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UC1843A: uc1843a

Kenneth Aquino
Expert 1330 points
Part Number: UC1843A

Tool/software:

What is Ro for the error amplifier?   Could it be as high as 1k.  I understand that Ro is the open loop output resistance  and that the actual output impedance decrease weth frequency but i just want to know what Ro is.

Also do you have any idea how much it varies from part to part?

  • 0
    TI__Guru 73825 points

    Hi,

    Its COMP pin output resistance is about 4.5k ohm based on its open-short measurement, varying about +/- 20% from part to part.

  • 0 in reply to Hong Huang
    Expert 1330 points

      We believe that switching noise is coupling into the COMP pin and causing erratic PWM behavior.  We are able to stop this by simply putting an oscilloscope probe on the COMP pin.  The high error amp out impedance does not attenuate the high dV/dt switching noise. Evidentially the scope capacitance does.

    My questions are:

    1.  Have customers reached out to TI about this?

    2.  What is the maximum capacitance that the error amplifier can drive without oscillating?

  • 0 in reply to Kenneth Aquino
    TI__Guru 73825 points

    Hi,

    You are the first customer having this noise issue.

    There are no specs on this capacitance limit, but typically <= 1nF is ok to use.