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BQ25895: strange behaviour of BQ25895's Vbat output

ju zhang56
Prodigy 130 points
Part Number: BQ25895

We use BQ25895, BQ29700 and STM8 to make our power management circuit.

The scenario is, when the battery's voltage is lower than 3V, the BQ29700 is turnoff and STM8 is poweroff too. 

With conditions like above, what we need is when plug charge into vbus, the VBAT pin of  BQ25895 will be raised up to 3V and the BQ29700 will be turn on.

But our tests on different boards got different bihaviours. some of them can turnon BQ29700, and the others cannot. Vbat will be 1.6V or 3~4.5v on different board.

What's the matter, and howto solve it?

  • 0
    TI__Guru 57551 points

    Hi,

    1. When the battery voltage is lower than 3V, is there any built-in or external battery protection circuitry disconnect the battery from VBAT pin of BQ25895?

    2. How much load is connected SYS pin?

    Thanks,

    Ning.

  • 0 in reply to Ning T
    Prodigy 130 points

    1. YES. bq29700 will be shutoff to protect the BAT.

    2. Wether or not. when charger pluged-in VSYS will raise to 4.4V, but some of bq25895's VBAT pin can wake up bq29700, some others can not. those CANNOTs' VBAT pin has 1.6V output.  

  • 0 in reply to ju zhang56
    TI__Guru 57551 points

    Hi,

    Looping BQ29700 support team.

    Thanks,

    Ning.

  • 0 in reply to Ning T
    TI__Expert 7455 points

    Hi,

    The BQ297000 has a under voltage protection of 2.8V. This means that the device will enter fault mode, and shut off, when the battery voltage is lower than 2.8V (for longer than a given amount of time). The device will not recover until the battery voltage exceeds the under voltage cutoff (Vuvp) plus a hysteresis (Hys). For this device, that value would be approximately 2.9V. For more details, see the figure below and refer to section 9.4.3. of the datasheet.