It's a buck converter, with input voltage range 5V to 40V. Its efficiency is above 80% for 1 or two LED, and above 90% for >3 LED's.

I was originally going to use an Arduino to generate the PWM control signals to do color fading, but it only had two high resolution PWM outputs, and I didn't feel like writing one in software. So I decided to use the mbed instead. It was a pretty good experience. I really enjoy the ease of use of the library.

Anyway, here's a quick video demonstrating my project: The LED in the video below is really three LED's in series per color, dropping around 10V each at a drive current of 350mA. That's 10W in a small package!

http://www.youtube.com/watch?v=1s1oACFbSwo

And here's my blog post describing my driver board (which in its final version will be in Arduino shield format). It'll have an onboard micro for generating PWM, and it'll be controlled by I2C. Of course, they will also be able to used standalone, without an Arduino or the onboard micro for least cost.

http://electronoblog.allanw.org/2010/10/triple-high-brightness-led-driver-arduino-shield/

Hope you guys like it!

Allan