Roland,

Running  a linear regulator after the switcher will give you good
ripple rejection and still low dissipation.

Whether the switcher ripple is low enough for you depends on your
circuit (naturally enough :-)).
eg What is the supply noise rejection capability of the op amp used
(in data sheet) and will the ripple component therefore be excessive
in this application?)

As the switcher typically runs at 20KHz plus (100K or above in some
modern designs)  ripple filtering is fairly easy. Filters are usually
just LC low pass although a notch reject could be useful *IF* your
switcher produced a constant frequency - this is not usually the case
as with eg mark-space modulation the frequency components vary with
the regulator mark-space ratio.

Several companies provide FREE software to assist in designing
switching regulator ccts including main inductor(s) and filters.

National and Philips (and LT?) all do so I think.
Look on net 1st but if no luck I can probably source several of these
programs.

You could also consider running  a small low dropout regulator from
your switcher output just for the especially noise sensitive parts of
your cct - I take it that the 5 amps is mainly not for the strain
gauges :-). The eg LM2936 (5v TO92 50ma) has very low dropout voltage
(a few tenths of a volt).
The LM29xx series in general is quite reasonable for use in a noisy
input voltage environment (eg automotive) if this is a problem.



        Russell McMahon


-----Original Message-----
From: Roland Andrag <randrag@ICON.CO.ZA>
To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
Date: Sunday, May 16, 1999 4:43 AM
Subject: [OT] Switchers (was high power voltage regulator)


>Firstly, thanks to everyone who responded to my original enquiry -
the
>advice is greatly appreciated.
>
>I have given switching power supplies a close look, and they are
definitely
>a much less crude solution than linear with huge heatsinks etc. I am
just
>worried about the switching noise, which should not be a problem for
the PIC
>part of the application or the part that requires the high current,
but will
>probably be a problem for a strain gauge amplifier (instrumentation
>amplifier) I am using.  I have some questions to anyone who has
experience
>mixing low level analogue circuitry with switchers:
>
>1. Can I use a separate 78lxx type regulator supplied straight from
the main
>24 V supply to supply the analogue part, and a switcher for the
rest, or
>will the switcher induced line noise on the power line still make my
life a
>misery?
>2. If I use only the switcher, will I be able to get the supply
clean enough
>using RC low pass filters (bypass caps), or an LC notch filter, or
will my
>life become a still become a misery?
>3. Any other advice/suggestions?
>
>Thanks
>Roland
>