Datasheet AL3644 (Diodes) - 10

AL3644 Application Information 1. General Operation
The AL3644 is a high-power white LED flash driver capable of delivering up to 1.5A in either of the two parallel LEDs. The device incorporates a 2MHz or 4MHz constant frequency-synchronous current-mode PWM boost converter and dual high-side current sources to regulate the LED current over the 2.5V to 5V input voltage range. The AL3644 PWM DC-DC boost converter switches and boosts the output to maintain at least VHR across each of the current sources (LED1/2).
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This minimum headroom voltage ensures that both current sources remain in regulation. If the input voltage is above the LED voltage plus current
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source headroom voltage, the device does not switch, but turns the PFET on continuously (Pass mode). In Pass mode the difference between (VIN
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− ILED × RPMOS) and the voltage across the LED is dropped across the current source.
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The AL3644 has three logic inputs including a hardware Flash Enable (STROBE), a hardware Torch Enable (TORCH/TEMP, TORCH = default),
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and a Flash Interrupt input (TX) designed to interrupt the flash pulse during high battery-current conditions. These logic inputs have internal 300kΩ
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(typical) pull down resistors to GND.
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Additional features of the AL3644 include an internal comparator for LED thermal sensing via an external NTC thermistor and an input voltage
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monitor that can reduce the Flash current during low VIN conditions. It also has a Hardware Enable (HWEN) pin that can be used to reset the state of the device and the registers by pulling the HWEN pin to ground. Control is done via an I2C-compatible interface. This includes adjustment of the Flash and Torch current levels, changing the Flash Timeout Duration, and changing the switch current limit. Additionally, there are flag and status bits that indicate flash current time-out, LED over temperature condition, LED failure (open/short), device thermal shutdown, TX interrupt, and VIN under voltage conditions.
2. Feature Description 2. 1 Flash Mode
In Flash Mode, the LED current sources (LED1/2) provide 128 target current levels from 10.9mA to 1500mA. Once the Flash sequence is activated the current source (LED) ramps up to the programmed Flash current by stepping through all current steps until the programmed current is reached. The headroom in the two current sources can be regulated to provide 10.9mA to 1.5A on each of the two output pins. There is an option in the register settings to keep the two currents in the output pins the same.

When the device is enabled in Flash Mode through the Enable Register, all mode bits in the Enable Register are cleared after a flash time-out event.
2.2 Torch Mode
In Torch mode, the LED current sources (LED1/2) provide 128 target current levels from 0.977mA to 179mA or 1.954mA to 360mA on AL3644TT. The Torch currents are adjusted via the LED1 and LED2 LED Torch Brightness Registers. Torch mode is activated by the Enable Register (setting M1, M0 to '10'), or by pulling the TORCH/TEMP pin HIGH when the pin is enabled (Enable Register) and set to Torch Mode. Once the TORCH sequence is activated, the active current sources (LED1/2) ramps up to the programmed Torch current by stepping through all current steps until the programmed current is reached. The rate at which the current ramps is determined by the value chosen in the Timing Register.

Torch Mode is not affected by Flash Timeout or by a TX Interrupt event.
2.3 IR Mode
In IR Mode, the target LED current is equal to the value stored in the LED1/2 Flash Brightness Registers. When the IR mode is enabled (setting M1, M0 to '01'), the boost converter turns on and set the output equal to the input (pass-mode). At this point, toggling the STROBE pin enables and disables the LED1/2 current sources (if enabled). The strobe pin can only be set to be Level sensitive, meaning all timing of the IR pulse is externally controlled. In IR Mode, the current sources do not ramp the LED outputs to the target. The current transitions immediately from off to on and then on to off. AL3644 10 of 23 July 2019 Document number: DS41558 Rev. 1 - 2
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