I'm trying to decide how to drive ~100 UV leds. The Vf is 3.2-3.4 V typ, 3.8 V max. I have a 36.5 VDC power supply (measured @ 0.1A). I need If of 20 mA (no control needed, just on or off) through each LED. Simple and cheap are preferred, provided the results are "good enough". The application is PCB photolithography.
So far, I've looked into a few options:
- Resistors. 10 LEDs in a string, 175 ohm resistor. Cheap and easy, but slight variations on Vf would cause big variations in If. Bigger resistors and shorter strings perhaps? 470 ohm w/ 8 LED strings? Still only 10% regulation, not sure if that's OK for LEDs (I don't have much info on the specific LEDs I got, other than what I've listed above).
- Infineon BCR402UE6182. This is a discrete single-channel current regulator, but doesn't seem to guarantee a low enough If.
- MAX6971 (or 2xMAX6970). Only needs a resistor and clock source to drive up to 16 strings at 20 mA, but the lead time is 4-5 weeks.
- MAX6957. In stock at Digikey, but requires an MCU to program it and common-base transistors to isolate it from the high voltages.
- Redoing the power supply for 18VDC to accomodate some other solution.