I'll be adding a constant-power DC-DC converter in the new version. Right now, I'm considering a MCP1624 switcher with an INA199 current-monitor amp for the feedback. More on that to come. Before sending out boards, I decided to simulate how a constant-power output would work powered from a capacitor, so I made a model in LTspice.
C1 is a 2.2F supercapacitor (final value may differ). With a DC-DC converter and current feedback, the LED will consume constant power - as the capacitor discharges, more current will be drawn to keep the LED current constant. To simulate this, I use B1, a voltage-dependent current source. The current is set to the LED power divided by the capacitor voltage and the converter efficiency. This will cause the capacitor current to increase as the voltage drops, just as it will with the converter. I drop the current to zero when the capacitor voltage reaches the minimum input voltage for the MCP1624, 0.35V. The capacitor size, LED power, and efficiency numbers are just guesses so far, but it's a nice way to play around with potential designs.
The waveforms look like this:
The capacitor voltage (cyan) starts at 2.7V and drops to 0.35V in 272 seconds. During that time, the capacitor current (yellow) goes from 10.7mA to 81mA, while the LED power (magenta) stays constant at 26mW. So, the LED is on with a constant brightness for four and a half minutes. That's not bad.
One thing that is not modeled is the converter efficiency change with input voltage. That depends very heavily on the converter itself, and I don't have a good model for that (yet?).
It's interesting to compare this with powering the circuit from a battery. With a battery, the internal resistance increases as the voltage drops, so the large currents required near the end wouldn't be possible. With the capacitor, the ESR remains constant, so it should be able to deliver this constant power down to 0.35V, when the boost converter stops.
I've chosen a white LED for the update to this project (when I made the original in 2001, I had never seen a white LED). I'll be using a Cree XPE-2 6500K led. I had a chance to test one of these LEDs on #Automated LED/Laser Diode Analysis and Modeling this morning, and here's what I found:
This LED is most efficient at around 15 mA, but you can achieve 90%+ of that between 4.5 and 40mA. I initially guessed I'd use 10mA. The analyzer measured a forward voltage of 2.63V at 10mA, which is where I got the 26mW power for the LTspice simulation above.
I'll have to play with this LED a little to see if this results in a good brightness.
I'm also thinking of a very efficient bright flashlight with an array of these LEDs driven near the efficiency peak. It will be very expensive due to the numerous LEDs required, but also run for a very long time compared to normal LED flashlights.