Re: Whats this op amp doing?


"Joerg" <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:0m7Wi.65796$YL5.35241@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Jon Slaughter wrote:
http://www.modtronix.com/products/prog/pickit2/pickit2%20data***.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...


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


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