LM3556_15 Datasheet, PDF (12/39 Pages) Texas Instruments – Synchronous Boost LED Flash Driver With High-Side Current Source

LM3556
SNVS796D – AUGUST 2011 – REVISED OCTOBER 2015
7 Detailed Description
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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 V_HR 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 (V_IN − I_LED × R_PMOS) 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-kΩ (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 V_IN 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 V_IN undervoltage conditions.
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