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Leakage current Compare pins

Martin Yu
Intellectual 1300 points

Hi champions,

My customer met a leakage current at Compare pins question, wish to get your help:

Customer take above circuit to compare the feedback current and fixed value with Comparer, there is a RC filter at comparer pin. Now customer found that when the feedback current was 0, there was about 30mV voltage detected at MCU's comparer pin.

Why the 30mV voltage exist at pin? Does it caused by leakage current of comparer pin?(this pin configured as Analog pin) 

Thanks!

  • 0
    TI__Genius 15171 points
    Martin, I was not able to view the schematic. Also please provide the device number if not clear from the schematic once you correct that.
    regards,
    Joe
  • 0 in reply to Joe Prushing
    TI__Intellectual 1300 points

    Joe,

    thanks for reply! Sorry that miss the device #, it's F28035, please find the SCH as blew:

    Thanks!

  • 0 in reply to Martin Yu
    TI__Genius 15171 points

    Martin, sorry for the delay, for some reason I never received notification of your reply.  I wouldn't expect the 30mV to be from pin leakage since that should be less than 2uA per the DS which would at most create a 3mV IR across the 1k.

    Below I have added some annotation to make sure I understand the setup properly, please confirm that I have accurately shown where you measure 0V and 30mV and that the comparator pin in question is the positive COMP input.

    You'll also note I added some conceptual detail on the effective feedback path in the comparator block due to the Schmitt trigger hysteresis. If you look in the notes below the "Electrical Characteristics of the Comparator/DAC" table in the data sheet, you see this effective resistance is approximately 100-kohm.  I'm not sure if you are using the DAC for the negative input or bringing it to a pin but if you have configured the COMP in a way the output is high, you could be seeing a voltage on the input from the feedback path.

    If the above is true I would think you could configure the COMP out to low and the voltage should go away or at least be significantly smaller.

    Also I believe (pre the DS note) if you disable hysteresis (assuming you have it enabled to being with) it should break the feedback path and that should eliminate the voltage as well.

    I hope this helps,

    Joe