Russell's design challenge is almost exactly what I need the results of.
However I think mine may have to be isolated. I have a +/-15V power rail
going to some analog stuff, and the ground rail of the PIC needs to be at
the ground level of the analogue stuff. I would prefer to have the pic power
generated from the 30V total of the supply so I am equally loading both
sides, and hence the reason the output may need to be isolated to get the
grounding correct. The input control to the PIC is to be through an isolated
I2C link.

Now for the fun part, the analogue output is allowed to have a 10khz (with
harmonics) on it, in fact in this application it is desirable). The level of
this needs to be controllable however, so it probably should not be
considered in the converter design, but may signal an operating frequency
for the converter, without needing heaps of filtering to keep the noise off
analogue supply rails.

I had been thinking in terms of using a fixed pulse width converter with a
minimum drop linear regulator, but would like to keep the current down if I
can. Does anyone have any experience with doing things like this?

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