Olin cant post this personally so I'm forwarding the reply on his behalf (I
hope I've done it in the right format etc ;-)

Here's my response to the backup battery post:

Peter Mcalpine wrote:
> Can anyone recommend a switch mode regulator that will do boost
> and buck regulation?
>
> Basic usage is for a system that runs in a car, but requires
> backup battery. It also requires good 3.6V regulation.
> Current consumption at 3.6V is around 200ma max. backup battery
> is presently 3xAAA NiMh 650mA

Why do you need battery backup in a car, since the car battery is always
there?  Are you trying to ride out short glitches or voltage droops when the
starter kicks in, or does your system have to run when the battery is
disconnected for extended periods of time?  If so, how long?

> My idea is the battery can also be feed into the input of
> the switch mode so to maintain good regulation.

You might consider the battery backup and charging together.  Select a set
of batteries so that their maximum voltage never exceeds your desired 3.6V
out.  This can be achieved with 2 NiMH batteries in series instead of 3.
Make a pure boost converter that produces the regulated 3.6V from the
varying battery voltage.  Now add a buck current regulater that always tries
to maintain the trickle charge current going into the batteries
(not also into the boost regulator).  Due to feedback magic, this will
automatically supply the power to operate the unit and trickle charge the
batteries when 12V power is present.  When 12V power goes away, the
batteries automatically take over.

Conversely, you can make sure the lowest operating voltage of your battery
pack is always a bit more than 3.6V.  5 NiMH cells in series should do this.
You end up with about the same topology, except there is a buck instead of
boost regulator between the battery and the load.  The buck from the 12V to
the battery remains the same.


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