I need to drive a white/blue led at low constant current (about 10mA.)
To do that I usually use the schematic below, but in my case I can only supply the LED with 3V3.
Since the typical forward voltage of white leds @10mA ranges from 2.7V to 3V, the voltage drop across R2 could range from about 0.5V to 0.3V.
For tolerance reasons I'm afraid that with this variability I cannot guaranteed a repeatable luminous flux with different leds.
Can you suggest to me a different way to drive a led that can control the resistance?
simulate this circuit – Schematic created using CircuitLab
Best Answer
The following might work with the compensation cap and resistors set to zero (i.e., no C1, wires for R3 and R4), but only if you choose an op-amp that is designed to drive capacitive loads, and a FET that has a particularly low-capacitance gate.
In addition to gate capacitance, your FET also needs to be "logic level", and should probably be rated for a 3.3V gate voltage.
The component count is distressingly high, but you should be able to get away with cheap parts.
simulate this circuit – Schematic created using CircuitLab