Datasheet

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
SNVS547G –NOVEMBER 2007–REVISED MAY 2013 www.ti.com
CONNECTION DIAGRAM
Figure 1. 28 Lead Plastic Exposed Pad TSSOP
Top View
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.
21 NDRV3 Base drive for the channel 3 current regulator. Connect to the NPN base or NFET gate.
22 SNS2 Current feedback for channel 2.
23 NDRV2 Base drive for the channel 2 current regulator.
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