> Ah, but that's only part of the problem. The positive side 
> of the hysterisis
> also changes with input voltage.

Split R6 into two. Take a diode from it's midpoint (anode) 
to the reference diode (D11/D12). This clamps the midpoint 
at 1 diode drop above reference and "stabilises" the on 
drive through R6_left. Two diodes or lower R6_left = more 
drive.

BUT somewhere about here you say enough is enough and 
Replace Q2 with two transistors in a long tailed pair. The 
reference feeds Q2_left_base  and the sense signal drives 
Q2_right_base with any hysteresis added to (probably) 
Q2_right. This was done by ?Natsemi? long ago. Different 
than either 'my' GSR or your/Roman's approach. Proper 
comparator should be much more regulated.

*BUT* unfortunately, the thing that makes this circuit 
almost certainly unsuited to me need is that, as didn't 
occur to me in my initial enthusiasm, the "regulation" at 
present occurs on the input and not the output side. ie the 
ocillation occurs when Iin reaches a limit set by the 
voltage across R2. At that stage the inductor energy is 
dumped into the LEDs (and a series diode with a capacitor on 
the KLED side of it would make the LEDs' life more gentle) 
and the cycle then repeats. You COULD add LED current 
sensing to make the process more defined but for now it's a 
bit open loop. I need something with reasonably well defined 
LED drive current over the battery operating range and 
suspect that this would be hard to tame. Maybe not 
impossible though. My CD40106 circuit uses a $US0.08 (25000 
volume IC) and 3 transistors (2 are FET gate drivers, one is 
for LED current sensing) so is certainly dearer. Efficiency 
is excellent.

> Coincidentally, I'm looking at trying to design a 
> "constant power" circuit
> to discharge a capacitor (33mF, 75V) quickly . Somewhat 
> similar requirement.

Why "constant power"?
Would constant current do?


        Russell

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