Vripple [permissible ripple] = 10% of 1.5V = 15mV

10% 0f 1.5V is 150mV not 15mV, probably just a typo :)

Kevin

On Fri, 26 May 2006, Philip Pemberton wrote:

> 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
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