how to generate a 0-120uA current (with a microcontroller) to adjust IC

PWM into a low-pass filter into a resistor.

Run a search on "addressable D/A converter" to find possible solutions.

A DAC and a resistor.

On a sunny day (Sat, 2 Nov 2013 16:23:39 -0700 (PDT)) it happened snipped-for-privacy@gmail.com wrote in :

PWM pin -> lowpass -> series resistor?

On a sunny day (Sun, 03 Nov 2013 06:52:13 GMT) it happened Jan Panteltje wrote in :

Actually if you do it that way may you as well drive the PWM ino a transitor with inductor and control the LED directly.

So, you'd want a sense resistor (I'd pick about 20k ohms) and sense the voltage across it (with 20 k ohms, 0-120 uA gets you a range of 0 to 2.4 volts).

I'm assuming the current will be terminated into some node that is held more than 2.4V plus a diode drop below an available (+) power supply. Take a matched pair of PNP transistors, with 20k ohm (also matched) emitter resistors. Pull those emitter resistors up to some (+) supply that's within the output range of an op amp. Wire the op amp to drive the base of both transistors, and balance when the collector current of the first (call it I_c1) and the micro controller's DAC output voltage (call it V_p) obey

V_p = R_x I_c1

(this is a VERY simple thing to arrange). Then, the second transistor will deliver, as its collector current, the 0 to 120 uA range of values when you set V_p *20k/R_x to values in the range of 0 to 2.4 volts.

There's lots of other ways to arrange a high-side current source, but this one is well behaved for low currents (where transistor self-heating can be ignored).