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BQ25703A: Additional battery fet to minimize battery drain

Ulf Liljeqvist
Prodigy 30 points
Part Number: BQ25703A
Other Parts Discussed in Thread: BQ25703, BQ25792

Dear support,

I have used your reference design TIDA-01515 and the battery charging part of this design in our project.

The system is a mobile electronic sign which is powered via an external 12V Pb-battery (usually about 70Ah or more). The system has an onboard small backup Pb-battery, 12V, 0.8Ah. I have made a 3-cell configuration which also is the default in the reference design. Charging cycles and power is regulated by our external MCU. 

The bq25703A is used to get a regulated voltage to the system and also charge the back-up battery. About once a week, personnel change the primary battery and the system will then be powered from the internal back-up battery. This works as expected and everything is fine.

But, I have one major issue since I want to completely disconnect the internal back-up battery to minimize current drain during storage. I know that we can use sleep-functions in the bq25703 to minimize the quisence current. The thing is that the other system which is powered via the SYS-rail will still drain power from the back-up battery. I see no option to disable the pfet driven by "BATDRV"? I also assume that current will still flow to the SYS-rail via the body diode in the pfet in the reference design. 

My idea is to use an additional pfet (Q8 in the schematic below) to completely disconnect the battery from SYS-rail and bq25703. Please let me know if this is possible or if I need to make any changes to my proposed battery disconnect circuit. Any other considerations i need to make?

Kind regards,

Ulf

  • 0
    TI__Expert 4285 points

    Hi Ulf,

    Have you tried low power mode? Does it satisfy your application?

    Sure, disconnect the battery will achieve the best result.

    Best Regards,

    Linhong

  • 0 in reply to Linhong Qin
    Prodigy 30 points

    Hi Linhong,

    Thanks for your reply. We have tried low power mode and the bq25703a behaves as expected. But it unfortunately do not satisfy or application. The system powered from the SYS rail cannot go in deep sleep and we will drain about 500uA even if the bq25703a is in sleep mode. 

    I hope the solution that I proposed in the schematic will solve the issue and cut the current drain from the battery when "BATTERY SWITCH" is not driven high.

    I need to verify that my proposed solution will work or if I need to modify the schematic in any way to get it working as expected. 

    Best regards,

    Ulf

  • 0 in reply to Ulf Liljeqvist
    TI__Expert 4285 points

    Hi Ulf,

    I don't see problem on your design. I was trying to save BOM for you because theoretically IC should not drain 500uA in low power mode. There must be other components draining the power. 

    Best Regards,

    Linhong

  • 0 in reply to Linhong Qin
    Prodigy 30 points

    Hi again Linhong,

    Yes, the IC itself can go into sleep mode and only consume abut 20uA which is fine. But the system connected to the SYS rail cannot go into deep sleep and will still drain about 500uA from the battery. It is no problem with the bq25703a, it acts as expected but I need to find a good solution to cut the Battery completely to the external system in order not to drain the battery when not in operational mode. Usually storage.

    Kind regards,

    Ulf

  • 0 in reply to Ulf Liljeqvist
    TI__Expert 4285 points

    Hi Ulf,

    understood! then the best way is to cuf off the line as you proposed. 

    Have you looked BQ25792 or its family products. It provides similar function.

    Best Regards,

    Linhong