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TPS65131: Issue with negative converter ouput

David Gash
Prodigy 30 points
Part Number: TPS65131


I am having an issue where the negative converter voltage VNEGis -20V instead of the -15V its supposed to.  I placed a decade box in place of R5 (see schematic below) but cannot get the voltage to lower, on top of that the voltage at pin 16 (FBN) is approx 0.4V instead of the 0V its supposed to be on the datasheet.  Any idea where i might be going wrong? The input voltage is 5V from a USB port.

Edit:

Here is voltage waveform I am seeing on the FBN pin:

  • 0
    TI__Guru 67130 points
    Hello David,

    I expect that you have a GND issue on your board. AGND needs to be connected to the VREF cap to keep noise from the switching part away from the feedback part. In addition, CN needs to be connected to AGND as well. Please check out the layout of the EVM to have a reference layout.
    In addition you could check if the behavior gets better by removing C14 for the moment to see if noise from the output is coupling into FBN.

    Best regards,
    Brigitte
  • 0 in reply to Brigitte
    Prodigy 30 points

    So i took C14 (the FF cap on the negative line) and my probelm seems resolved, Is my value to high and the impedance to low at the switching frequency?

  • 0 in reply to David Gash
    TI__Guru 67130 points
    Hello David,

    Ok, so it seems that you are getting noise injected in the feedback node because of the feedforward capacitor.
    We do not recommend to leave this cap away in the system because of stability reasons, this was just a test, if the problem is caused by noise injected through the cap.
    Is it possible that your feedback connection is closer to the diode than to the output capacitor?
    In addition, is your output capacitor an electrolytic one? If yes, you should add a ceramic cap in parallel to it to reduce ripple.
    Please check if the problem stays away, if you add the cap the way that it is closely connected to the output capacitor and the output capacitor is a low impedance one.

    Best regards,
    Brigitte
  • 0 in reply to Brigitte
    TI__Expert 8010 points
    Hello David,

    Do you have an update on Brigitte's comments?

    Thank you.
    Best Regards.
    Ilona