[sdiy] current sink idea

[Roman Sowa]

Since criticizing circuit ideas is a new trend, here's something I came 
up with 2 days ago.
Special needs require special solutions - I needed to control display's 
backlit intensity from internal DAC of the microcontroller, which 
unfortunately is unbuffered and rather high impedance. Obvious simplest 
solution with emitter follower works, but even high beta transistor 
loads the DAC too much. I could replace it with Darlington, but then 
there's even more of DAC range lost and possibly even worse 
repeatability between units.
Next step was well known current source/sink using opamp driving 
transistor with current sensing resistor. But that's 2 active parts and 
space is limited. So I made this (hoping ithis shows up in the email 
body or attachment):
Current sink schematics
I moved the sensing resistor to opamp's power input thus eliminating the 
need of external transistor. Quiescent current is 60uA so it doesn't 
affect performance. It works up to 23mA of LED current, which is all I 
needed. It can also be controlled by PWM with 1st order very-low pass RC 
filter, like 1M+100nF adding free fade in/out effects.

And here's how it looks:
assembled prototype

Roman