J
Joerg
- Jan 1, 1970
- 0
Jon said: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.