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TPS7A7001 output is around 200 millivolt

kerb huang
Intellectual 690 points
Other Parts Discussed in Thread: TPS7A7001, TPS3831, TPS3890

Hello, 

when power on, TPS7A7001 output 5 volts, but after tens minutes, TPS7A7001 output only 270 millivolts.

Thank you,

Kerb

  • 0
    TI__Guru 55940 points

    Hi Kerb,

    Thank you for the schematic; however, to help debug your application, more information is needed.  A scope shot showing Vin, Vout, and Iout would be helpful.

    Please note that a common cause for Vout drooping as you describe is an unintentional current path caused by flux.  Please check for flux residue particularly around the feedback network.

    One thing to point out with regards to your schematic is that your resistor divider is not within the operating range listed in the datasheet:

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Intellectual 690 points
    Hi Ryan,

    we have changed R2 to 33k, but it still happened sometimes, TPS7A7001 output is around -0.4V.

    Thank you!
  • 0 in reply to kerb huang
    TI__Guru 55940 points
    Hi Kerb,

    Can you please provide a scopeshot with Vin, Vout, and Iout?

    Were you able to check your board for unintentional current paths such as flux or unintentional biases?

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Intellectual 690 points

    Hi  Ryan,

    we cann't measure the output current of TPS7A7001, the below is Vin/ Vout waveform. Ch2 is input, Ch1 is output.

    we find a phenomenon:  when there is no output, power down  and connect output to the ground with a wire,  then power up, TPS7A7001 output around 5V.

    why does this happen?

    Thank you!

  • 0 in reply to kerb huang
    Intellectual 690 points
    BTW before power up again, remove the wire.
  • 0 in reply to kerb huang
    TI__Guru 55940 points

    Hi Kerb,

    Thank you for the scope shot.  It looks like you are pulling the output of the LDO negative before it starts up.  As a result, the LDO is not able to properly startup.  This implies that you have a negative voltage in your application that is starting up before your TPS7A7001 and is biasing the output via a leakage path (likely in the load of the LDO.  Most likely all you need to do to fix this is to sequence the TPS7A7001 before the negative rail.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Intellectual 690 points
    Hi Ryan,

    Thank you for your support.
    Yes, TPS7A7001 is for the operational amplifier, we have a negative voltage(TPS7A3301) for the OP in our application.

    Regards,
    Kerb
  • 0 in reply to kerb huang
    TI__Guru 50090 points

    HI Kerb,

    The TPS7A3301's enable can be driven from a +5V. So if you either put an RC from VCC6 or give a fixed delay with something like the TPS3831 or TPS3890, you can ensure that the positive rail turns on before the negative.

    Regards,

  • 0 in reply to JCHK
    Intellectual 690 points
    Hi John,
    In our application, positive rail turns on before the negative.
    when power down, after seconds power up again, tps7a7001 output is negative.
    when power down, there is a negative voltage on the output of tps7a7001, then power up again, tps7a7001 output is negative(around -0.4V).

    When power down, how can we make the negative voltage on the output of tps7a7001 disappear as soon as possible?

    Regards,
    kerb
  • 0 in reply to kerb huang
    TI__Guru 55940 points

    Hi Kerb,

    There are a few application work around concepts that can help when an LDO is negatively biased.  It seems that the last two will be more useful for your application.

    • Enable the device before the negative regulator and disable the device after the negative regulator.
    • Use an external pulldown resistor.
    • Place a zener diode from IN to OUT to provide a small positive dc bias on the output when the input is supplied to the device.

    • Use a PFET to isolate the output of the device from the load causing the negative bias when the device is off.

    Very Respectfully,

    Ryan