Hi folks,

        I'd like to use an LM2576 adjustable "Simple Switcher" (National) as a
buck regulator to give me voltages from 1.2V to 55V, with an input of
60V. There is a design in the datasheet that does just this, using a 50K
POT in series with 1.2K fixed for the voltage feedback/control. The 50K
+ 1.2K is from the output voltage to ground, and the feedback "tap" is
at the junction of the 50K & 1.2K.

        For soft control of the voltage with a PIC, I'd like to use a digital
POT, but the most tolerant ones I've found so far have a max Vdd of 30V
(from AD).

        Are there any tricks to use a limited voltage digipot in such a
situation (float Vdd?). Alternatively, what other ways could I
"simulate" a digipot, e.g. maybe PWM a MOSFET with an RC filter? Or
jerry rig some sort of op-amp in between, with the digipot controlling
the opamp?

        For reference, the design is on page 18 of
http://cache.national.com/ds/LM/LM2576.pdf.

Thanks,
-Ishaan

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