Dropped a 1000uF can across the inputs of the driver to rule out any harmonic/inductive issues with the PSU and wires (16AWG about 12 inches with an inline toggle switch), and the line voltage was super flat even at 100% draw so I reckon that was the problem with the previous test.
For some reason I was seeing about a 59KHz ringing everywhere though, which is weird as that’s an order of magnitude lower than the switching speed anywhere inside the light and a little over twice my PWM pulserate. Might have been noise coming off the PSU?
I didn’t measure LED currents, sticking to calculated values, so my efficiency numbers are technically useless, but I’m seeing upwards of 90% peaking around 96% at 50% duty cycle and dropping to 88% at the full 3A output. Not bad for an SC70 regulator and, admittedly, not a great inductor (29mOhm DCR). Note that the output includes a 25mOhm sampling resistor, so if we include the power loss from this part the total conversion efficiency is going to be a bit lower (at 3A, the resistor eats 2% of input power). I will be replacing the 25mOhm sampling resistor with a 16mOhm so I can use the same 50mV PWM signal on 2A and 3A builds, and this will also reduce resistor power loss by about 36% (or 0.7% of input power at max draw). I may not even bother continuing the 2A design since 3A costs basically the same and is more capable, but 3A will need temperature limiting.
At 3A, by the time the temperature reached steady state, the light was too hot to hang onto for more than five to eight seconds.
Thought of a different UI I want to try out sometime also. It might require a hardware change if the microcontroller isn’t responsive enough, but I want to test out ramping. My off-time for mode switching is set to about 0.5 seconds right now. I figure if I can tap on for 0.5 seconds, off for 0.5 seconds, and then hold on, the light would ramp from 0-100% and back in 5-10 second cycles. When the button is released, the micro would lock in that intensity for next time the light is turned on. I need to test if the battery sense circuit will drain fast enough to trigger an interupt causing the micro to write levels to EEPROM fairly reliably before the 2.5V LDO and output bypass cap drain low enough to turn it off. Else it’ll require a hardware change (at minimum, a bigger output cap on the LDO; possibly a shottky to prevent stray drainage) to make this work. Datasheet says max 5mS to do a full write operation.
Oh also I need to do a bit more research but I figure I’ll make the ISP pads dimensionally compatible with HQ’s pogo key. The PIC needs a minimum of five pins and, if I’m seeing right, that one’s got a generic header so it should be able to cable to any ISP device including my PICKit3.
June’s going to be a busy month at work so I may not have a lot of flashlight time inbetween regular maintenance, manufacturing and something like three other design projects, but I want to at least get a final hardware design on the 3A driver (including driver-level temp sensor), code in a thermal-throttling routine and see about getting my buck-boost concept tested. Maybe add a single-AAA light to the project, nothing fancy, maybe 300mA peak output. Also I need to buy a good thermal-conductive adhesive.