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TPS7A49 Does Not Shutdown

Erran Sousa
Prodigy 20 points
Other Parts Discussed in Thread: TPS7A49, TPS736

I am using the TPS7A49 and cannot shut down the regulator even when the EN pin is directly connected to ground.

The same topology being used is the same as suggested in the data sheet along with the suggested values for all of the capacitors.

Operating Parameters:
Vin = 3.3-5.5V
Vout = 3.0V
Iout < 0.075mA

Component Values:

Vin = 3.3-5.5V
R1 = 124k
R2 = 80.6k
CIN = 10uF, X7R, 0603
COUT = 10uF, X7R, 0603
CNR/SS = 10nF, X7R, 0603
CFF = 10nF, X7R, 0603

Does anyone have any ideas?

Thanks in advance for your help.

  • 0
    TI__Guru 55940 points
    Hi Erran,

    Please keep in mind that TPS7A49 will not discharge the output capacitor. As you are using a rather light load, it may take some time before the output capacitor is discharged. Does the output rail come down after some time or does it always remain high? If the rail remains high, is there anything downstream from the LDO that could be biasing the output of the LDO?

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Prodigy 20 points

    Hi Ryan,

    Thanks for your prompt reply and wisdom.

    Yes, as it turns out, there was something down stream of the LDO biasing its output.  The TPS7A49 supplies an AD7714 Analog to Digital Converter and a voltage reference in the analog domain.  There is a TPS736 that supplies power to the digital domain.  Some of the logic inputs (/CS,/RESET,/SYNC) to the AD7714 are held high through pull-up resistors that are in the digital domain.  When the TPS7A49 was disabled the output was being biased by the pull-up resistors.

    To solve this issue, the pull-up resistors were removed.  On the microcontroller, the /CS,/RESET and /SYNC pins were configured to use the internal pull-up resistors.  Then, to shut down the TPS7A49, the /CS,/RESET and /SYNC pins were configured as analog inputs with no pull-ups.

    Now, the TPS7A49 shuts down properly.


    On another note, I mistakenly wrote that the current drain on the TPS7A49 was 0.075mA.  It is in fact, 0.5mA.

    Thanks again for asking the right question which lead to a solution.

    I very much appreciate your help.

    Best Regards,

    Erran