D1

Current

Sink

D2

Current

Sink

Current

Control

RSET

MIN. Dx

VOLTAGE

SELECT

D1

D2

TORCH CTRL

FLASH CTRL

TIME-OUT CTRL

I

2

C INTERFACE/

CONTROL LOGIC/

REGISTERS

+

-

0.35V or

0.450V

VIO

SCL

SDA

M/F

RESET/GPIO

RZ

CC

¦

gm

SWITCH

CONTROLLER

OSC

RAMP

CURRENT

LIMIT

THERMAL

SHUTDOWN

OVP

OVP

SW

Tx

FEN

VREF

VIN

LM3553

SW

Driver

GND

ISTEP CTRL

LM3553

SNVS414B – FEBRUARY 2008 – REVISED MAY 2013

www.ti.com

BLOCK DIAGRAM

Circuit Description

CIRCUIT COMPONENTS

FEN Pin

The flash enable pin, FEN, provides an external method (non-I

2C) for starting the flash pulse. When F

EN is pulled

high, logic '1', the flash current level defined through the I2C interface, will be delived to the Flash LED. If the FEN

pin is driven low during the flash pulse, the flash event will stop. In the event that FEN is not pulled low during the

flash pulse, the LM3553 will continue to deliver the flash current until the safety timer duration (set through the

I2C interface) is reached.

The LM3553 does not provide a fixed off-time after the flash pulse has ended. Most flash LED manufacturers

require that the flash pulse duration be 10% of the total Flash cycle. Example: If the flash pulse duration is set to

be 200 milliseconds (Flash Duration Code= 0011), the recommended off time for the LED would be 1.8 seconds.

Please consult the LED manufacturers datasheet for exact timing requirements.

If the LM3553 is placed in indicator mode or torch mode through the I2C interface and the FEN pin is pulled high

and then low, at the end of the flash event, the LM3553 will return to the mode stored in the General Purpose

Register.

It is recommended that an external pull-down be placed between the FEN pin and GND to prevent unwanted LED

flashing during system start-up due to unknown control logic states.

8

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