Like the circuit in Figure 1, it operates with 3.6 to 6.5V inputs and drives one LED with currents as high as 1A. For the single-LED circuit of Figure 3, the starting voltage of the MAX1685 defines ...

With the circuit values shown, R2 will drop a bit less than 5 V before D3 turns on. Prior to turn on, D2 remains back-biased and no current flows through the LED (D1).

Consequently, a boost converter is often used to create the LED string compliance voltage from a car battery that can vary from 6V under start-stop and cold-crank condition to 19V. The boost topology ...