We know that most flashlights require three cells because they are regulating output current with only series resister. The odd number of cells except one is not good, and a margin between VLED and VBAT is too small, intensity of light is affected by variation of input voltage within the battery life.
The project is a white LED head lamp which can operate with only two AA batteries. The LEDs are driven in constant current, so that the brightness is maintained until end of battery life. The project designed by Chan
White Led Driver
Download White Led Lamp Driver Document and schematic