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Original question:

BQ25756: The Reverse mode

BQ25756E: Reverse mode enable and reset behavior

Jim G
Intellectual 756 points

Part Number: BQ25756E
Other Parts Discussed in Thread: BQ25750

Tool/software:

I have a system using BQ25756E to implement battery backup in a gateway product.

Ideally, the BQ25756E automatically handles battery backup via its reverse operation mode by driving its buck-boost. I have some concerns about the reverse mode behavior in the event of AC adapter power loss and am looking for some clarification.

According to the datasheet I enable this mode by setting the EN_REV bit in the Power Path and Reverse Mode Control Register:

So for me this means as soon as I'm powered up I need to configure the BQ25756E and set this bit to enable backup power mode. 

However, the datasheet also states that this bit is reset by a reset command, the watchdog, or an adapter plug in event.

This related E2E post from 2 years ago elaborates more with   saying
"The reverse mode will indeed be reset when charging is enabled and will again have to be set once the charge is disabled."

So let's say my AC adapter is plugged in, the host is powered, and I am charging the battery pack.
Since I am charging, reverse mode is reset. If I lose power during this moment it seems like backup power mode will not function, correct?

Is there any way to enforce better persistent reverse backup behavior?

  • 0
    TI__Mastermind 39506 points

    Hello Jim,

    Thanks for working on this. It sounds like your system needs auto-reverse mode?

    The BQ25750 and BQ25856 ICs both have auto-reverse mode.

    The BQ25756E will not automatically go into reverse mode when the VAC voltage falls.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Intellectual 756 points

    Thanks Ethan, I'm looking at BQ25750 now.

    If I'm understanding correctly for BQ25750 compared to BQ25756E, we now have additional NMOS FETs which decouple both the AC input and the battery from the system voltage, and have their own drive pins.

    I'm trying to understand the actual reverse behavior, because it appears the BQ25750 can supply power to the system in one of two ways:

    - directly via the high side BATFET driven by BAT_DRV pin
    - via reverse mode using the buck-boost

    In datasheet 8.3.2 it says the system can directly connect the BATFET if there is no AC adapter sensed and if the Vsrn is in an acceptable range.

    I would actually prefer to disable this direct BATFET behavior as my downstream load is only rated to run at 12V and my pack voltage would exceed this.

    Can I achieve the behavior I want with BQ25750 by:

    1. Omitting the BAT FET entirely, setting the FORCE_BATFET_OFF bit. (If so, what should I do with the BAT_DRV and BAT_SRC pins? float or gnd?)

    2. Setting VSYS_REV to set the regulated voltage output

    3. Setting EN_REV and AUTO_REV bits to enable auto reverse mode

  • 0 in reply to Jim G
    TI__Mastermind 39506 points

    Hello Jim,

    Jim G said:
    1. Omitting the BAT FET entirely, setting the FORCE_BATFET_OFF bit. (If so, what should I do with the BAT_DRV and BAT_SRC pins? float or gnd?)

    This is the best solution. We have an FAQ for the pins without the BATFET.

    Jim G said:

    I'm trying to understand the actual reverse behavior, because it appears the BQ25750 can supply power to the system in one of two ways:

    - directly via the high side BATFET driven by BAT_DRV pin
    - via reverse mode using the buck-boost

    This is correct.

    We also have an FAQ for the power path switchover.

    Best Regards,
    Ethan Galloway