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TPS3840: Monitoring 12V supply

Alan Hoehn
Intellectual 460 points
Part Number: TPS3840

I am using the TPS3840DL49DBVR to monitor a 12V power supply for power down.  I am using similar circuit to one on Figure 52.  The difference being I have a larger resistor divider and no CT cap for the smallest reset delay.  The circuit I have does not work as intended as the voltage at the resistor divider is 4.8V instead of 5.5V and the 4.8V volts in trips the reset at 0V all the time.  What is it I am overlooking?

  • 0
    TI__Mastermind 27495 points

    Hi Alan, 

    The circuit you have shown should be working.  Can you provide a scope photo of "VDD" that is going the TPS3840?  Also, can you provide a scope shot of the output (12V_PWR_DN_N)?

    Just for sanity check, is it possible for you to remove R35 and apply a voltage between R35 and R33?  This test will provide us some information whether the part is functional or not.

    Ben

  • 0 in reply to Ben C
    TI__Mastermind 27495 points

    Hi Alan,

    One more thing, can you reduce the resistor values of R33 and R35 by half for each of them and repeat your test?  Thanks!

    Ben  

  • 0 in reply to Ben C
    TI__Mastermind 27495 points

    Hi Alan,

    Just wondering if you have any updates?

    Ben

  • 0 in reply to Ben C
    Intellectual 460 points

    Hi Ben,

    Yes the part on the BOM is the TPS3840PL48DBVR.  I am getting in the ‎TPS3840DL46DBVR to hopefully get this going.  I'll take it the "PL" variants are not desired to use with the voltage divider?  There seems to always be a 0.7V drop on VDD even with lower value resistors or could also be a combination of the pull up resistor on the RESET_N line out.  I stopped further testing once I realized the wrong part was on the board and verified my output was never going above 3.3V to save the 3.3V MCU.  I'll let you know how things work once I get the new part in. ‎

  • 0 in reply to Alan Hoehn
    TI__Mastermind 27495 points

    Hi Alan, 

    Thanks for the update.  Yes, you dont want to have a voltage divider for the -PL variant.  Also, you would not need a pull up resistor on /RESET for the -PL variant because it has an internal push-pull device that will output a "H" or "L" depending if the VDD voltage is above or below the threshold voltage.

    Yes, please keep me posted with your new results.

    Ben

  • +1 in reply to Ben C
    Intellectual 460 points

    I put the new part in and everything now works as expected.  Thanks!

  • 0 in reply to Alan Hoehn
    TI__Mastermind 27495 points

    Hi Alan,

    Awesome!  Glad that everything is working as expected.  :-)

    If there are no other questions, please close the thread by clicking on "resolved".  Good luck to you!

    Ben