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BQ25798: Reverse Boost Lithium-Ion Charger Selection

Dan Rechichi
Prodigy 70 points
Part Number: BQ25798

Tool/software:

Hi,

Will BQ25798 work with an architecture like the one shown in the image below but with PMID and SYS disconnected?

Or is there a better option?

We're looking for a 2A Lithium-Ion battery charger that can do the following.

  • When the main input power is connected, the charger IC charges the battery from the 5V output of the buck converter.
  • When the main input power is removed, the charger IC powers the 5V rail from the battery (reverse boost, probably has other names).
  • (wish list, but optional) There is an always on output that can power an external RTC regardless of whether or not the battery is charging or discharging.

I know I've used a TI USB OTG in the past to do exactly this in the past, but I remember it needing a HDI layout. This project can't afford the added lead time for HDI.

Otherwise, I've only found chargers that convert the input or battery voltages to a third separate output, or chargers that require intervention from the MCU to change over from charge to discharge which in out case won't work because the MCY won’t have any warning that the main input power is going to be removed.

Battery Charger Architecture

  • 0
    TI__Guru**** 166626 points

    Hi Dan,

    BQ25798 has backup mode.  As shown below, your 5V system must connected to PMID pin.  The 5V connected VBUS passes through to PMID to the system.  When 5V is removed, the converter automatically and quickly turns on reverse/OTG mode to provide 5V.  Backup mode must be first configured by the MCU and then rearmed after it triggers.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 70 points

    Thanks Jeff,

    Does this mean we connect the battery to the BAT pin, and the 5V system (load and main power) to VBUS, with the RTC on PMID?
    e.g. does BQ25798 automatically swich from charging from VBUS to BAT to discharging from BAT to VBUS while PMID is always powered?

    Or does it mean we connect the battery to the BAT pin, and the 5V system to the PMID pin, leaving VBUS unused?
    e.g. VBUS loses power when the main supply is removed, but PMID stays live.

    Kind regards,

    Dan

  • 0 in reply to Dan Rechichi
    Prodigy 70 points

    I think this part of the datasheet supports my first statement. e.g. Battery on BAT, the system and main supply on VBUS, and the RTC on PMID. Can you please confirm?

    Although, I haven't found anything that says the BQ25798 will automatically switch from charging to discharging on VBUS without host intervention. Only that discharging to PMID is automatic, as long as we rearm it as you pointed out.

  • 0 in reply to Dan Rechichi
    TI__Guru**** 166626 points

    Hi Dan,

    All chargers have a form of reverse blocking to prevent battery voltage from appearing on VBUS when input power is removed.  Battery is automatically connected to SYS when VBUS is removed.  The only way to ensure 5V on PMID is to use backup mode or add an external boost converter.

    Regards,

    Jeff