Whats this op amp doing?

[Joerg]

http://www.modtronix.com/products/prog/pickit2/pickit2 datasheet.pdf

(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?

Do you mean page 25 in the real document? There ain't no page 29...

Anyhow, if yes: U2 controls the gate of Q1 so the voltage at the drain
of Q1 equals twice the voltage at its pin 3 (on account of the divider
R5/R6). Looks like the PIC spits out a PWM signal at CCP1 where the duty
cycle is proportional to the voltage. R4 and C8 smooth that out to a
nice DC level which then becomes the reference for the voltage regulator
that U2/Q1 comprise. It's a low-dropout (LDO) regulator scheme, not
exactly my favorite. This is also why pos/neg inputs appear reversed.
They have to because a rising output voltage at the opamp actually
reduces +V-TGT which probably is some kind of desired target voltage.

Hope this helps.

 

[Joerg]

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

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.

 

[Jon Slaughter]

http://www.modtronix.com/products/prog/pickit2/pickit2 datasheet.pdf

(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

 

[Jon Slaughter]

http://www.modtronix.com/products/prog/pickit2/pickit2 datasheet.pdf

(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.

 

[Jon Slaughter]

Do you mean page 25 in the real document? There ain't no page 29...

yeah, I was looking at the absolute page.

Anyhow, if yes: U2 controls the gate of Q1 so the voltage at the drain of
Q1 equals twice the voltage at its pin 3 (on account of the divider
R5/R6). Looks like the PIC spits out a PWM signal at CCP1 where the duty
cycle is proportional to the voltage. R4 and C8 smooth that out to a nice
DC level which then becomes the reference for the voltage regulator that
U2/Q1 comprise. It's a low-dropout (LDO) regulator scheme, not exactly my
favorite. This is also why pos/neg inputs appear reversed. They have to
because a rising output voltage at the opamp actually reduces +V-TGT which
probably is some kind of desired target voltage.

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

 

[DJ Delorie]

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?

 

[Joerg]

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.