At 09:50 AM 99.07.22 -0300, you wrote:
>Hi,
>
>I am trying to design a PIC-programmable pwer supply. I'd like to select by
>software any voltage between 1.5V and 15V. PCB space is VERY small.
> ( ... stuff deleted ...)

I've done something similar - not exactly a programmable supply, but a PIC
controlled voltage regulator.

What I did was build a simple voltage regulator using an op-amp and a
series pass transistor. The reference voltage for this type of circuits is
normally taken from a Zener diode or a reference IC, and a pot is used to
sample to output voltage and feed it back to the op amp. (This is basically
how the 317 works, and you are talking about having the pic control the
resistor in the sampling circuit.)

What I did was have a D/A (MAX522) driven by the PIC (you could use a PWM
technique instead, I suppose) and used that to provide a variable reference
to the regulator.

Space: the D/A is an 8-pin dip, as is the op amp. The series pass
trasnistor is whatever you need to carry the load. A couple of resistors
and capacitors and away you go.

If you wish to pursue this idea, send me E-mail, and I'll e-mail you a
diagram.

Larry
Larry Bradley
Orleans (Ottawa), Ontario, CANADA