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Driving a Cree Q5 LED with one NiMH cell - anything much better than an LTC3490?
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floobydust:
At low input voltages the (input) ripple current is higher.
Don't forget a weak battery has high internal resistance, so the 10uF input cap you have is probably not big enough. Upsize it and see if that helps.
0xdeadbeef:
Well, I don't really care for a little ripple. Should I? Actually the capacitor is only 4.7µF but I don't see how that would improve the startup behavior.
I just don't get why the circuit behaves completely different with a bench supply and compared to a real cell and/or the LTSpice simulation.
With the exactly same circuit, the spice simulation starts at 0.9V. Also with a real Eneloop it starts at <1.2V. Just with the bench supply it doesn't.
I checked the voltage supply but it's spot on (down to 1mV). I also measured my wiring but it's below 0.06 Ohm. I'm cluesless.

Anyway, my best guess is that there is a contact resistance involved. Actually when I increase the serial resistance of the voltage supply in LTSpice to something like 0.3Ohm (from 0.025Ohm which should fit an Eneloop), things start to get ugly at 0.9V. I guess my measurements are flawed but it's kinda tricky to get a low ohmic connection to the internals of the lamp without soldering (which I tried to avoid). Probably I have to go that way to get more sensible measurements.

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