First idea that comes to my mind is to use 2 digital pots in series.
Voltage would divide between them as long as they stayed close to equal
resistance...

$.02 - KF4HAZ - Lonnie

----- From: "Ishaan Dalal" <izx@

> 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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