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BQ24296: Switch charger on and off

Guigui
Prodigy 10 points
Part Number: BQ24296

Hello,

I am working on a device that uses a BQ24296 charger to charge its internal Li-ion battery. The device draws current from a 24V car battery (through a step-down converter that feeds 5V into the charger's VBUS).

To protect the car battery we want the device to stop drawing current when its voltage goes below 22V. The device must work on its internal battery in that case. It can start using the car battery again only when its voltage goes above 24V.

The device has a microcontroller connected to the charger and able to measure the car battery's voltage. Unfortunately the step-down converter is always (hard-wired) enabled and we cannot modify the PCB. Is there a way to force the BQ24296 to power the device from its battery and not draw current from VBUS (even though there is 5V on VBUS)? It seems we can set the charger into HiZ mode but I'm not sure this is the right way to do what we want.

Thanks!

  • +1
    TI__Mastermind 22295 points

    Hi Guigui,

    If you want to force the charger to provide battery power, you can use I2C from the host controller to put REGN LDO into HiZ mode through REG00[7].

    REG00[7]

    The default value for this bit is 0b so if the BQ24296 resets, the REGN LDO will be turned on according to the 3 requirements listed in Section 9.3.1.3.1 of the datasheet. If your car battery is still below 22V, you would need to rewrite REG00[7] to 1b after the reset event.

    Regards,

    James

  • 0 in reply to James Steenbock
    Prodigy 10 points

    Hi James,

    Thank you for your explanation.

    However I just realised this will not work. The host controller is powered by the charger SYS, so when HiZ is enabled and the internal battery is eventually depleted the host will never be able to disable HiZ again to recharge the battery and the whole device will be stuck.

  • 0 in reply to Guigui
    TI__Mastermind 22295 points

    Hi Guigui,

    Glad I could answer your question!

    You would need external power for the host to maintain control of the charger at all times according to your setup, unless your buck is capable of supplying both the charger and the host.

    Regards,

    James