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TPS782: PMOS Reverse Bias Long Recovery Time

Tianyu Lin
Prodigy 40 points
Part Number: TPS782

Hi,

In the design I am working on the input side of my TPS78233 can be switch between two different power sources.

The switch between two power sources can cause a 230us "gap" in between the 1st source off to the 2nd source reach operating voltage (3.6V).

The lowest voltage on the input pin during the "gap" is 2.1V. But the output pin have 3V at this point. 

After the power source received. On oscilloscope the TPS78233 seems like turned off for 4ms output drooped all the way to 2.8V. And it take another 3ms to get back to 3.3V

The Light-blue is input. The Pink is output. Dark-blue is switch power sources trigger.

So what is causing the TPS78233 to turn off?

What is the reverse bias recovery time for the PMOS in TPS78233?

  • 0
    TI__Guru 55940 points
    Hi Tianyu,

    Please note that when the Vin to Vout headroom is not greater than Vout + Vdo, the load of the LDO will discharge the output capacitors causing Vout to drop. TPS782 also has an active pulldown on Vout that could also be pulling down the output voltage if the LDO is disabled.

    In order to help debug, could you provide a scope shot of both input supplies, Vout, and Ven? Please set the timescale such that we can see what is occurring with all four rails during both the Vin droop and the Vout droop.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Prodigy 40 points

    Hi Ryan,

    Thank you for your reply. 

    In the schematic U8 and U12 is the switches to switch between power supplies.

    In the scope shot yellow is TPS_EN on U12. Light blue is VOK on U8.

    Purple is Input and Ven. Dark blue is Vout.

    As you can see the purple line ( Vin )recovered in 240us.

    But the Dark blue( Vout) keep dropping voltage. It dropping for 3ms to 2.8V.

    Then takes another 3ms to recover to 3.3V.

  • 0 in reply to Tianyu Lin
    TI__Guru 55940 points
    Hi Tianyu,

    The initial drop is caused by the LDO going into dropout; however, as your input voltage continues to drop it goes below minimum Vin (2.2 V). When this occurs the active pulldown of TPS782 is no longerfully operating and may engage partially. Due to the large amount of output capacitance in your application, it is reasonable for it to take some time to recharge the output capacitors.

    Very Respectfully,
    Ryan