OK folks, I've spent the past week meddling with my battery charger design,
and I've come to the following conclusions:

  - 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).

  - 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.
    - Assuming it is slow switching that's cooking the FET, how could I speed
up the switching circuit? Some of the messages on AVRFreaks suggest an LM2725
high side driver, but they're obsolete and the replacement (LM27222) is
"just a bit" expensive (plus it's designed for push-pull MOSFET drivers, not
single-ended).
    - 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? :)

  - Microchip AN793 seems to be pretty accurate. The formulae on
<http://www.smps.us/> (buck converter section) seem to agree with the
appnote, and a lot of the other stuff I've looked at says the same thing.

  - I've come up with an inductor value of 68uH for this scenario:
    Vin = 6V, Vout = 1.5V, Iout(max) = 1.5A, Iout(min) = 10uA, Fosc=100kHz,
    Vripple [permissible ripple] = 10% of 1.5V = 15mV
      - Does this seem about right?      

      - 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).

Thanks.
-- 
Phil.                         | Kitsune: Acorn RiscPC SA202 64M+6G VF+UniPod
philpem@dsl.pipex.com         | Cheetah: Athlon64 3200+ A8VDeluxeV2 1G+180G
http://www.philpem.me.uk/     | Tiger: Toshiba SatPro4600 Celeron700 256M+40G
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