With an LED present, as the output voltage ramps to approximately 3.4V, the LED will turn on causing current to flow through R7. The voltage across R7 is amplified by a factor of 7X by the SOT-5 LM321MF and the gain resistors R5 and R6. This produces a voltage of 1.24 volts at the junction of R4 and R5 when 350mA flows through the LED. The 1.24 volts causes the LM2623 to stop switching which regulates the LED current at a constant value. This current loop controlled boost circuit requires 1 or 2 cells and will operate with full brightness down to 1.0V. It uses the LM2623MM, a mini-SO8 package boost switcher with an internal 1.2amp switch, to boost the input voltage from 2-3V to the required LED voltage of 3.4V.

The LM2623A should be used for single cell operation since it guarantees a peak switch current greater than 2A.

The open circuit voltage with no LED present is set to 5V by the resistor ladder of R3-R6. If the LED is permanent, R3 can be omitted.

Voutmax = Vfb*(1+R3/(R4+R5+R6))

LED current = Vfb/R7*R6/(R6+R5)

The inductor is a Coilcraft DS1608 size, the capacitors are small ceramic, and the diode is a 0.5A rated Schottky in the SOD package.

Notes: Please note that A1 in the schematic is represented as U in the board layout.