I have two groups, one that puts out 5.5V at 500mA, and one that puts
    out 5.3 at 500mA. My problem is this: since they are really close to
    5V, I can't use a 7805 on them. Would a SMPS be voltage stable by
    definition?

Yes, an SMPS would be voltage stable (over some range of output
current) by definition.  Are your numbers their rated output, or is
that what you're measuring?  I'm thinking that 5.5 and 5.3V are "odd"
votages, and perhaps you're just not loading the supplies sufficiently
to get into full regulation.  (SMPS require some relatively
significanr load before they become fully regulated.)

    Do I need try regulation, or would a regular diode or two to drop
    voltage be enough?

A single silicon diode (in series with V+ lead) should be enough.
You'll get .6V or so of voltage drop, bringing the outputs well within
the range that PICS can tolerate.  Since the input is regulated, the
output is also effectively regulated.


    Should I use a 5.1V zener instead?

No, you'd have to do some circuit design to use a zener, since it's a
"shunt" regulator (connected in parallel with the supply) and needs
current limitting as well.

BillW

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