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TPS25751: LDO_3V3/VIN_3V3 switch-over when VIN_3V3 is lost?

Vincent Politzer
Prodigy 60 points
Part Number: TPS25751
Other Parts Discussed in Thread: BQ25731,

Hello,

I am working on a system which has its host MCU power source downstream of the USB-C controller and Charger ICs.
ie. TPS25751 --> BQ25731 --> 3V3_SMPS --> HOST MCU

I found that with the dead battery configuration (AlwaysEnableSink), and the charger IC's PoR configuration, the host can receive power from the charger output even when USB-C is plugged in without the battery installed (thanks to CELL_BATPRESZ: VSYS_MIN & 256mA ChargeCurrent()).

However, there will be the following scenario: the TPS25751 is configured and operating normally, USB-PD is already negotiated and it is passing VBUS to the charger. The battery is done charging, so the host sends charge_inhibit to the charger. User unplugs the battery. Since charge_inhibit=TRUE, the charger is not switching. VSYS goes to 0V, so 3V3_SMPS goes 0V. Thus, TPS25751 VIN_3V3 goes to 0V. (Host MCU also, therefore, loses power.)

What happens in this case? Will TPS25751 reset? Is this undefined, and should I avoid this? If so, I could add a 3.3V supply off of PPHV and a power mux so the 3.3V system rail can be supplied by either VBUS(USB, output from TPS25751) or VSYS(battery on output of BQ25731).

Thanks,
Vincent

  • +1
    TI__Mastermind 31462 points

    Hi Vincent, 

    The TPS25751 is powered either from VIN3V3 or VBUS. See below from Section 8.3.2 Power Management in the TPS25751 datasheet: 

     

    Vincent Politzer said:
    What happens in this case? Will TPS25751 reset? Is this undefined, and should I avoid this? If so, I could add a 3.3V supply off of PPHV and a power mux so the 3.3V system rail can be supplied by either VBUS(USB, output from TPS25751) or VSYS(battery on output of BQ25731).

    In this case, the TPS25751 will reset and switch over the power input from VIN3V3 to VBUS and will assert the dead battery flag. When VIN3V3 becomes available again, the dead battery flag would need to be cleared. We do not recommend adding a 3.3V supply off of PPHV. 

    Best Regards, 

    Aya Khedr 

  • 0 in reply to Aya Khedr
    Prodigy 60 points

    OK, reset is a desirable behavior in our application, thanks. When it resets, will the sink path also momentarily switch off?

    Thanks,
    Vincent

  • 0 in reply to Vincent Politzer
    TI__Mastermind 31462 points

    Hi Vincent, 

    Yes, the sink path will switch off momentarily. 

    Best Regards, 

    Aya Khedr