The CAT3626 is a nice little I2C device from ON Semiconductor. It comes in a 14 pin TQFN package which is probably the most difficult thing i’ve ever soldered! Each pad is separated from it’s neighbour by about 100 micrometers! To make matters even more difficult there’s a ground pad right in the middle for sinking heat away. Really you should get some sort of breakout board or reflow it but that would be boring! Here’s a picture of the device before soldering:
…and here’s the finished product! The LED is 10mm diffuse with a common anode and a maximum current rating of 20mA per channel. As soon as i’d soldered the wires to the CAT3626 i glued it to the board with epoxy, threaded the wires through the holes and then glued those down with more epoxy. After that i realised i’d forgotten where pin 1 had gone so i had to scrape the glue off the top of the chip!
The wire coming through the hole in the middle of this picture is soldered to the CAT3626 and acts as a crude heat sink. In practice it seems to work well enough although i probably should have used copper. I pulled the ceramics from an old projector i had lying around. Not the neatest job but it works fine!
After I’d built that i tested it quickly using I2C tools and then wrote a kernel driver. The CAT3626 allows a maximum of 32mA per channel regulated in steps of 0.5mA. Unfortunately because my LED can take a maximum of 20mA per channel i can only generate ~64,000 colours. To make matters worse the light intensity is not in general a linear function of the input current so the achievable fade sequences are not as nice as those done with PWM but they still look pretty good. Here’s a video!
Update: The kernel driver for the CAT3626 is now available for free on github!