You can use Pulse Width Modulation and suitable power FET devices and a single current sense resistor for each of the series strings, and then you are not going to need to design any analog control circuits. Just set the voltage for each string at a level that gives about 90% of max output at 100% on time. Then you can do it all in software.Just use a high enough PWM frequency to avoid visible flicker. AND heat-sink those series switch transistors. Probably the cheapest, easiest, most compact way of doing it.
by Jeff Child
by Duane Benson
by Jake Hertz