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UC1843B-SP: External error amplifier to override internal error amplifier

Stephen Savulak
Prodigy 50 points
Part Number: UC1843B-SP
Other Parts Discussed in Thread: UC1843-SP

I am using the UC1843-SP in an application where the error amplifier reference needs to be variable, so I am wondering what is the best way to override the internal error amplifier but also stay within the ratings of the VFB and COMP pins.  Any suggestions?

  • 0
    TI__Mastermind 26055 points

    Hey Stephen,

    Most of the time the error amplifier is overridden with an external amplifier by forcing the COMP pin of the device with an external EA.

    The VFB pin of the device can be connected to a stable voltage such that the internal EA doesn't interfere too much with the external EA.

    I understand that the COMP pin doesn't have an explicit voltage rating, but I would suggest using the 6.3 V of VFB as the rating here as they are connected to similar rails.

    Thanks,
    Daniel

  • 0 in reply to Daniel Hartung
    Prodigy 50 points

    Would it be preferable to drive the internal EA output low or high to minimize its interference with the external EA?

  • 0 in reply to Stephen Savulak
    TI__Mastermind 26055 points

    Hey Stephen,

    The output of the internal EA is what you would drive with the external EA, so no.

    Thanks,
    Daniel

  • 0 in reply to Daniel Hartung
    Prodigy 50 points

    Sorry, my question was badly written.  You suggested the VFB pin should be driven by a stable voltage so that the internal EA doesn't interfere with the external EA too much.  Since this will put the internal EA into open loop mode, a VFB above 2.5V will drive the internal EA output low and vice versa (VFB<2.5V, EA output goes high) - which state do you believe will interfere the least with the external EA's output trying to override the internal EA?  Sorry, it's hard to put that in words...

  • +1 in reply to Stephen Savulak
    TI__Mastermind 26055 points

    Hey Stehpen,

    I have seen people pull VFB high, low, and to the voltage reference.
    All of these seem like viable options.

    The change will simply be whether or not the EA is sinking or sourcing its current.

    Thanks,
    Daniel