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TPS62873: Start up and shut down time different

Allen Wu
Prodigy 50 points
Part Number: TPS62873

Tool/software:

Hi all, I have tested TPS62873 start up and shut down time. I found out there is a huge different between these two. Can someone explain why this would happen?

                                

It takes 784us to start up the device. But it only takes 183us to shut down the devices.

Thank you

  • 0
    TI__Guru**** 150880 points

    Hi,

    They should not be the same.  What is your requirement for the timings?

    The start-up time is controlled by the soft start time circuitry.  This will be a fixed time.

    The shut down time is not controlled.  The device activates its output discharge, which interacts with the output load and output capacitance to determine the shut down timing.

    Chris

  • 0 in reply to Chris Glaser
    TI__Prodigy 50 points

    Hi Chris,

      Thank you for your reply, it great helps. This is already meet the requirement. I notice that there is a discharge description in the data sheet. However, it doesn't give too much detail. Do you mean that discharge time is also related to output cap and output load? Thank you for your reply again.

    Best,

    Allen

  • +1 in reply to Allen Wu
    TI__Guru**** 150880 points

    Hi Allen,

    Yes, the discharge time is normally very dependent on the output load and cap.  When the device is disabled, it stops switching and turns on the discharge.  So, Vout drops to 0V by removing the stored charge in the output caps.  I = C * dV/dt.  The I is the current being drawn out of the output caps, which is the sum of the discharge current and the load current.  The C is the total Cout.

    We did add the discharge spec to the D/S recently, which shows that the discharge behaves as a constant current discharge at higher Vouts and then transitions to a constant resistance discharge as Vout decreases.

    Chris

  • 0 in reply to Chris Glaser
    TI__Prodigy 50 points

    Hi Chris

      Thank you so much for your help!

    Best,

    Allen