Andy Kunz wrote:

>>If the whole circuit is operating from a single supply that *changes*
>> from 3.3V to 5.5V, you could just manipulate the Vadj pin on your
>> voltage regulator.  This works for both 317 type linear regs as well
>> as LM2575 type switchers.

> Both are coming from a 13-16V supply.  They can be two regulators, or
> one switched between two values, or a 5V supplying the 3V.  Doesn't
> matter.

  Fine.  So use the idea you just quoted!

  The LM317 ( http://www.national.com/pf/LM/LM117.html ) uses a fixed
resistor, nominally 240 ohms, between output and reference, while the
resistor from reference to ground determines the drop from reference to
ground.

  For 5V output, this would be 3.75V and the resistor 720 ohms. For
3.3V, the drop is 2.05V and the total resistance, 390 ohms.  However,
presuming your resistor for this setting is paralleled with the 720
ohms, you need 870 ohms.

  I'd try this:

             __________
   V+ o-----|In     Out|--+---------------o Vdd
            |  LM317   |  |
            |          |  <
            |   Ref    |  < 240
            |__________|  <
                 |        |   820
                 +--------+--VVVVV--------o PIC control
                 |                           Open for 5V
                 <                           Ground for 3.3V
                 < 750 (or 680)
                 <
                 |
   V- o----------+------------------------o Vss

--
  Cheers,
        Paul B.