selection of switching frequency is based on the tradeoffs

between size, cost, and efficiency. In general, a lower fre-

quency means larger, more expensive inductors and capac-

itors will be needed. A higher switching frequency generally

results in a smaller but less efficient solution, as the power

MOSFET gate capacitances must be charged and dis-

charged more often in a given amount of time. For this appli-

cation, a frequency of 600 kHz was selected as a good

compromise between the size of the inductor and efficiency.

PCB area and component height are restricted in this appli-

cation. Following the equation given for RT in the Applications

Information section, a 27.4 kΩ 1% resistor should be used to

switch at 600 kHz.

MOSFET

Selection of the power MOSFET is governed by tradeoffs be-

tween cost, size, and efficiency. Breaking down the losses in

the MOSFET is one way to determine relative efficiencies be-

tween different devices. For this example, the SO-8 package

provides a balance of a small footprint with good efficiency.

Losses in the MOSFET can be broken down into conduction

loss, gate charging loss, and switching loss.

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LM3430

Conduction, or I2R loss, PC, is approximately:

The factor 1.3 accounts for the increase in MOSFET on re-

sistance due to heating. Alternatively, the factor of 1.3 can be

ignored and the on resistance of the MOSFET can be esti-

mated using the RDSON Vs. Temperature curves in the MOS-

FET datasheets.

Gate charging loss, PG, results from the current required to

charge and discharge the gate capacitance of the power

MOSFET and is approximated as:

PG = VCC x QG x fSW

QG is the total gate charge of the MOSFET. Gate charge loss

differs from conduction and switching losses because the ac-

tual dissipation occurs in the LM3430 and not in the MOSFET

itself. If no external bias is applied to the VCC pin, additional

loss in the LM3430 IC occurs as the MOSFET driving current

flows through the VCC regulator. This loss, PVCC, is estimated

as:

PVCC = (VIN – VCC) x QG x fSW

Switching loss, PSW, occurs during the brief transition period

as the MOSFET turns on and off. During the transition period

both current and voltage are present in the channel of the

MOSFET. The loss can be approximated as:

PSW = 0.5 x VIN x [IO / (1 – D)] x (tR + tF) x fSW

Where tR and tF are the rise and fall times of the MOSFET

For this example, the maximum drain-to-source voltage ap-

plied across the MOSFET is VO plus the ringing due to para-

sitic inductance and capacitance. The maximum drive voltage

at the gate of the high side MOSFET is VCC, or 7V typical.

The MOSFET selected must be able to withstand 33V plus

any ringing from drain to source, and be able to handle at least

7V plus ringing from gate to source. A minimum voltage rating

of 40VD-S and 10VG-S MOSFET will be used. Comparing the

losses in a spreadsheet leads to a 60VD-S rated MOSFET in

SO-8 with a typical RDSON of 22 mΩ, a gate charge of 18 nC,

and rise and falls times of 10 ns and 12 ns, respectively.

BOOST DIODE

The boost regulator requires a boost diode D1 (see the Typ-

ical Application circuit) to carrying the inductor current during

the MOSFET off-time. The most efficient choice for D1 is a

Schottky diode due to low forward drop and zero reverse re-

covery time. D1 must be rated to handle the maximum output

voltage plus any switching node ringing when the MOSFET

is on. In practice, all switching converters have some ringing

at the switching node due to the diode parasitic capacitance

and the lead inductance. D1 must also be rated to handle the

average output current, IO.

The overall converter efficiency becomes more dependent on

the selection of D1 at low duty cycles, where the boost diode

carries the load current for an increasing percentage of the

time. This power dissipation can be calculating by checking

the typical diode forward voltage, VD, from the I-V curve on

the diode's datasheet and then multiplying it by ID. Diode

datasheets will also provide a typical junction-to-ambient ther-

mal resistance, θJA, which can be used to estimate the oper-

ating die temperature of the Schottky. Multiplying the power

dissipation (PD = IO x VD) by θJA gives the temperature rise.

The diode case size can then be selected to maintain the

Schottky diode temperature below the operational maximum.

In this example a Schottky diode rated to 40V and 0.5A will

be suitable, as the maximum diode current will be 180 mA. A

small case such as SOD-123 or SOT-23 can be used if a small

footprint is critical. Larger case sizes generally have lower

θJA and lower forward voltage drop, so for better efficiency, a

larger case size such as SMA can be used. In applications

with a high boost ratio, such as 1:4, the reverse recovery time,

tRR, has a large impact on losses and efficiency. The Schottky

diode selected should therefore have a tRR value below 15 ns.

BOOST INDUCTOR

The first criterion for selecting an inductor is the inductance

itself. In fixed-frequency boost converters this value is based

on the desired peak-to-peak ripple current, ΔiL, which flows in

the inductor along with the average inductor current, IL. For a

boost converter in CCM IL is greater than the average output

current, IO. The two currents are related by the following ex-

pression:

IL = IO / (1 – D)

As with switching frequency, the inductance used is a tradeoff

between size and cost. Larger inductance means lower input

ripple current, however because the inductor is connected to

the output during the off-time only there is a limit to the re-

duction in output ripple voltage. Lower inductance results in

smaller, less expensive magnetics. An inductance that gives

a ripple current of 30% to 50% of IL is a good starting point for

a CCM boost converter. Minimum inductance should be cal-

culated at the extremes of input voltage to find the operating

condition with the highest requirement:

By calculating in terms of amperes, volts, and megahertz, the

inductance value will come out in microhenrys.

In order to ensure that the boost regulator operates in CCM

a second equation is needed, and must also be evaluated at

the corners of input voltage to find the minimum inductance

required:

By calculating in terms of volts, amps and megahertz the in-

ductance value will come out in microhenrys.

For this design ΔiL will be set to 40% of the maximum IL. Duty

cycle is evaluated first at VIN(MIN) and at VIN(MAX). Second, the

average inductor current is evaluated at the two input volt-

ages. Third, the inductor ripple current is determined. Finally,

the inductance can be calculated, and a standard inductor

value selected that meets all the criteria.

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LM3430

Inductance for Minimum Input Voltage

DVIN(MIN) = (33 – 9.0 – 0.5) / (33 – 0.5) = 72%

IL-VIN(MIN) = 0.18 / (1 – 0.72) = 0.64A

ΔiL = 0.4 x 0.64A = 0.26A

Inductance for Maximum Input Voltage

DVIN(MAX) = (33 – 20.9) / 33 = 37%

IL-VIN(MIAX) = 0.18 / (1 – 0.37) = 0.29A

ΔiL = 0.4 x 0.29A = 0.12A

Maximum average inductor current occurs at VIN(MIN), and the

corresponding inductor ripple current is 0.26AP-P. Selecting

an inductance that exceeds the ripple current requirement at

VIN(MIN) and the requirement to stay in CCM for VIN(MAX) pro-

vides a tradeoff that allows smaller magnetics at the cost of

higher ripple current at maximum input voltage. For this ex-

ample, a 47 µH inductor will satisfy these requirements.

The second criterion for selecting an inductor is the peak cur-

rent carrying capability. This is the level above which the

inductor will saturate. In saturation the inductance can drop

off severely, resulting in higher peak current that may over-

heat the inductor or push the converter into current limit. In a

boost converter, peak current, IPK, is equal to the maximum

average inductor current plus one half of the ripple current.

First, the current ripple must be determined under the condi-

tions that give maximum average inductor current:

Maximum average inductor current occurs at VIN(MIN). Using

the selected inductance of 47 µH yields the following:

ΔiL = (9 x 0.72) / (0.6 x 47) = 230 mAP-P

The highest peak inductor current over all operating condi-

tions is therefore:

IPK = IL + 0.5 x ΔiL = 0.64 + 0.115 = 0.76A

Hence an inductor must be selected that has a peak current

rating greater than 0.76A and an average current rating

greater than 0.64A. One possibility is an off-the-shelf 47 µH

±20% inductor that can handle a peak current of 0.9A and an

average current of 0.93A. Finally, the inductor current ripple

is recalculated at the maximum input voltage:

ΔiL-VIN(MAX) = (20.9 x 0.36) / (0.6 x 47) = 267 mAP-P

OUTPUT CAPACITOR

The output capacitor in a boost regulator supplies current to

the load during the MOSFET on-time and also filters the AC

portion of the load current during the off-time. This capacitor

determines the steady state output voltage ripple, ΔVO, a crit-

ical parameter for all voltage regulators. Output capacitors are

selected based on their capacitance, CO, their equivalent se-

ries resistance (ESR) and their RMS or AC current rating.

The magnitude of ΔVO is comprised of three parts, and in

steady state the ripple voltage during the on-time is equal to

the ripple voltage during the off-time. For simplicity the anal-

ysis will be performed for the MOSFET turning off (off-time)

only. The first part of the ripple voltage is the surge created

as the output diode D1 turns on. At this point inductor/diode

current is at the peak value, and the ripple voltage increase

can be calculated as:

ΔVO1 = IPK x ESR

The second portion of the ripple voltage is the increase due

to the charging of CO through the output diode. This portion

can be approximated as:

ΔVO2 = (IO / CO) x (D / fSW)

The final portion of the ripple voltage is a decrease due to the

flow of the diode/inductor current through the output

capacitor’s ESR. This decrease can be calculated as:

ΔVO3 = ΔiL x ESR

The total change in output voltage is then:

ΔVO = ΔVO1 + ΔVO2 - ΔVO3

The combination of two positive terms and one negative term

may yield an output voltage ripple with a net rise or a net fall

during the converter off-time. The ESR of the output capacitor

(s) has a strong influence on the slope and direction of ΔVO.

Capacitors with high ESR such as tantalum and aluminum

electrolytic create an output voltage ripple that is dominated

by ΔVO1 and ΔVO3, with a shape shown in Figure 5. Ceramic

capacitors, in contrast, have very low ESR and lower capac-

itance. The shape of the output ripple voltage is dominated by

ΔVO2, with a shape shown in Figure 6.

13 www.national.com

LM3430

20209626

FIGURE 5. ΔVO Using High ESR Capacitors

20209627

FIGURE 6. ΔVO Using Low ESR Capacitors

For this example, the load is fairly constant, and the height

restriction favors the low profile of ceramic capacitors. The

output ripple voltage waveform of Figure 6 is assumed, and

the capacitance will be selected first. The desired ΔVO is ±2%

of 33V, or 1.32VP-P. Beginning with the calculation for ΔVO2,

the required minimum capacitance is:

CO-MIN = (IO / ΔVO) x (DMAX / fSW)

CO-MIN = (0.18 / 1.32) x (0.72 / 600000) = 164 nF

Ceramic capacitors rated 1.0 µF ±20% are available from

many manufacturers. The minimum quality dielectric that is

suitable for switching power supply output capacitors is X5R,

while X7R (or better) is preferred. Careful attention must be

paid to the DC voltage rating and case size, as ceramic ca-

pacitors can lose 60%+ of their rated capacitance at the

maximum DC voltage. For example, the typical loss in ca-

pacitance for a 1.0 µF, 50V, 1206-size capacitor is 50% at

30V. This is the reason that ceramic capacitors are often de-

rated to 50% of their capacitance at their working voltage.

The ESR of the selected capacitor has a typical value of 3

mΩ. The worst-case value for ΔVO1 occurs during the peak

current at minimum input voltage:

ΔVO1 = 1.26 x 0.003 = 3.8 mV

The worst-case capacitor charging ripple occurs at maximum

duty cycle, taking into account an output capacitance of 50%

x 1 µF = 500 nF:

ΔVO2 = (0.18 / 5 x 10-7) x (0.72 / 600000) = 432 mV

Finally, the worst-case value for ΔVO3 occurs when inductor

ripple current is highest, at maximum input voltage:

ΔVO3 = 0.398 x 0.003 = 1.2 mV (negligible)

The output voltage ripple can be estimated by summing the

three terms:

ΔVO = 3.8 mV + 432 mV - 1.2 mV = 435 mV

The RMS current through the output capacitor(s) can be es-

timated using the following, worst-case equation:

The highest RMS current occurs at minimum input voltage.

For this example the maximum output capacitor RMS current

is:

IO-RMS(MAX) = 1.13 x 0.64 x (0.72 x 0.28)0.5 = 0.32ARMS

Ceramic capacitors in 1206 case sizes are generally capable

of sustaining RMS currents in excess of 2A, making them

more than adequate for this application.

VCC DECOUPLING CAPACITOR

The VCC pin should be decoupled with a ceramic capacitor

placed as close as possible to the VCC and GND pins of the

LM3430. The decoupling capacitor should have a minimum

X5R or X7R type dielectric to ensure that the capacitance re-

mains stable over voltage and temperature, and be rated to

a minimum of 470 nF. One good choice is a 1.0 µF device

with X7R dielectric and 1206 case size rated to 25V.

INPUT CAPACITOR

The input capacitors to a boost regulator control the input

voltage ripple, ΔVIN, hold up the input voltage during load

transients, and prevent impedance mismatch (also called

power supply interaction) between the LM3430 and the in-

ductance of the input leads. Selection of input capacitors is

based on their capacitance, ESR, and RMS current rating.

The minimum value of ESR can be selected based on the

maximum output current transient, ISTEP, using the following

expression:

For this example, no specific load transient is given, hence

ISTEP is set equal to the maximum load current of 180 mA. The

desired ΔVIN is 4%P-P. ΔVIN and duty cycle are taken at min-

imum input voltage to give the worst-case value:

ESRMIN = [(1 – 0.72) x 0.36] / 0.36 = 0.28Ω

The minimum input capacitance can be selected based on

ΔVIN, based on the drop in VIN during a load transient, or

www.national.com 14

LM3430

based on prevention of power supply interaction. In general,

the requirement for greatest capacitance comes from the

power supply interaction. The inductance and resistance of

the input source must be estimated, and if this information is

not available, they can be assumed to be 1 µH and 0.1Ω, re-

spectively. Minimum capacitance is then estimated as:

As with ESR, the worst-case, highest minimum capacitance

calculation comes at the minimum input voltage. Using the

default estimates for LS and RS, minimum capacitance is:

The closest standard 20% capacitor value is 1.5 µF, but be-

cause the actual input source impedance and resistance are

not known, a 3.3 µF capacitor will be used. In general, dou-

bling the calculated value of input capacitance provides a

good safety margin. The final calculation is for the RMS cur-

rent. For boost converters operating in CCM this can be

estimated as:

IRMS = 0.29 x ΔiL(MAX)

From the inductor section, maximum inductor ripple current is

267 mA, hence the input capacitor(s) must be rated to handle

0.29 x 0.267 = 77 mARMS.

The input capacitors can be ceramic, tantalum, aluminum, or

almost any type, however the low capacitance requirement

makes ceramic capacitors particularly attractive. As with the

output capacitors, the minimum quality dielectric used should

X5R, with X7R or better preferred. The voltage rating for input

capacitors need not be as conservative as the output capac-

itors, as the need for capacitance decreases as input voltage

increases. For this example, the capacitor selected will be 3.3

µF ±20%, rated to 25V, in a 1206 case size. The RMS current

rating is over 1A, more than enough for this application.

CURRENT SENSE FILTER

Parasitic circuit capacitance, inductance and gate drive cur-

rent create a spike in the current sense voltage at the point

where Q1 turns on. In order to prevent this spike from termi-

nating the on-time prematurely, every circuit should have a

low-pass filter that consists of CCS and RS1, shown in Figure

1. The time constant of this filter should be long enough to

reduce the parasitic spike without significantly affecting the

shape of the actual current sense voltage. The recommended

range for RS1 is between 10Ω and 500Ω, and the recom-

mended range for CCS is between 100 pF and 2.2 nF. For this

example, the values of RS1 and CCS will be 100Ω and 1 nF,

respectively.

RSNS AND CURRENT LIMIT

The current sensing resistor RSNS is used for steady state

regulation of the inductor current and to sense over-current

conditions. The resistance value selected must be low

enough to keep the power dissipation to a minimum, yet high

enough to provide good signal-to-noise ratio for the current

sensing circuitry. The resistance should be set so that the

current limit comparator, with a threshold of 0.5V, trips before

the sensed current exceeds the peak current rating of the in-

ductor.

For this example the inductor peak current rating is 0.9A. The

threshold for current limit, ILIM, is set slightly below to account

for tolerance of the circuit components, at a level of 0.8A. The

required resistor calculation must take into account both the

switch current through RSNS and the compensation ramp cur-

rent flowing through the internal 2 kΩ and external 100Ω

resistors:

RSNS = [0.5 - 45µ x (2000 + 100)] / 0.8 = 0.51Ω

Power dissipation in RSNS can be estimated by calculating the

average current. The worst-case average current through

RSNS occurs at minimum input voltage/maximum duty cycle

and can be calculated as:

PCS = [(0.18 / 0.27)2 x 0.51] x 0.73 = 0.16W

For this example a 0.51Ω ±1%, thick-film chip resistor in a

1206 case size rated to 0.33W will be used.

CONTROL LOOP COMPENSATION

The LM3430 uses peak current-mode PWM control to correct

changes in output voltage due to line and load transients.

Peak current-mode provides inherent cycle-by-cycle current

limiting, improved line transient response, and easier control

loop compensation.

The control loop is comprised of two parts. The first is the

power stage, which consists of the pulse width modulator,

output filter, and the load. The second part is the error ampli-

fier, which is an op-amp configured as an inverting amplifier.

Figure 7 shows the regulator control loop components.

20209634

FIGURE 7. Power Stage and Error Amp

15 www.national.com

LM3430

One popular method for selecting the compensation compo-

nents is to create Bode plots of gain and phase for the power

stage and error amplifier. Combined, they make the overall

bandwidth and phase margin of the regulator easy to deter-

mine. Software tools such as Excel, MathCAD, and Matlab

are useful for observing how changes in compensation or the

power stage affect system gain and phase.

The power stage in a CCM peak current mode boost con-

verter consists of the DC gain, APS, a single low frequency

pole, fLFP, the ESR zero, fZESR, a right-half plane zero, fRHP,

and a double pole resulting from the sampling of the peak

current. The power stage transfer function (also called the

Control-to-Output transfer function) can be written:

Where the DC gain is defined as:

Where:

RO = VO / IO

The system ESR zero is:

The low frequency pole is:

The right-half plane zero is:

The sampling double pole quality factor is:

The sampling double corner frequency is:

ωn = π x fSW

The natural inductor current slope is:

Sn = RSNS x VIN / L

The external ramp slope is:

Se = 45 µA x (2000 + RS1 + RS2)] x fSW

In the equation for APS, DC gain is highest when input voltage

is at the maximum and output current is at the lower threshold

of CCM operation. (IL = 0.5 x ΔiL) In this the example those

conditions are VIN = 20.9V and IO = 180 mA.

Maximum DC gain is 44 dB. The low frequency pole fP =

2πωP is at 1.9 kHz, the ESR zero fZ = 2πωZ is at 80 MHz, and

the right-half plane zero fRHP = 2πωRHP is at 230 kHz. The

sampling double-pole occurs at one-half of the switching fre-

quency. Gain and phase plots for the power stage are shown

in Figure 8.

20209641

FIGURE 8. Power Stage Gain and Phase

The single pole causes a roll-off in the gain of -20 dB/decade

at lower frequency, which then flattens out due to the RHP

zero. The sharp drop in gain beginning around 200 kHz is a

result of the sampling double pole. The phase tends towards

-90° at lower frequency but then increases to -180° from the

www.national.com 16

LM3430

RHP zero and the sampling double pole. The effect of the

ESR zero is not seen because its frequency is several

decades above the switching frequency. The combination of

increasing gain and decreasing phase makes converters with

RHP zeroes difficult to compensate. Setting the overall con-

trol loop bandwidth to 1/3 to 1/10 of the RHP zero frequency

minimizes these negative effects. If this loop were left un-

compensated, the bandwidth would be 312 kHz and the

phase margin -100°. The converter would oscillate, and there-

fore is compensated using the error amplifier and a few

passive components.

The transfer function of the compensation block, GEA, can be

derived by treating the error amplifier as an inverting op-amp

with input impedance ZI and feedback impedance ZF. The

majority of applications will require a Type II, or two-pole one-

zero amplifier, shown in Figure 7. The LaPlace domain trans-

fer function for this Type II network is given by the following:

Many techniques exist for selecting the compensation com-

ponent values. The following method is based upon setting

the mid-band gain of the error amplifier transfer function first

and then positioning the compensation zero and pole:

1. Determine the desired control loop bandwidth: The

control loop bandwidth, f0dB, is the point at which the total

control loop gain (H = GPS x GEA) is equal to 0 dB. For

this example, a low bandwidth of 30 kHz, or

approximately 1/8th of the RHP zero frequency, is

chosen because of the wide variation in input voltage.

2. Determine the gain of the power stage at f0dB: This

value, A, can be read graphically from the gain plot of

GPS or calculated by replacing the ‘s’ terms in GPS with

‘2πf0dB’. For this example the gain at 30 kHz is

approximately 20 dB.

3. Calculate the negative of A and convert it to a linear

gain: By setting the mid-band gain of the error amplifier

to the negative of the power stage gain at f0dB, the control

loop gain will equal 0 dB at that frequency. For this

example, -20 dB = 0.1V/V.

4. Select the resistance of the top feedback divider

resistor RFB2: This value is arbitrary, however selecting

a resistance between 10 kΩ and 100 kΩ will lead to

practical values of R1, C1 and C2. For this example,

RFB2 = 20 kΩ 1%.

5. Set R1 = A x RFB2: For this example: R1 = 0.1 x 20000

= 2 kΩ

6. Select a frequency for the compensation zero, fZ1:

The suggested placement for this zero is at the low

frequency pole of the power stage, fLFP = ωLFP / 2π. For

this example, fZ1 = fLFP = 1.9 kHz

7. Set

For this example, C2 = 41.2 nF

8. Select a frequency for the compensation pole, fP1:

The suggested placement for this pole is at one-half of

the switching frequency. For this example, fP1 = 200 kHz

9. Set

For this example, C1 = 401 pF

10. Plug the closest 1% tolerance values for RFB2 and

R1, then the closest 10% values for C1 and C2 into

GEA and model the error amp: The open-loop gain and

bandwidth of the LM3430’s internal error amplifier are 75

dB and 4 MHz, respectively. Their effect on GEA can be

modeled using the following expression:

ADC is a linear gain, the linear equivalent of 75 dB is

approximately 5600V/V. C1 = 390 pF 10%, C2 = 39 nF

10%, R1 = 2 kΩ 1%

11. Plot or evaluate the actual error amplifier transfer

function:

17 www.national.com

LM3430

20209648

FIGURE 9. Error Amplifier Gain and Phase

12. Plot or evaluate the complete control loop transfer

function: The complete control loop transfer function is ob-

tained by multiplying the power stage and error amplifier

functions together. The bandwidth and phase margin can

then be read graphically or evaluated numerically.

20209649

FIGURE 10. Overall Loop Gain and Phase

The bandwidth of this example circuit is 34 kHz, with a phase

margin of 60°.

Efficiency Calculations

A reasonable estimation for the efficiency of a boost regulator

controlled by the LM3430 can be obtained by adding together

the loss is each current carrying element and using the equa-

tion:

The following shows an efficiency calculation to complement

the circuit design from the Design Considerations section.

Output power for this circuit is 33V x 0.18A = 5.9W. Input

voltage is assumed to be 12V, and the calculations used as-

sume that the converter runs in CCM. Duty cycle for VIN = 12V

is 63%, and the average inductor current is 0.49A.

CHIP OPERATING LOSS

This term accounts for the current drawn at the VIN pin. This

current, IIN, drives the logic circuitry and the power MOSFETs.

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LM3430

The gate driving loss term from the power MOSFET section

of Design Considerations is included in the chip operating

loss. For the LM3430, IIN is equal to the steady state operating

current, ICC, plus the MOSFET driving current, IGC. Power is

lost as this current passes through the internal linear regulator

of the LM3430.

IGC = QG X fSW

IGC = 18 nC x 600 kHz = 11 mA

ICC is typically 3.5 mA, taken from the Electrical Characteris-

tics table. Chip Operating Loss is then:

PQ = VIN X (IQ + IGC)

PQ = 12 X (3.5m + 11m) = 0.17W

MOSFET SWITCHING LOSS

PSW = 0.5 x VIN x IL x (tR + tF) x fSW

PSW = 0.5 x 12 x 0.49 x (10 ns + 12 ns) x 6 x105 = 39 mW

MOSFET AND RSNS CONDUCTION LOSS

PC = D x (I2L x (RDSON x 1.3 + RSNS))

PC = 0.63 x (0.492 x (0.029 + 0.51) = 82 mW

INPUT CAPACITOR LOSS

This term represents the loss as input ripple current passes

through the ESR of the input capacitor bank. In this equation

‘n’ is the number of capacitors in parallel. The 3.3 µF input

capacitor selected has an ESR of approximately 3 mΩ, and

ΔiL for a 12V input is 268 mA:

IIN-RMS = 0.29 x ΔiL = 0.29 x 0.268 = 0.08A

PCIN = 0.082 x 0.003 = 0.3 mW (negligible)

OUTPUT CAPACITOR LOSS

This term is calculated using the same method as the input

capacitor loss, substituting the output capacitor RMS current

for VIN = 12V:

IO-RMS = 1.13 x 0.49 x (0.37 x 0.63)0.5 = 0.267A

PCO = 0.267 x 0.003 = 1 mW

BOOST INDUCTOR LOSS

PDCR = IL2 x DCR

PDCR = 0.492 x 0.18 = 43 mW

Core loss in the inductor is assumed to be equal to the DCR

loss, adding an additional 43 mW to the total inductor loss.

TOTAL LOSS

PLOSS = Sum of All Loss Terms = 0.38W

EFFICIENCY

η = 5.9/(5.9 + 0.38) = 94%

Layout Considerations

To produce an optimal power solution with the LM3430, good

layout and design of the PCB are as important as the com-

ponent selection. The following are several guidelines to aid

in creating a good layout.

FILTER CAPACITORS

The low-value ceramic filter capacitors are most effective

when the inductance of the current loops that they filter is

minimized. Place CINX as close as possible to the VIN and

GND pins of the LM3430. Place COX close to the load. CCS

should be placed right next to RSNS, and CF next to the VCC

and GND pins of the LM3430.

19 www.national.com

LM3430

SENSE LINES

The top of RSNS should be connected to the CS pin with a

separate trace made as short as possible. Route this trace

away from the inductor and the switch node (where D1, Q1,

and L1 connect). For the voltage loop, keep RFB1/2 close to

the LM3430 and run a trace from as close as possible to the

positive side of COX to RFB2. As with the CS line, the FB line

should be routed away from the inductor and the switch node.

These measures minimize the length of high impedance lines

and reduce noise pickup.

COMPACT LAYOUT

Parasitic inductance can be reduced by keeping the power

path components close together and keeping the area of the

loops that high currents travel small. Short, thick traces or

copper pours (shapes) are best. In particular, the switch node

should be just large enough to connect all the components

together without excessive heating from the current it carries.

The LM3430 (boost converter) operates in two distinct cycles

whose high current paths are shown in Figure 11:

20209652

FIGURE 11. Boost Converter Current Loops

The dark grey, inner loops represents the high current paths

during the MOSFET on-time. The light grey, outer loop rep-

resents the high current path during the off-time.

GROUND PLANE AND SHAPE ROUTING

The diagram of Figure 11 is also useful for analyzing the flow

of continuous current vs. the flow of pulsating currents. The

circuit paths with current flow during both the on-time and off-

time are considered to be continuous current, while those that

carry current during the on-time or off-time only are pulsating

currents. Preference in routing should be given to the pulsat-

ing current paths, as these are the portions of the circuit most

likely to emit EMI. The ground plane of a PCB is a conductor

and return path, and it is susceptible to noise injection just as

any other circuit path. The continuous current paths on the

ground net can be routed on the system ground plane with

less risk of injecting noise into other circuits. The path be-

tween the input source, input capacitor and the power switch

and the path between the output capacitor and the load are

examples of continuous current paths. In contrast, the path

between the grounded side of the power switch and the neg-

ative output capacitor terminal carries a large pulsating cur-

rent. This path should be routed with a short, thick shape,

preferably on the component side of the PCB. Multiple vias in

parallel should be used right at the negative pads of the input

and output capacitors to connect the component side shapes

to the ground plane. Vias should not be placed directly at the

grounded side of the power switch (or RSNS) as they tend to

inject noise into the ground plane. A second pulsating current

loop that is often ignored but must be kept small is the gate

drive loop formed by the OUT and VCC pins, Q1, RSNS and

capacitor CF.

www.national.com 20

LM3430

BOM for Example Circuit

ID Part Number Type Size Parameters Qty Vendor

U1 LM3430 Low-Side Controller LLP-12 1 NSC

Q1 Si4850EY MOSFET SO-8 60V, 31mΩ, 18nC 1 Vishay

D1 CMHSH5-4 Schottky Diode SOD-123 40V, 0.5A 1 Central Semi

L1 SLF7045T-470M90-1P

Inductor 7.0 x7.0 x 4.5mm 47µH, 0.9A, 180mΩ1 TDK

Cin C3216X7R1E335M Capacitor 1206 3.3µF, 25V, 3mΩ1 TDK

Co C3216X7R1H105M Capacitor 1206 1µF, 50V, 3mΩ1 TDK

Cf C2012X7R1E105K Capacitor 0805 1µF, 25V 1 TDK

Cinx

Cox

VJ0805Y104KXXAT Capacitor 0805 100nF 10% 1 Vishay

C1 VJ0805A391KXXAT Capacitor 0805 390pF 10% 1 Vishay

C2 VJ0805Y393KXXAT Capacitor 0805 39nF 10% 1 Vishay

Css VJ0805Y103KXXAT Capacitor 0805 10nF 10% 1 Vishay

Ccs VJ0805Y102KXXAT Capacitor 0805 1nF 10% 1 Vishay

R1 CRCW08052001F Resistor 0805 2kΩ 1% 1 Vishay

Rfb1 CRCW08057870F Resistor 0805 787Ω 1% 1 Vishay

Rfb2 CRCW08052002F Resistor 0805 20kΩ 1% 1 Vishay

Rs1 CRCW0805101J Resistor 0805 100Ω 5% 1 Vishay

Rsns ERJ8BQFR51V Resistor 1206 0.51Ω 1%, 0.33W 1 Panasonic

Rt CRCW08052742F Resistor 0805 27.4kΩ 1% 1 Vishay

Ruv1 Ruv2 CRCW08051002F Resistor 0805 10kΩ 1% 2 Vishay

21 www.national.com

LM3430

Physical Dimensions inches (millimeters) unless otherwise noted

12-Lead LLP Package

NS Package Number SDF12A

www.national.com 22

LM3430

Notes

23 www.national.com

LM3430

Notes

LM3430 Boost Controller for LED Backlighting

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