VOVP

PWM

Control

Thermal

Shutdown

+150°C

IN

VREF

I

2

C

Interface

Error

Amplifier

Slope

Compensation

SDA

SCL

Control

Logic/

Registers

GND

OUT

LED

TEMP

TX

STROBE

TORCH

ENABLE

SW

65 m:

85 m:

Over Voltage

Comparator

NTC VTRIP

INTC

ILED

Soft-Start

4 MHz

Oscillator

UVLO

Input Voltage

Flash Monitor

OUT-VHR

Current Sense/

Current Limit

LM3556

www.ti.com

SNVS796D – AUGUST 2011 – REVISED OCTOBER 2015

7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 Power-Amplifier Synchronization (TX)

The TX pin is a power-amplifier synchronization input. It is designed to reduce the flash LED current and thus

limit the battery current during high battery-current conditions such as PA transmit events. When the LM3556 is

engaged in a flash event, and the TX pin is pulled high, the LED current is forced into Torch Mode at the

programmed torch current setting or shutdown. If the TX pin is then pulled low before the flash pulse terminates,

the LED current returns to the previous flash current level. At the end of the flash time-out, whether the TX pin is

high or low, the LED current turns off. The polarity of the TX input can be changed from active high to active low

through the Configuration Register (0x07) and can be disabled/enabled by setting the TX Enable bit in the

Enable Register (0x0A) to a '0'.

7.3.2 Input Voltage Flash Monitor (IVFM)

The LM3556 device can adjust the flash current based upon the voltage level present at the IN pin using an input

voltage flash monitor. Two adjustable thresholds (IVM-D and IVM-U) ranging from 2.9 V to 3.6 V in 100-mV

steps, and four different usage modes (Report Mode, Stop and Hold Mode, Adjust Down Only Mode, Adjust Up

and Down Mode), are provided. The Flags Register has the fault flag set when the input voltage crosses the

IVM-D value. In Report Mode, apart from the fault flag triggering, no action is taken on the LED current.

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Product Folder Links: LM3556