I recently built a single-LED TV remote that works ok, but would like to design a version with two LEDs in series to produce twice the IR power from the same current. And I want it to be constant current. The basic setup includes a 3.3V 8MHz Adruino Pro Mini with the power indicator LED and voltage regulator removed, with both the Arduino and the LED string powered directly from an 18650 Li-ion cell. The output would be typical NEC format 38KHz with a 30% duty cycle. The LEDs are TSAL6100, and the LED current is to be about 500mA.
The circuit I've come up with is shown below. I will need to order some parts for it, but before doing that I wanted to see if anyone sees anything that obviously won't work, or which could be done better.
Of course the woolly mammoth in the room is the voltage headroom as the battery discharges. The remote I built uses a TSAL6200, which I believe is the same LED but with a different lens. At 500mA, its Vf is 1.7V. So two of them would be 3.4V, and add another 0.1V for whatever, and in theory the circuit should still work down to 3.5V battery voltage.
But for constant current, I can't use the typical circuit of a source resistor that drops 0.7V in order to feed the NPN transistor base that provides negative feedback. That 0.7V headroom "waste" would make it impossible to use two LEDs. Instead, I put one of the LEDs in the source circuit, and depend on the idea that the Vf of the LED increases with current. And I also provide for, but may not need, a 0.2R resistor which will drop that extra 0.1V at 500mA.
I would tune the other four resistors so the NPN has no effect at 3.5V battery voltage, but at any higher voltage the constant current feedback would kick in to take the mosfet out of saturation. It doesn't have to stay at exactly 500mA, but hopefully pretty close. I have no idea what the other resistor values should be. And actually, I'm not sure that the feedback will work at all, and not oscillate, but seems like it ought to - it just needs to damp the increased current that would otherwise flow, on up to a 4.2V battery voltage.
It looks like the mosfet will easily handle 500mA with 1.7V GS voltage, which is half a volt above its maximum threshold voltage, and almost a full volt above typical threshold.
I would appreciate any comments and suggestions.