The LM3645 is a high-power white LED flash driver capable of delivering up to 2 A in any of the four parallel LEDs. The device incorporates a 2-MHz or 4-MHz constant frequency-synchronous current-mode PWM boost converter and quad high-side current sources to regulate the LED current over the 2.3-V to 5.5-V input voltage range.

The LM3645 PWM DC/DC boost converter switches and boosts the output to maintain at least V_HR across each of the current sources (LED1/2/3/4). This minimum headroom voltage ensures that the current sources remain in regulation. If the input voltage is above the LED voltage + current source headroom voltage the device does not switch, but turns the P-channel MOSFET 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 LM3645 has logic inputs including two hardware Flash Enables (STROBE1 and STROBE2), and a hardware Torch Enable muxed with a Flash Interrupt input (TX) designed to interrupt the flash pulse during high battery-current conditions (TORCH/Tx, Tx = default). These logic inputs have internal 300-kΩ (typical) pulldown resistors to GND.

Additional features of the LM3645 include internal comparators for LED thermal sensing via external NTC thermistors (TEMP1, TEMP2) and an input voltage monitor that can reduce the Flash current during low V_IN conditions. It also has a Hardware Enable (EN) pin that can be used to reset the state of the device and the registers by pulling the EN 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 V_IN undervoltage conditions.