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TPS25750: Startup concerns

Nicholas Carley
Genius 9895 points
Part Number: TPS25750
Other Parts Discussed in Thread: BQ25731,

We're using the TPS25750 as a DRP with the BQ25731. When used as a source the load has a large current spike at the beginning, then tapers down to the expected 60-100W. 

During the large current spike we see the VBUS and PP_HV voltage get pulled pretty low for ~4ms (from 20V to 15V or even 10V). This causes the circuit to turn off, I'm not sure if it's because of over current (we have the TPS25750 to allow a 200% over current) or under voltage. In any case is there a way to program the TPS25750 to automatically turn back on? Other USB-C supplies we've tested also drop out but then come right back on. Just wondering if there is a setting in the TPS25750 that allows this?

The USB-C device we are powering sometimes has some residual voltage on VBUS (~1-3V). This seems to prevent the TPS25750 from negotiating with the device (it does work once the voltage drops below 1V). Other supplies we've tested don't seem to have this limitation. Is there a way to set the TPS25750 to tolerate this condition?

Thanks,
Nick

  • 0
    TI__Mastermind 32611 points

    Nick,

    Is this transient happening over the type C cable or within your internal power structure?  If it is happening when the TPS25750 is a UFP (Sink) then the current limit should prevent the PP_HV voltage from dipping that low.  You should not set the current to 200%, this will allow a current spike of as much as 10A to pass through they TPS25750.

    The TPS25750 is designed to provide a slow ramp in the Sink case, so with a normal current limit, and the specified capacitances on PP_HV and VBUS, this should not be happening.  The only way that the TPS25750 will not turn back on is if the current is so high that it enters the hard_short state where it thinks that VBUS is shorted to another supply or ground and they system needs to enter into a robust protection state.

    Are you able to capture the VBUS and CC lines of the transaction that you are describing where VBUS has from 1-3V still on the bus.  This is actually a violation of the spec, but I can look into this as a possible feature enhancement in the future.

    Regards,

    Chuck

  • 0 in reply to Chuck Branch
    TI__Genius 9895 points

    This transient happens when the TPS25750 is the source (DFP).

    Let me check on the VBUS 1-3V issue.

  • 0 in reply to Nicholas Carley
    TI__Mastermind 32611 points

    Nicholas,

    If the transient is fast enough and the TPS2570 reacts as if this is a hard short, then it will not automatically turn back on until the cable is disconnected.

    This can be mitigated by adding additional capacitance on PPHV1 to stabilize the initial large power draw further.  Do you happen to know what the instantaneous current load is and when it is applied?