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TPS24751: XOR two devices

Part Number: TPS24751
Other Parts Discussed in Thread: TPS65990,

I'm trying to 'or' two TPS24751 where if first one has a valid source voltage applied, the second one is shut down. I have the first one's PG# connected to EN pin of second device.  After I applied a valid source voltage to first device and an open source on second device, the second device slowly burned itself up, literally with a blackened board around its VCC pin. What is the correct way to 'OR' these two? [ FYI when the TPS65990 is no longer preliminary I intend to replace just the second device. My intent is to have one USB C source and one optional wall/car adapter to run/charge my device. The output to the right feeds 50uF of ceramic caps and a BQ257503 ]

  • Hi William Rector,

    Welcome to E2E!

    Do you want to prioritize one source over the other source?
    When you apply valid source voltage to the first device and an open source on second device - Have you observed any reverse current through the second device which is leading to failure. For ORing, we would need back-to-back FET configuration for reverse current blocking (like Figure 46 in the datasheet).

    The app note www.ti.com/.../slva811a.pdf may help you further.

    Best Regards,
    Rakesh
  • Thanks, I definitely need the reverse blocking and that should also protect the device as well when there's no input voltage to it.

    I'm wondering without the reverse blocking FET when the input is floating and having voltage at the output, that it's not happy about having the VCC pin floating. Maybe use two diodes to OR the two input sources to the VCC pins so that both TPS devices are powered when one of the input sources is missing.

  • Yes, correct William Rector.
    Diodes can be used, If you don't have concern with the power loss.

    Best Regards,
    Rakesh
  • Doing this for just the VCC has got to be a fairly insignificant loss since the operating Current of the device is small and these diodes are not in line with the main current passing through the FET.

    Thanks.