# Driving a high-power LED at very low levels *efficiently*

So, I have these garden lights powered by solar cells. I also happen to have a power-LED over from a broken flashlight, I think its a Cree R2 or Q5 or similar. These lights charge a 1.5V 600mah Nimh battery during the day, and then light up these rectangular low-power leds.

Now I would want to experiment with simply driving a high-power LED at similar, or slightly lower power levels, meaning a very very low current. I have not measured the drive current for these, but I imagine its well below 50ma. The theory being that it will be brighter still because of the excellent efficiency of power LEDs in this power band.

I know power leds generally require current limiting or control, and that the battery voltage is not enough so I will need some sort of boost circuit if not already present.

What I am wondering is what is the most efficient way of doing this, without being overly complicated? Is it possible to do this with just a resistor, or am I still prone to current run-away at these low levels?

• I think your 2nd to last paragraph needs revision. If (since) 1.5V is not enough to drive high-power white LED, you'll require a boost circuit. A buck converter lowers the voltage, a boost converter increases it. If you were using "buck circuit" to mean a generic DC-DC LED driver, you may want to clarify this. May 3, 2011 at 14:25
• @reemrevnivek: Out of curiosity, are there any good flyback-mode LED drivers that can operate independent of whether the supply is above or below the LED voltage? I have a project where the LED voltage is about 5 volts and the supply is about 4-15 (depending upon battery configuration and condition). At present, the LED driver shares the main supply switcher (which converts to 3.3 volts) so it always gets 3.3 volts, but it would be nicer if it could run directly off the raw supply. May 3, 2011 at 15:49
• @supercat - I've not used any personally, but I'm sure there are. Flyback is almost always a buck-boost configuration. See this National article, specifically the Flyback/Buck-Boost/Boost ICs heading, where they explain that a flyback buck-boost converter is essentially the same as a generic boost converter. Here's an example of a Linear Tech part for this purpose. But we're getting off topic. May 3, 2011 at 16:13
• @reemrevnivek: In some applications, a flyback and boost converter would be similar, but LED drivers I've seen use a low-side current sensing input which they would expect to measure current below the positive rail. The LT part looks interesting, though it requires more external components than some other drivers. As for topicality, the OP's goal seems to be to drive an LED using some type of switcher; having a switching topology which is agnostic to whether the input voltage is above or below the LED forward voltage would seem a useful option. May 3, 2011 at 18:03