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Subject: Re: Getting rid of that damn noise.
From: Andrew Jamieson (ajamiesn_at_mira.net)
Date: 2001-12-28


> Ok. I have finally seen the schematic. One comment - the MC34063 chip
(U15)
> on the i/f and power sheet is a switch-mode power supply chip from
Motorola
> (an old trashy chip in my opinion - there are much nicer ones than this
> part). It looks as if they are using it in a buck configuration (step
> down) - acting purely as a battery charger (6V?). The short between pins 7
&
> 8 mean there is no current limiting for the switcher.

Yeah. Hmmm ....

This is weird.

The 10k resistor (on the high side of T3 and the 34063 supply) seems to be
the current limiter for the battery charger. This seems a bit high, but the
energy supplied by the inductor may be doing something I'm not factoring in
(in fact there must be something I am missing, cos my calcs say the
batteries would take ~ 3111 hours to charge with a 9V source and a 10K feed
resistor!).

The comp input of the switcher seems to be connected up to a strange circuit
that connects to one of the A/D inputs of the CPU (AN1), and also PB5, and
then fed back to the comp circuit via a couple of resistors and a diode. So
what's it do?

Well, it seems that the state of PB5 determines if the switcher is on or
not. Set PB5 high, and the switcher will turn off (as comp will go high).
Set it low, and the switcher will turn on (assuming there is a drain of the
voltage left by setting PB5 high (PB5 won't pull it low because of the
diode) - this may be a big assumption, and the switcher could take awhile to
turn off after changing the state of PB5). Therefore, the battery charger
can be controlled by the CPU.

BTW: with no direct feedback from the switcher output to comp, there is
absolutely no voltage regulation; the switcher is being used more as a
current source than a voltage source.



Page was last modified "Jan 10 2012" The Rockbox Crew
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