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.

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

Submit Documentation Feedback

Figure 1. Efficiency with 9 Series LEDS AT 1A

1

Copyright © 2009–2013, Texas Instruments Incorporated

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

Voltage

Input Voltage Test point for input voltage.

Input the LM3424 switching frequency to the external clock.

www.ti.com

2

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

Copyright © 2009–2013, Texas Instruments Incorporated

Submit Documentation Feedback

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

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

www.ti.com

4 Schematic

Schematic

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

Submit Documentation Feedback

Copyright © 2009–2013, Texas Instruments Incorporated

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

www.ti.com

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

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

Submit Documentation Feedback

Copyright © 2009–2013, Texas Instruments Incorporated

www.ti.com

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 through a series
the LED current sense resistor.

Connect to GND and place 6 ­pour.

sensing or to

DS-ON

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

Submit Documentation Feedback

Copyright © 2009–2013, Texas Instruments Incorporated

5