VIN 1

PGND 2

VCC 3

LG 4

CS 5

ILIM 6

MODE/F 7

FF 8

17 CFB

18 SC

15 SS/SH

16 DLY

19 LEDOFF

26 THM

27 DIM

28 EN

RT 9

REF 10

REFIN 11

COMP 12

SGND 13

AFB 14

20 SNS3

21 NDRV3

22 SNS2

23 NDRV2

24 SNS1

25 NDRV1

Exposed Pad

Connect to SGND

LM3431

www.ti.com

SNVS547F – NOVEMBER 2007 – REVISED MARCH 2009

These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam

during storage or handling to prevent electrostatic damage to the MOS gates.

CONNECTION DIAGRAM

Top View

Figure 1. 28 Lead Plastic Exposed Pad TSSOP

See Package Number PWP0028A

PIN DESCRIPTIONS

Pin No.

Pin Name

Description

1

VIN

Power supply input.

2

PGND

Power ground pin. Connect to ground.

3

VCC

Internal reference voltage output. Bypass to PGND with a minimum 4.7 µF capacitor.

4

LG

Boost controller gate drive output. Connect to the NFET gate.

5

CS

Boost controller current sense pin. Connect to the top side of the boost current sense resistor.

6

ILIM

Boost controller current limit adjust pin. Connect a resistor from this pin to the Boost current sense resistor to set

the current limit threshold.

7

MODE/F

Dimming mode selection pin. Pull high for digital PWM control. Or connect to a capacitor to GND to set the

internal dimming frequency.

8

FF

Feedforward pin. Connect to a resistor to ground to control the output voltage over/undershoot during PWM

dimming.

9

RT

Frequency adjust pin. Connect a resistor from this pin to ground to set the operating frequency of the boost

controller.

10

REF

Reference voltage. Use this pin to provide the REFIN voltage.

11

REFIN

This pin sets the LED current feedback voltage. Connect to a resistor divider from the REF pin.

12

COMP

Output of the error amplifier. Connect to the compensation network.

13

SGND

Signal ground pin. Connect to ground.

14

AFB

Anode feedback pin. The boost controller voltage feedback during LED off time. Connect this pin to a resistor

divider from the output voltage.

15

SS/SH

Soft-start and sample-hold pin. Connect a capacitor from this pin to ground to set the soft-start time.

16

DLY

Fault delay pin. Connect a capacitor from this pin to ground to set the delay time for shutdown.

17

CFB

Cathode feedback pin. The boost controller voltage feedback. Connect through a diode to the bottom cathode of

each LED string.

18

SC

LED short circuit detection pin. Connect through a diode to the bottom cathode of each string.

19

LEDOFF

A dual function pin. The LEDOFF signal controls external drivers during PWM dimming. Or connect to ground to

enable automatic fault restart.

20

SNS3

Current feedback for channel 3. Connect to the top of the channel 3 current sense resistor.

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