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TPS2116: Secondary power not switching in

Ismail Hawk-Bey
Prodigy 10 points
Part Number: TPS2116
Other Parts Discussed in Thread: TPS2121

I am currently trying to perform simulations using the TPS2116DRLR. The primary voltage is 3.3V and the secondary is 3V. I currently have the device set up in primary switchover mode where the device where the primary voltage will be the output until the primary voltage drops below PR1 threshold. In my current setup I have the PR1 threshold set up at 2.9 because the Output can not dip below 2.55V no matter what in this design. I know that when the primary drops out it takes a few microseconds for the secondary voltage to switch in.  During that time, the voltage output will dip. I was hoping to prove in simulation that choosing a big enough output cap and a high enough PR1 threshold that I could minimalize that dip so that the voltage stays above 2.55V but I am not able to simulate the cross over in the simulation once the primary voltage drops out the output  drops out and doesn’t switch over to the secondary voltage. Even if I don’t connect the primary voltage it never works off the secondary voltage please see graph below and set up below.

  • 0
    TI__Mastermind 20260 points

    Hi Ismail,

    Welcome to e2e!

    Thanks for bringing this to my attention - I will need to look into the model to see why it is inaccurate.

    Are you planning on having 1A typical load current? On silicon, using a 50-100uF cap on the output should keep it above 2.55V. I can quickly verify this tomorrow, but in the meantime please see the below section and equation from the TPS2121 data sheet which can be applied here.

    Thanks,

    Patrick

  • 0 in reply to Patrick Shalton
    TI__Mastermind 20260 points

    Hi Ismail,

    100uF of output capacitance should be enough to keep VOUT above 2.55V. But, given that it is application-dependent, please place another pad or otherwise ensure that you can put up to 200uF on the output.

    Also, depending on the load transient capability of your inputs, you may need a significant amount of input capacitance as well to make sure there is not too much droop upon switchover.

    Thanks,

    Patrick