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TPS386596L33 RESETn Signal Problem

Tchaya
Prodigy 90 points
Other Parts Discussed in Thread: TPS386596

I'm using a TPS386596L33 to monitor 3.3V (Sense 1), 2.5V (Sense 2), 1.5V (Sense 3), and 1.2V (Sense 4) rails in my design. The datasheet shows that the RESETn signal should assert if any of Sense 2,3,and 4 inputs are below 400mV; however, my circuit does not seem to behave as described.

I can get RESETn to assert (~200mV) if I pull down the MRn input, but I can't get RESETn to assert if I pull any of the sense inputs below the threshold; I've gone as low as 300mV on Sense 2 - 4, but RESETn pin never goes below ~2.2V. I've attached my circuit.

Regards

  • 0
    TI__Expert 8905 points
    Hi,

    Does the reset get asserted if you ground one of the inputs?

    Regards,
    David
  • 0 in reply to David Hopkins
    Prodigy 90 points

    David

    I tried that, but got the same result. I also tried shorting the MRn input to GND and noticed that the behavior is the same as the Sense inputs.

    The RESETn output of the TPS386596 is driving the input of a a TI SN74LVC2G17DBVR Schmitt Trigger buffer (U600), which in turn drives an LED. The MRn input is also being driven by the output of another TI SN74LVC2G17DBVR Schmitt Trigger buffer (U6), whose input is driven by a microcontroller RSTOUT; the output of U6 also fans out as a system RESET signal to other ICs on my board.

    I've observed the following behavior:

    1. If I short the input of U600 to GND, I can get the LED to turn off, as expected
    2. If I short any of the TPS386596 Sense inputs to GND, the RESETn pin does not assert; I measure 2.2V
    3. If I short the output of U6 to GND (effectively shorting the TPS386596 MRn input to GND), the TPS386596 RESETn pin does not assert; I measure 2.2V. All other ICs on my board reset, as expected.
    4. If I short the input of U6 (uC RSTOUT) to GND, I observe the same behavior as #2 above
    5. If I issue a system reset from the uC, I can get the TPS386596 RESETn pin to assert. This is the only action that asserts RESETn

    It looks like my issue is related to the interface between the TPS386596 and the SN74LVC2G17DBVR; I've been poring over datasheets, but I can't find anything obvious.

     

     

  • 0 in reply to Tchaya
    TI__Expert 8905 points
    With the RESET/ output at 2.2V, the part is trying to assert itself but is unable to drive the output down to 0V. Other than that, it appears to be functioning correctly. If you increase your pull-up resistors to 100kOhm are you able to get it to function?
  • 0 in reply to David Hopkins
    Prodigy 90 points
    I increased the pull-up to 100K, but no luck. I also tried 200K, but that didn't work either. There is a plausible explanation for the MRn input not asserting reset when I tie it to GND; it has to do with the drive from the uC reset out pin. I'm still baffled why shorting the Sense pins doesn't assert RESETn; could it be the size of my voltage divider resistors and/or the 100nF filter caps at the Sense inputs? The datasheet recommends 1nF to 10nF filter caps.
  • 0 in reply to Tchaya
    TI__Expert 8905 points
    It could be your voltage dividers - you are sitting right above the threshold voltages.
    Could you increase the 10k resistors to 20k? Have you also measured your 3.3V rail to make sure that it is at 3.3V?
  • 0 in reply to David Hopkins
    Prodigy 90 points

    Thanks for all your responses; I found what the issue was. There was another output buffer in the design that was driving the RESETn signal and preventing it from going low. Issue resolved!