Hysterisys feedback PFM LED stepdown converter

I built this and it works well. Drives an ultrabright red LED from 2.0VDC, but could be modified for white.

More Info

Stephan: A few questions... How efficient is it? Since it has an inductor coil, can it do stepup and stepdown? What is the lowest Vin it can run at?

This circuit only steps the voltage down. If your LED voltage drop at 18mA is 1.8V then the circuit will go into regulation at 1.9V. The efficiency is somwhere around 76% because of the 1K resistor, but you can half the losses by going to a 2.2K resistor. The penalty is that you then you need more battery voltage to go into regulation; 2.0V in the example above. Use of a FET and a 4584 hex inverter really helps the efficiency but then you are looking at higher voltage operation around 6Volts, though that would be needed anyhow if white LED used.

So the circuit goes in to regulation at 0.1V above the Vf of the LED in use. Below that, it just runs wide open until the battery is supplying about 1V? Or am I reading the circuit wrong?

Yup.

Forgot to ask about max Vin.

Regulation SLOWLY starts to peter out above 3V as designed.
With a white LED and an altered 1.0K resistor, I would expect it to easily run up to 6 volts or so, but that's untested. I would suggest a low VgsON FET to greatly improve efficiency.