Texas Instruments Incorporated AN-1967 User's Guide

VIN (V)

EFFICIENCY (%)

100

95 90 85 80 75 70

0 16 32 48 64 80

1 Introduction

This wide range evaluation board showcases the LM3424 NFET controller used with a buck-boost current regulator. It is designed to drive 4 to 10 LEDs at a maximum average LED current of 1A from a DC input voltage of 10 to 70V.

The evaluation board showcases many of the LM3424 features including thermal foldback, analog dimming, external switching frequency synchronization, and high frequency PWM dimming, among others. There are many external connection points to facilitate the full evaluation of the LM3424 device including inputs, outputs and test points. Refer to Table 1 for a summary of the connectors and test points.

The buck-boost circuit can be easily redesigned for different specifications by changing only a few components (see the Alternate Designs section). Note that design modifications can change the system efficiency for better or worse.

This application note is designed to be used in conjunction with the LM3424 datasheet as a reference for the LM3424 buck-boost evaluation board. Refer to the LM3424 Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs (SNVS603) data sheet for a comprehensive explanation of the device, design procedures, and application information.

User's Guide

SNVA397A–August 2009–Revised May 2013

AN-1967 LM3424 Buck-Boost Evaluation Board

2 Key Features

• Input: 10V to 70V

• Output: 4 to 10 LEDs at 1A

• Thermal Foldback / Analog Dimming

• PWM Dimming up to 10 kHz

• External Synchronization > 500 kHz

• Input Under-voltage and Output Over-voltage Protection

All trademarks are the property of their respective owners.

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Figure 1. Efficiency with 9 Series LEDS AT 1A

1

External Connection Descriptions

3 External Connection Descriptions

Table 1. Connectors and Test Points

Qty Name Description Application Information

J1 V

IN

J2 GND Input Ground Connect to negative terminal of supply voltage (GND). J3 EN Enable On/Off Jumper connected enables device. J4 LED+ LED Positive Connect to anode (top) of LED string. J5 LED- LED Negative Connect to cathode (bottom) of LED string. J6 BNC Dimming Input Connect a 3V to 10V PWM input signal up to 10 kHz for PWM dimming the LED load. J7 OUT Output with NTC Alternative connector for LED+ and LED-. Pins 4 and 11 are used for connecting an

TP1 SW Switch Node Test point for switch node (where Q1, D1, and L1 connect).

TP3 SGND Signal Ground Connection for GND when applying signals to TP5, TP8, and TP9. TP4 LED+ LED Positive Test point for anode (top) of LED string.

TP5 nDIM Inverted Dim Signal Test point for dimming input (inverted from input signal). TP6 V

IN

TP8 SYNC Synchronization Connect a 3V to 6V PWM clock signal > 500 kHz (pulse width of 100ns) to synchronize

TP9 NTC Temp Sense Input Connect a 0V to 1.24V DC voltage to analog dim the LED current. TP10 PGND Power Ground Test point for GND when monitoring TP1, TP4, or TP6.

Input Voltage Connect to positive terminal of supply voltage.

external NTC thermistor. Refer to schematic for detailed connectivity.

Voltage

Input Voltage Test point for input voltage.

Input the LM3424 switching frequency to the external clock.

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SS

TGAIN

OVP

LM3424

nDIM

GND

TSENSE

TREF

DDRV

VS

DAP

GATE

EN

COMP

VIN

CSH

RT/SYNC

IS

HSN

SLOPE

VCC

HSP

Q7

D1

L1

R14

C9

R6

Q1

C7

R17

R11

R13

R5

R15

NTC

R19

R21

R22

1

2

3

4

5

6

7

8

9

10

20

19

18

17

16

15

14

13

12

11

R8

R7

C22

R9

R20

C12

C11

C15

Q3

PWM

R4

Q6

Q5

Q4

D2

V

IN

LED+

V

IN

DIM

R24

R23

Q2

C4, C5, C6, C17, C19

C2, C3, C16, C18, C23

C1

R3

J3

R12

J6

TP5

R2

C8

R1

R10

3

4

5

C10

C13

C14

R25

TP3 TP10

TP8

TP6

TP1

TP4

V

IN

J1

GND

J2

R26

C20

C21

8 9

10

12 13 14

7 6 5

3 2 1

J7

11 4

LED-

J4

J5

NTC

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4 Schematic

Schematic

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3

LM3424 Pin Descriptions

5 LM3424 Pin Descriptions

Pin Name Description Application Information

1 V

2 EN Enable the device or to < 0.8V for low

3 COMP Compensation

4 CSH Current Sense High

5 RT Resistor Timing synchronize external clock as

6 nDIM Not DIM input

7 SS Soft-start

8 TGAIN Temperature Foldback Gain

9 TSENSE Temperature Sense Input

10 TREF VSto set the temperature

11 V

12 OVP Over-Voltage Protection Turn-off threshold is 1.24V and

13 DDRV Dimming Gate Drive Output

14 GND Ground

15 GATE Gate Drive Output

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Bypass with 100 nF capacitor

IN

Input Voltage

to GND as close to the device as possible in the circuit board layout.

Connect to > 2.4V to enable power shutdown.

Connect a capacitor to GND to compensate control loop.

Connect a resistor to GND to set the signal current. Can also be used to analog dim as explained in the Thermal

Foldback / Analog Dimming

section of the datasheet. Connect a resistor to GND to

set the switching frequency. Can also be used to

explained in the Switching Frequency section of the datasheet.

Connect a PWM signal for dimming as detailed in the PWM Dimming section of the datasheet and/or a resistor divider from VINto program input under-voltage lockout (UVLO). Turn-on threshold is

1.24V and hysteresis for turnoff is provided by 20 µA current source.

Connect a capacitor to GND to extend start-up time.

Connect a resistor to GND to set the foldback slope.

Connect a resistor/ thermistor divider from VSto sense the temperature as explained in the Thermal Foldback / Analog Dimming section of the datasheet.

Temperature Foldback

Reference

Connect a resistor divider from foldback reference voltage.

2.45V reference for

S

Voltage Reference temperature foldback circuit

and other external circuitry. Connect to a resistor divider

from VOto program output over-voltage lockout (OVLO).

hysteresis for turn-on is provided by 20 µA current source.

Connect to gate of dimming MosFET.

Connect to DAP to provide proper system GND

Connect to gate of main switching MosFET.

4

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Pin Name Description Application Information

16 V

17 IS Main Switch Current Sense switch for R

18 SLOPE Slope Compensation

19 HSN resistor to the negative side of

20 HSP resistor to the positive side of

DAP (21) DAP Thermal pad on bottom of IC 9 vias to bottom layer ground

6 Bill of Materials

Qty Part ID Part Value Manufacturer Part Number

4 C1, C5, C20, C23 0.1 µF X7R 10% 100V TDK C2012X7R2A104K 4 C2, C3, C16, C18 4.7 µF X7R 10% 100V TDK C5750X7R2A475K 4 C4, C6, C17, C19 10 µF X7R 10% 50V TDK C5750X7R1H106K 2 C7, C22 0.47 µF X7R 10% 16V MURATA GRM21BR71C474KA01L 0 C8 DNP 1 C9 2.2 µF X7R 10% 16V MURATA GRM21BR71C225KA12L 1 C10 1 µF X7R 10% 16V MURATA GRM21BR71C105KA01L 1 C11 47 pF COG/NPO 5% 50V AVX 08055A470JAT2A 1 C12 0.22 µF X7R 10% 16V MURATA GRM219R71C224KA01D 3 C13, C14, C21 100 pF COG/NPO 5% 50V MURATA GRM2165C1H101JA01D 1 C15 1 µF X7R 10% 16V MURATA GRM21BR71C105MA01L 1 D1 Schottky 100V 12A VISHAY 12CWQ10FNPBF 1 D2 Zener 10V ON-SEMI BZX84C10LT1G 4 J1, J2, J4, J5 Banana Jack KEYSTONE 575-8 1 J3 1x2 Header Male SAMTEC TSW-102-07-T-S 1 J6 BNC connector AMPHENOL 112536 1 J7 2x7 Header Male Shrouded SAMTEC TSSH-107-01-SDRA

1 L1 33 µH 20% 6.3A COILCRAFT MSS1278-333MLB 2 Q1, Q2 NMOS 100V 32A FAIRCHILD FDD3682 1 Q3 NMOS 60V 260mA ON-SEMI 2N7002ET1G 1 Q4 PNP 40V 200mA FAIRCHILD MMBT5087 1 Q5 PNP 150V 600 mA FAIRCHILD MMBT5401 1 Q6 NPN 300V 600mA FAIRCHILD MMBTA42 1 Q7 NPN 40V 200mA FAIRCHILD MMBT6428 2 R1, R11 12.4 kΩ 1% VISHAY CRCW080512K4FKEA 0 R2 DNP 3 R3, R20, R26 10Ω 1% VISHAY CRCW080510R0FKEA 1 R4 17.4 kΩ 1% VISHAY CRCW080517K4FKEA 1 R5 1.43 kΩ 1% VISHAY CRCW08051K43FKEA

CC

RA

Internal Regulator Output

High-Side LED Current Sense

Negative

High-Side LED Current Sense

Positive

Bill of Materials

Bypass with a 2.2 µF–3.3 µF, ceramic capacitor to GND.

Connect to the drain of the main N-channel MosFET

a sense resistor installed in the source of the same device.

Connect a resistor to GND to set slope of additional ramp.

Connect through a series the LED current sense resistor.

Connect to GND and place 6 pour.

sensing or to

DS-ON

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5

Bill of Materials

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1 R6 0.04Ω 1% 1W VISHAY WSL2512R0400FEA 2 R7, R8 1.0 kΩ 1% VISHAY CRCW08051K00FKEA 1 R9 0.1Ω 1% 1W VISHAY WSL2512R1000FEA 1 R10 14.3 kΩ 1% VISHAY CRCW080514K3FKEA 4 R12, R13, R14, R15 10.0 kΩ 1% VISHAY CRCW080510K0FKEA 1 R17 499 kΩ 1% VISHAY CRCW0805499KFKEA 3 R19, R21, R22 49.9 kΩ 1% VISHAY CRCW080549K9FKEA 1 R23 499Ω 1% VISHAY CRCW0805499RFKEA 1 R24 4.99 kΩ 1% VISHAY CRCW08054K99FKEA 1 R25 150Ω 1% VISHAY CRCW0805150RFKEA 8 TP1, TP3, TP4, Turret Keystone 1502-2

TP5, TP6, TP8, TP9, TP10

1 U1 Buck-boost controller TI LM3424

6

AN-1967 LM3424 Buck-Boost Evaluation Board SNVA397A–August 2009–Revised May 2013

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7 PCB Layout

PCB Layout

Figure 2. Top Layer

Figure 3. Bottom Layer

SNVA397A–August 2009–Revised May 2013 AN-1967 LM3424 Buck-Boost Evaluation Board

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7

R10 = =

= 14.4 k:

1 + 1.95e-8 x f

SW

1.40e

-10

x f

SW

1 + 1.95e-8 x 500 kHz

1.40e

-10

x 500 kHz

D

MAX

==

677.0

=

V21

V10V21 +

V

O

VV

IN-MINO

+

21V

=

21V + 70V

= 0.231

D

MIN

=

VO + V

IN-MAX

V

O

533.0467.01D1'D

=

-

=

-

=

D

==

467.0

=

V21

V24V21 +

V

O

VV

INO

+

:

=

:x

=

x

=

95.1m3256rNr

LEDD

V21V5.36VNV

LEDO

=

x

=

x

=

Design Procedure

8 Design Procedure

8.1 Specifications

N = 6 V

= 3.5V

LED

r

= 325 mΩ

LED

VIN= 24V V

= 10V

IN-MIN

V fSW= 500 kHz V I

Δi Δi Δv

I V V V V TBK= 45°C T t

IN-MAX

SNS

= 1A

LED

L-PP

LED-PP

IN-PP

= 6A

LIM

TURN-ON

HYS

TURN-OFF

HYSO

= 125°C

END

= 40 ms

TSU

= 70V

= 100 mV

= 700 mA

= 12 mA

= 100 mV

= 10V

= 3V

= 50V

= 10V

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8.2 Operating Point

8.3 Switching Frequency

8

Solve for VOand rD:

(1) (2)

Solve for D, D', D

MAX

, and D

MIN

:

(3) (4)

(5)

(6)

Solve for RT:

(7)

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PP-==L

DVINx

f1LSWx

kHz045H33 xP

467.0V42 x

mA674

=

'i

==

DVINx

fSWx

467.0V42 x

PH

32

=

1L

PP-'iL

kHz045700 mA x

R15 = 10 k: R19 = R21 = R22 = 49.9 k:

R

GAIN

=

R19

x 2.45V

100 PA

R19 + R21

-

¸

¸ ¹

·

¸ ¸

¹

·

R

NTC-END

R

NTC-END

+ R22

I

CSH

= 9.49 k:

R

GAIN

=

1

x 2.45V

2

-

¸

¸ ¹

·

¸ ¸

¹

·

6.34 k: + 49.9 k:

6.34 k:

R8 = R7 = 1 k:

R1 = 12.4 k:

R9 = 0.1:

LED

=

= = 1.0A

1.24V x R8 R9 x R1

1.24V x 1.0 k:

0.1: x 1.24 k:

= 1.0 k:

I

LED

x R1 x R9

1.24V

1A x 1.24 k: x 0.1:

= 0.1:

V

SNS

100 mV

1A

I

LED

R10 = 14.3 k:

fSW =

= 504 kHz

1.40e

-10

x 14.3 k: - 1.95e

-8

1

fSW =

1.40e

-10

x R10 - 1.95e

-8

1

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The closest standard resistor is 14.3 kΩ therefore fSWis:

The chosen component from step 2 is:

8.4 Average LED Current

Design Procedure

(8)

(9)

Solve for R

Assume R

:

SNS

= 12.4 kΩ and solve for R

CSH

The closest standard resistor for R

The chosen components from step 3 are:

8.5 Thermal Foldback

Using a standard 100k NTC thermistor (connected to pins 4 and 11 of J7), find the resistances corresponding to TBKand T datasheet. Assuming R

Solve for R

GAIN

:

END(RNTC-BK

= R

REF1

:

HSP

is actually 0.1Ω and for R

SNS

= 243 kΩ and R

= 49.9 kΩ, then R

REF2

NTC-END

BIAS

is actually 1 kΩ therefore I

HSP

= 71.5 kΩ) from the manufacturer's

= R

NTC-BK

= 243 kΩ.

LED

(10)

(11)

is:

(12)

(13)

The chosen components from step 4 are:

8.6 Inductor Ripple Current

Solve for L1:

The closest standard inductor is 33 µH therefore Δi

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is:

L-PP

(14)

(15)

(16)

(17)

9

= 9.9 k:

R15 =

VO x R10 x R9

R15 =

1.5e13 x L1

35V x 14.3 k: x 0.1:

1.5e13 x 33 PH

R6 = 0.04:

=

0.04:

= 6.13A

I

LIM

=

R6

245 mV 245 mV

=

6A

= 0.041:

R6 =

I

LIM

245 mV 245 mV

C4 = C6 = C17 = C19 = 10 PF

x

A1

=

I

LED

I

RMSCO-

=

1- 0.677

677.0

1.45A

x

1- D

MAX

D

MAX

=

DI

LED

x

=

'i

PP-LED

SW

fxrDx

C

O

2

= =

kHz04595.1 xx:

1 mA

467.0A1 x

F40 P

'i

PP-LED

f

'i

r

SWPP-LEDD

xx

DI

LED

x

CO=

2

= F39.6P=

kHz045mA195.1 xx:

467.0A1 x

C

O

H331L P

=

A89.1

12

1

I

RMSL

=

+

x

=

-

I

LED

RMSL

x

=

-

12

1

2

x

+

¸

¸ ¹

·

¨

¨ ©

§

Di

PPL

c

x

'

-

I

LED

D

c

533.0mA674

2

x

¸

¸ ¹

·

¨

¨ ©

§

A1

x

533.0

A1

Design Procedure

Determine minimum allowable RMS current rating:

The chosen component from step 5 is:

8.7 Output Capacitance

Solve for CO:

The closest capacitance totals 40 µF therefore Δi

LED-PP

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(18)

(19)

(20)

is:

Determine minimum allowable RMS current rating:

The chosen components from step 6 are:

8.8 Peak Current Limit

Solve for R

The closest standard resistor is 0.04 Ω therefore I

The chosen component from step 7 is:

LIM

:

8.9 Slope Compensation

Solve for R

SLP

:

LIM

(21)

(22)

(23)

(24)

is:

(25)

(26)

The chosen component from step 8 is:

10

AN-1967 LM3424 Buck-Boost Evaluation Board SNVA397A–August 2009–Revised May 2013

(27)

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C2 = C3 = C16 = C18 = 4.7 PF

x

A1

=

I

LED

I

RMSIN-

=

1- 0.677

677.0

1.45A

x

1- D

MAX

D

MAX

=

C

IN

==

kHz504mV100 x

467.0A1 x F27.9 P=

f

'v

SWPPIN-

x

DI

LED

x

C12 = 0.22 PF

C10 = 1 PF

R20 = 10:

C12 = =

= 0.28 PF

1

10:xZP3

1

rad

sec

10:x 360k

1010(max

1P

xZ

=

x

=

,

1Z1P

ZZ

sec

rad

k36

sec

rad

k36010=x

=

3P

Z

3P

Z

)

C10 = =

= 0.30 PF

1

Z

P2

x 5e

6

:

1

0.675 x 5e

6

:

rad sec

= =

sec

rad

675.0=

sec

rad

k19

56305 x56305x

1Z

Z

2P

=Z

),min(

1Z1P

ZZ

T50Ux

TU0 =

= = 5630

D' x 620V

(1 + D) x I

LED

x R6

0.533 x 620V

1.467 x 1A x 0.04:

sec

rad

k36===

533.095.12x: H33467.0 Px

Dr

2

D

c

x

L1Dx

1Z

Z

sec

rad

k19===

1.467 F40

1.95:

Px

COrDx

D1+

1P

Z

R

SLP

= 10 k:

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8.10 Loop Compensation

ω

is approximated:

P1

ω

is approximated:

Z1

TU0is approximated:

To ensure stability, calculate ωP2:

Solve for C

CMP

:

Design Procedure

(28)

(29)

(30)

(31)

(32)

To attenuate switching noise, calculate ωP3:

Assume RFS= 10Ω and solve for CFS:

The chosen components from step 9 are:

8.11 Input Capacitance

Solve for the minimum CIN:

To minimize power supply interaction a 200% larger capacitance of approximately 20 µF is used, therefore the actual Δv capacitors are chosen.

Determine minimum allowable RMS current rating:

is much lower. Since high voltage ceramic capacitor selection is limited, four 4.7 µF X7R

IN-PP

(33)

(34)

(35)

(36)

(37)

The chosen components from step 10 are:

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(38)

(39)

11

V

TURN-ON

=

R5

1.24V x (1.43 k: + 10 k:)

1.43 k:

1.24V x (R5 + R13)

= 9.91V

V

TURN-ON

=

1.24V x R13

V

TURN-ON

- 1.24V

= 1.42 k:

R5 = =

1.24V x 10 k: 10V - 1.24V

HYS

=

R5

20 PA x 17.4 k: x (1.43 k: + 10 k:)

1.43 k:

HYS

=

+ 20 PA x R13

20 PA x R4 x (R5 + R13)

+ 20 PA x 10 k: = 2.98V

R4 =

R5 x (V

HYS

- 20 PA x R13)

20 PA x (R5 + R13)

= 17.5 k:

R4 =

1.43 k: x (3V - 20 PA x 10 k:) 20 PA x (1.43 k: + 10 k:)

D1 o 12A, 100V, DPAK

mW600mV600A1VIP

FDDD

=

x

=

x

=

A1II

LEDMAXD==-

V91V21V70VVV

OMAXINMAXRD

=

+

=

+

=

--

Q1 o 32A, 100V, DPAK

mW82m50A28.1RIP

2

DSON

2

RMSTT

=

:x

=

x

=

-

x

I

RMST=-

I

LED

D

c

=

x

A28.1

=

0.467

A1

533.0

D

=

A2.1A1=x

677.01-

677.0

I

MAXT-

V91V21V70VVV

OMAXINMAXT

=+=+=

--

Design Procedure

8.12 NFET

Determine minimum Q1 voltage rating and current rating:

A 100V NFET is chosen with a current rating of 32A due to the low R PT:

The chosen component from step 11 is:

8.13 Diode

Determine minimum D1 voltage rating and current rating:

= 50 mΩ. Determine I

DS-ON

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and

T-RMS

(40)

(41)

(42)

(43)

(44)

(45)

A 100V diode is chosen with a current rating of 12A and VD= 600 mV. Determine PD:

The chosen component from step 12 is:

8.14 Input UVLO

Solve for R

The closest standard resistor is 150 kΩ therefore V

Solve for R

UV2

UV1

(46)

(47)

(48)

:

(49)

is:

HYS

(50)

:

The closest standard resistor is 21 kΩ making V

12

AN-1967 LM3424 Buck-Boost Evaluation Board SNVA397A–August 2009–Revised May 2013

(51)

:

TURN-ON

(52)

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CSS = 0.47 PF

CSS =

= = 540 nF

(40 ms - 29.2 ms)

(t

TSU

- t

SU-SS-BASE

)

20 k: 20 k:

V

O

I

LED

t

SU-SS-BASE

= 168: x C9 + 28 k:x C10 + x C

O

t

SU-SS-BASE

= 29.2 ms

21V

1A

t

SU-SS-BASE

= 168: x 2.2 PF + 28 k:x 1.0 PF + x 40 PF

V

O

I

LED

tSU = 168: x C9 + 36 k:x C10 + x C

O

tSU = 37.2 ms

21V

1A

tSU = 168: x 2.2 PF + 36 k:x 1.0 PF + x 40 PF

R11 = 12.4 k: R17 = 499 k:

= 51.1V

12.4 k:

1.24V x (12.4 k: + 499 k:)

V

TURN-OFF

=

R11

1.24V x (R11 + R17)

V

TURN-OFF

=

R11 = =

= 12.5 k:

V

TURN-OFF

- 1.24V

1.24V x R17

1.24V x 499 k: 50V - 620 mV

V

HYSO

= R17 x 20 PA = 499 k: x 20 PA = 9.98V

R17 =

=

= 500 k:

V

HYSO

10V

20 PA20 PA

R4 = 17.4 k:

R13 = 10 k:

R5 = 1.43 k:

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The chosen components from step 13 are:

8.15 Output OVLO

Design Procedure

(53)

Solve for R

The closest standard resistor is 499 kΩ therefore V

Solve for R

The closest standard resistor is 15.8 kΩ making V

The chosen components from step 14 are:

8.16 Soft-Start

Solve for tSU:

OV2

OV1

:

(54)

is:

HYSO

(55)

:

(56)

TURN-OFF

:

(57)

(58)

If tSUis less than t

, solve for t

TSU

Solve for CSS:

The chosen component from step 15 is:

SNVA397A–August 2009–Revised May 2013 AN-1967 LM3424 Buck-Boost Evaluation Board

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SU-SS-BASE

:

(59)

(60)

(61)

(62)

13

I

LED

(A)

V

SW

(V)

60

40

20

0

1.0

0.5

0.0

I

LED

2 Ps/DIV

V

SW

I

LED

(A)

V

DIM

(V)

10

5

0

1.0

0.0

-1.0

I

LED

V

DIM

4 ms/DIV

Typical Waveforms

9 Typical Waveforms

TA= +25°C, VIN= 24V and VO= 21V.

Figure 4. Standard Operation Figure 5. 200Hz 50% PWM Dimming

TP1 Switch Node Voltage (VSW) TP5 Dim Voltage (V

LED Current (I

) LED Current (I

LED

www.ti.com

)

DIM

)

10 Alternate Designs

Alternate designs with the LM3429 evaluation board are possible with very few changes to the existing hardware. The evaluation board FETs and diodes are already rated higher than necessary for design flexibility. The input UVLO, output OVP, input and output capacitance can remain the same for the designs shown below. These alternate designs can be evaluated by changing only R9, R10, and L1.

Table 2 gives the main specifications for four different designs and the corresponding values for R9, R10,

and L1. PWM dimming can be evaluated with any of these designs.

Specification / Design 1 Design 2 Design 3 Design 4

Component

V

IN

V

O

f

SW

I

LED

R9 0.05Ω 0.2Ω 0.04Ω 0.08Ω

R10 12.1 kΩ 10.2 kΩ 14.3 kΩ 10.2 kΩ

L1 22µH 68µH 15µH 33µH

Table 2. Alternate Design Specifications

10V - 45V 15V - 50V 20V - 55V 25V - 60V

14V 21V 28V 35V

600kHz 700kHz 500kHz 700kHz

2A 500mA 2.5A 1.25A

14

AN-1967 LM3424 Buck-Boost Evaluation Board SNVA397A–August 2009–Revised May 2013

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Texas Instruments Incorporated AN-1967 User's Guide

Specifications and Main Features

Frequently Asked Questions

User Manual

1 Introduction

2 Key Features

3 External Connection Descriptions

4 Schematic

5 LM3424 Pin Descriptions

6 Bill of Materials

7 PCB Layout

8 Design Procedure

8.1 Specifications

8.2 Operating Point

8.3 Switching Frequency

8.4 Average LED Current

8.5 Thermal Foldback

8.6 Inductor Ripple Current

8.7 Output Capacitance

8.8 Peak Current Limit

8.9 Slope Compensation

8.10 Loop Compensation

8.11 Input Capacitance

8.12 NFET

8.13 Diode

8.14 Input UVLO

8.15 Output OVLO

8.16 Soft-Start

9 Typical Waveforms

10 Alternate Designs