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TPS25751: PP_5V Power Rail Generation

Brian Sampoh
Prodigy 50 points
Part Number: TPS25751
Other Parts Discussed in Thread: BQ25798,

Tool/software:

Hello,

Context

I'm using the TPS255751D USB PD controller in a battery powered DRP USB PD device in tandem with the BQ25798 battery charger.
In the design process I realized that the dedicated `PP_5V` rail requires a dedicated buck converter to boost the 1S Li-Ion battery voltage from ~3.7 V to 5 V.
I understand that the TPS25751 was not designed to operate exclusively in a battery-only environment.
So there are cases where a dedicated buck converter or LDO is necessary.

But in my case, where there is a battery charger like the BQ25798 that already integrates a buck booster, I find the introduction of a new buck conversion stage quite redundant and expensive (PCB space and money). 


Questions

  1. Does the TPS25751 support the temporary configuration of the BQ25798 to make it source 5 V/3 A on its `VBUS` pin to feed `PPHV` which the TPS25751 would internally route to the `PP_5V` path?
    Is this possible in autonomous mode?
  2. If not, can I implement this logic manually, which would mean that the `PP_5V` would float after successful PDO negotiation and the `PPHV` path is used to sink or source `VBUS` of the BQ25798?

    This means if the USB connectors would be powered by `PP_5V` it would also be powerless for a very brief moment (the time it takes to disconnect  `PP_5V` and reprogram the BQ25798 based on the negotiated PDO).
    Would this small interruption be acceptable in terms of USB PD compliance?
  3. Can you suggest a solution tat allows to completely omit the dedicated 5 V power rail or the dedicated 5 V buck conversion stage?

Kind regards,
Brian

  • +1
    TI__Mastermind 34256 points

    Hi Brian, 

    Brian Sampoh said:
    Does the TPS25751 support the temporary configuration of the BQ25798 to make it source 5 V/3 A on its `VBUS` pin to feed `PPHV` which the TPS25751 would internally route to the `PP_5V` path?
    Is this possible in autonomous mode?

    PP_5V and PPHV are independent from one another, PPHV does not internally route to PP5V. 

    Brian Sampoh said:
    If not, can I implement this logic manually, which would mean that the `PP_5V` would float after successful PDO negotiation and the `PPHV` path is used to sink or source `VBUS` of the BQ25798?

    PP_5V requires a constant 5V rail at all times when sourcing even after a PD contract has been negotiated. 

    Brian Sampoh said:
    1. Can you suggest a solution tat allows to completely omit the dedicated 5 V power rail or the dedicated 5 V buck conversion stage?

    the 5V power rail is required for TPS25751, this is used internally to power the CCx logic to do PD negotiation, messages, and Vconn if applicable. 

    To save BOM cost, you can use a 5V LDO instead of a 5V buck. For the LDO, we recommend around 300mA -400mA capability. 

    Let us know if you have any further questions or concerns! 

    Thanks and Regards,
    Raymond Lin

  • 0 in reply to Raymond Lin
    Prodigy 50 points

    Thanks for your great reply. I'm afraid an LDO won't solve the problem as I need to step up voltage from ~3.5-4.2 V of a 1S battery configuration to 5 V. Also the LDO would be less efficient for sourcing the 3 A that PP_5V expects (ok, I think the minimum was 300 mA).

    That's a bit of an annoyance that the USB specs kind of clash with a 3.3 V system. I mean staying backwards compatible with 1996 USB 1.0 TTL tech is paradox when your goal is to develop moder or latest tech that will never host or serve museum tech.
    Buck boosting is just so bulky. So I was hoping for a solution that allows me to avoid that.

    Thank you for your help and providing all the information.

    Kind regards,
    Brian