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TPS25750: Do we need back-to-back fets between VBUS and VSYS

Kiran Kerai
Prodigy 200 points
Part Number: TPS25750
Other Parts Discussed in Thread: TIDA-050047, , BQ25792

We have a design similar to TIDA-050047.

The TPS25750 is connected to BQ25792 and there is no other load for VSYS apart from BQ25792.

In this configuration I see no logical reason for the back-to-back fets.
BQ25792 can accept from -2 to 30V (absolute maximum) on input and TPS25750 -0.3 to 28V (absolute maximum) and USB-C can deliver 20V max.

We have no voltages sources on our design and will be a sink only therefore cannot generate higher voltages.

Is it acceptable to remove the back-to-back fets in our design?
What is the risk, if any.

  • 0
    TI__Mastermind 32891 points

    Kiran,

    The TPS25750 has extensive protection circuitry that assumes that the back to back fets are in place and controlled by the TPS25750, so removing them is not possible.

  • 0 in reply to Chuck Branch
    Prodigy 200 points

    Can you list the protection and relate to sink only application that connects to bq25792 and has no sys load on bq25792 charger.

  • +1 in reply to Kiran Kerai
    TI__Mastermind 32891 points

    Kiran,

    Per the type C spec, the VBUS can only have 10uF of exposed capacitance, so at least 1 blocking fet is necessary to meet the specification.

    With our 2 fet solution, we provide OVP and RCP protection to the sink path.

    Regards,

    Chuck

  • 0 in reply to Chuck Branch
    Prodigy 200 points

    Which fet provides blocking fet for exposed capacitance?

    With regards to OVP, the TPS25750 is rated to handle 28V on vbus, this is lower than downstream BQ25792 which is rated for 30V therefore this protection is not benefitial. The TPS25750 will be damaged before BQ25792.

    For RCP, this implies negative voltage from usb-c adapter. If this is the case, the TPS25750 Vbus input can only withstand -0.3V and will be damaged. Whether the BQ25792 is damaged or not makes no difference, the sink application will be non-functional.