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Whats this op amp doing?

Discussion in 'Electronic Design' started by Joerg, Oct 31, 2007.

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  1. Joerg

    Joerg Guest

    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.
  2. Joerg

    Joerg Guest

    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.
  3. 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?

  4. I imagine I coudl also probably replace all that with a DC-DC converter if I
    wanted or even an external supply.
  5. 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.

  6. DJ Delorie

    DJ Delorie Guest

    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?
  7. Joerg

    Joerg Guest

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