Whats this op amp doing?

Unless this voltage must be under software-control. Then you'd have to at least make the external supply remote-controllable. I didn't read the whole app note so I don't know what that voltage rail is supposed to do.

(updated pdf on microchips site but I think its almost exactly the same)

I'm trying to understand what the opamp on page 29 is doing. It looks to be acting as a comparator but I can't make much sense of it. It seems to be controlling the power to the target device(i.e., simply turning on and off the mosfet). Not sure why they are using the op amp though but I guess it might have something ot do with the charge pump on the next page?

What I think is going is maybe that the op amp is comparing the voltage V_TGT and turning on and off the mosfet which feeds the inductor to create a charge pump and so on... the control being done by the pic itself. I can't see why though the op amp is necessary unless its needed to "drive" the mosfet for some reason?

Any ideas?

Thanks, Jon

I imagine I coudl also probably replace all that with a DC-DC converter if I wanted or even an external supply.

yeah, I was looking at the absolute page.

Well, I don't understand it all but I see the big picture. Maybe I'll try to figure out the details later. I think I'm probably going to just design my own since I can program a pic with the pc port anyways, I just need to make it more robust. Probably 90% of that circuit is just fixing up the power to the target pic and I'm sure there are several ways I can do that.

Thanks, Jon

The documentation does imply that the programmer can provide target power if needed, and has a dialog box to select what voltage. Maybe they're just using that as a programmable LDO?

Yep, that's its function. Be careful when connecting capacitors to the output. These things can easily start singing the blues.