LM3556

SNVS796D – AUGUST 2011 – REVISED OCTOBER 2015

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7 Detailed Description

7.1 Overview

The LM3556 is a high-power white LED flash driver capable of delivering up to 1.5 A into a single high-powered

LED. The device incorporates a 4-MHz constant-frequency synchronous current-mode PWM boost converter,

and a single high-side current source to regulate the LED current over the 2.5-V to 5.5-V input voltage range.

The LM3556 PWM converter switches and maintains at least VHR across the current source (LED). This

minimum headroom voltage ensures that the current source remains in regulation. If the input voltage is above

the LED voltage plus current source headroom voltage, the device does not switch, and turns the PFET on

continuously (Pass Mode). In Pass Mode the difference between (VIN − ILED × RPMOS) and the voltage across the

LED is dropped across the current source.

The LM3556 has three logic inputs including a hardware flash enable (STROBE), a hardware torch enable

(TORCH) used for external torch mode control and custom LED indication waveforms, and a flash interrupt input

(TX) designed to interrupt the flash pulse during high battery-current conditions. All three logic inputs have

internal 300-k

Ω (typical) pulldown resistors to GND.

Additional features of the LM3556 include an internal comparator for LED thermal sensing via an external NTC

thermistor and an input voltage monitor that can reduce the Flash current (during low VIN conditions).

Control of the LM3556 is done via an I2C-compatible interface. This includes adjustment of the flash and torch

current levels, changing the flash time-out duration, changing the switch current limit, and enabling the NTC

block. Additionally, there are flag and status bits that indicate flash current time-out, LED overtemperature

condition, LED failure (open or short), device thermal shutdown, TX interrupt, and VIN undervoltage conditions.

12

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