Mauricio Jancic wrote:

>   Does anyone have a good idea on how to make a digital controlled
> power supply (a good one), I've already see the example on National's
> LM317 datasheet, but I'm looking for something better.

  Something better?  Wouldn't be difficult!  For starters, that design
should use FETs rather than transistors.  But the potentially good trick
about it is that each FET controls one bit, and it can be in BCD rather
than binary (though with a PIC and the ability to perform binary <->
decimal conversion in software, that is trivial).

  The important questions:

 1} What do you want it to do and why?  What programmable tricks?

 2} Maximum and minimum voltage and current?

 3} What resolution?  Are you sure?

 4} What accuracy?

 5} How fast do you want it to change?

  Three approaches come to mind:

 a} Make the LM317 datasheet circuit work.

 b} Use a voltage reference feeding a DAC to derive the voltage you
want, buffer (and ratio multiply) this with a power buffer.

 c} Use an inverse DAC to divide the output and compare this with a
reference, controlling feedback to the pass transistors (which might be
an LM317 or similar) accordingly.
--
  Cheers,
        Paul B.