Texas Instruments Incorporated AN-1764 User's Guide

1 Introduction

The LM3431 is a combination switching boost voltage regulator and multi-channel LED current controller.
The LM3431 Evaluation Board is designed to drive four parallel strings of eight series LEDs at 140mA per
string from an input voltage range of 8V to 18V. The boost switcher regulates LED cathode voltage for
optimum efficiency, while allowing the output voltage to vary according to the LED forward voltage. The
LM3431 incorporates complete fault protection features including LED short and open protection, over­current protection, over-voltage and under-voltage protection, and thermal shutdown. The error detection
flag can be monitored at the DLY post, which has a programmable delay time.

The board includes a DIM input terminal for a selectable digital or analog dimming control signal. PWM
dimming frequencies up to 1kHz at 100:1 contrast ratio and contrast ratios up to 500:1 at 200Hz are
possible.

The evaluation board is designed to operate at ambient temperatures up to 70°C.
The complete evaluation board schematic is shown in Figure 2. Typical waveforms and performance

curves are shown in Figure 3 through Figure 8.
Although a wide variety of LED combinations and currents can be driven with the LM3431, the schematic

must be modified to achieve optimum performance with any given array. However, for initial evaluation
and demonstration, some modifications from the default settings are possible. A few of the possible
variations are described below and listed in Table 2 and Table 3.

LED current can be set as low as 100mA per string without significantly degrading performance; the
number of strings can be reduced from 4 to 3 or 2; and the number of series LEDs can be set between 6
and 9.

To design for LED arrays other than the default, and for complete device and design information, refer to
the LM3431/31A/31Q/31AQ 3-Channel Constant Current LED Driver With Integrated Boost Controller
(SNVS547) data sheet.

User's Guide

SNVA309A–January 2008–Revised May 2013

AN-1764 LM3431 Evaluation Board

2 Before Powering Up

Before powering up the LM3431 evaluation board, a few options must be selected.
First, verify that the default LED current setting of 140mA is appropriate to drive the LEDs. To drive LEDs

at other currents, refer to Table 2. If the required LED current is not listed, refer to the LM3431 data sheet
(SNVS547), as more extensive modifications will be required.

By default, the LM3431 evaluation board has channels 1-4 enabled. To drive fewer channels, first remove
the sense resistor or NPN on the channel to be disabled (R10-R13 or Q2-Q5). Then refer to Table 3 and
make any other required changes. This table also gives options for driving fewer LEDs per string.

Finally, select the type of dimming control signal to be used. By default, R-MODE is installed, which
enables digital dimming control mode. To use this mode, connect a digital PWM signal to the DIM post.
The dimming signal must have a minimum high logic level of 2.3V.

To enable analog dimming, remove the MODE resistor and connect an analog voltage level to the DIM
post. In analog mode, the dimming duty cycle is varied by the voltage at the DIM post within a range of

0.37V to 2.5V. The default analog dimming frequency is 1kHz. To modify this frequency, change C5 as
described in the datasheet.

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SNVA309A–January 2008–Revised May 2013 AN-1764 LM3431 Evaluation Board

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VA

VA

VC3

VC2

VC1 VC4 GND

THM

board outline

8-pin header

Before Powering Up

Any dimming frequency between 180Hz and 20kHz can be used in either digital or analog dimming control
mode. However the default schematic is not designed for dimming frequencies above 1kHz.Once the
evaluation board has been configured, connect the LED strings as shown in Figure 2. The first anode of
each string connects to the VA post and the last cathode of each string connects to VC1 through VC4.
LEDs can be connected either via the 8-pin header or to the terminal posts. Figure 1 shows the pinout of
the header.

Connect the input voltage supply to the VIN and GND posts.
When all connections and polarities have been verified, power can be applied. When the input voltage

rises above 7.7V typically, the EN pin will cross the UVLO threshold and the LM3431 will startup. EN can
be pulled low (via the EN post) to shutdown the evaluation board.

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Figure 1. LED Connection, Top View Header Pinout

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AN-1764 LM3431 Evaluation Board SNVA309A–January 2008–Revised May 2013

Copyright © 2008–2013, Texas Instruments Incorporated

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LM3431

VIN
EN

FF

SGND

AFB

VCC

REF
REFIN

SNS1

NDRV3

SNS3

NDRV1

NDRV2

SNS2

LG

CS

PGND

COMP

MODE/F

RT

CFB

THM

SC

SS/SH
DLY

DIM

ILIM

Vin: 8V to 18V

LEDOFF

+

-

REFIN

LEDOFF

VCC

D2-5 3003C

D6-9 1001A

D1 40V 2A

L1

7 PH 3.1A

Q1

Si4446

C10*

R19

562k

R18

33.2k

R3

51m:

R4 5.76k

C1

1 PF

C2

10 PF

R2

20k

R1

105k

R7 88.7k

R8 13k

R9 909:

C4 47 nF

R5b

15.8k

C3

4.7 PF

R6

34.8k

C6

47 nF

C7

4.7 nF

VCC

R10

2.2:

Q2

2N2222

Q6

2N7002k

LED ARRAY = 4 x 8 @140 mA

R15 1k

VA

VC1

VC2

VC3

VC4

RMODE

0:

C5

10 nF

THM

R17 95.3k

VCC

Op1

LMV710

THM

External

Thermistor

C11 *

EP

C8,C9

4.7 PF

Q3

2N2222

Q4

2N2222

Q5

2N2222

R11

2.2:

R12

2.2:

R14
10k

* = not installed

R16 10k

Rrestart

*

C13 *

+

10 nF

8V to 18V input

700 kHz switching, 180 Hz ± 1 kHz dimming

4 strings of 8 LEDs, 140 mA per string

R13

2.2:

VCC

R24 *

GND

C14

*

D10 *

C12 *

R5a

0:

R21 - R23

*

RVCC

*

C16*

DIM

R20 *

EN

R25 0:

C15

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Before Powering Up

SNVA309A–January 2008–Revised May 2013 AN-1764 LM3431 Evaluation Board

Figure 2. Complete LM3431 Eval Board Schematic

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