I was originally intending to drive 4 luxeons from 7.2 v (2 li-ion cells) but realized that 2 would be less bulky and provide ample light output for close quarters work. I am currently driving them using a 7 ohm dropping resistor giving me about 250 ma of current. I will probably switch to a 5 ohms or so, but my calculations state that I'll lose about 0.6 watt in the new resistor which means only ~75% efficiency. Is there any chip out there using only a few components that is an efficient step-down voltage converter? I looked through Maxim's line and none of them seem to stand out.