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BQ25798BKUPEVM: VSYS source query

Part Number: BQ25798BKUPEVM

Dear TI,

I hope that you are doing well. I have a query that when the charger input is removed, in that case, will VSYS be retained through battery or it will also become zero?

Can you guide one using multiple chargers with common VSYS as well?

I am looking forward for the response.

Regards,
Ahmed Ali

  • Hi Ahmed,

    The chargers with output of the converter and battery FET from BAT to SYS are called NVDC chargers.  The system load flows through the charger's dc/dc converter to the SYS output and the remaining power goes to charge the battery.  When the input power to the converter is removed, the battery FET allows current to flow from battery to SYS, so the battery provides the system load.

    When using multiple NVCD chargers in parallel, their BAT pins can be tied directly together but the SYS pins cannot be tied directly together.  The SYS pins need to be connected with (ideal) diode OR in order to prevent one charger with higher battery back feeding to a charger with lower battery.

    Regards,

  • Dear Jeff,
    I am trying to build the following system.

    All batteries are of the same specifications. Will it cause any problems? I will be setting the VSYS=8.4V.

  • Hi Ahmed,

    Yes.  When one charger's battery voltage is higher than the other, the higher charger's SYS will back drive the lower charger SYS and potentially provide unregulated current to the battery.  If you current is low, you can connect the SYS pins through Schottky diodes.  If your current is high, you can used ideal diode as explained at

    https://www.ti.com/power-management/power-switches/ideal-diodes-oring-controllers/products.html

    Regards,

    Jeff 

  • But the batteries are independent. Not in parallel. Would this still cause any problems?

  • Yes.  If the SYS voltage one charger is higher than the SYS voltage of another, the higher voltage SYS drive the lower voltage and pushes unregulated current into the lower battery.  The only time this will not occur is when the battery is below MINSYS voltage and the charger is linearly regulating the charge current through the BATFET.  

    Regards,

    Jeff