>
> - The Atmel AVR450 appnote is just plain wrong. The section on charge
> profiles and charging methods is OK (but check against the Sanyo and
> Panasonic charging specs before you go writing any code).


I've told other people this..

- Apparently the AVR450 drive circuit doesn't turn the MOSFET on fast
> enough, so it ends up cooking (there are a few people on the AVRFreaks
> forums
> saying this). The circuit is a 680R from gate to source, a BC847 (SMD
> BC547)
> pulling the gate down (E to GND, C to FET gate). The 847 gets its base
> drive
> from the MCU via a 1k series resistor, and a 10k pulldown on the base side
> of
> the 1k. Power goes into the source, drain is wired to the inductor/diode
> junction. If my description isn't good enough, google for "AVR450 pdf" and
> take a look at page 27.



You need a better fet driver, I used a driver chip that gave me something
like 2A on the gate. Micrel MIC4429BM


   - I was going to drive the gate of the FET directly from the PIC (and
> maybe use an N-channel instead of a P-channel FET), but I'm not sure if
> this
> would work. It certainly doesn't work in SPICE, but since when have SPICE
> simulations ever accurately modelled real life? :)


Trust me, it sucks.  At least it does when running at 500 kHz.


>      - If I pick a maximum Vout of 3V (i.e. two NiMH cells), I get L=75uH.
> If I want to optimise my charger so it charges one or two cells at a
> reasonable efficiency (70% or better), should I stick to the lower value,
> or
> pick one in the middle (if one is available)? Or maybe pick the higher
> value?
> Common sense suggests "pick the one in the middle" but I'm not sure how
> things are generally done (a lot of the stuff I've found is a bit hazy on
> this).


Most of your losses will end up being diode drops, or resistive I^2R losses.

Try to keep those down.
If I had it to do again, I would use a conventional switcher chip configured
as a constant current source, and then use the micro to switch between
current levels as needed, maybe with a Dpot, maybe with a D/A arraingement.
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