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BQ25708: CELL_BATPRES / Operation with no battery

Julius Schmidt
Prodigy 30 points
Part Number: BQ25708

The datasheet states that the voltage at CELL_BATPRES is sensed on power-up to determine SYSOVP / various other settings.

However, if I pull CELL_BATPRES low when the battery is removed, how would the device start with no battery?

  • 0
    TI__Expert 5330 points

    Hello Julius,

    BQ25708 will power off of VBUS if an adapter is present, even if no battery is attached.  It will regulate VSYS to VSYS_MIN as set in register 0x3E.

    However, BQ25708 is not meant to be powered up (Power-on Reset) with CELL_BATPRES driven to GND (no battery.)  This is meant to be used to signal hot-removal of the battery but not for booting the device with no battery attached.  The reason is because, when CELL_BATPRES is GND at boot, the default settings of SYSOVP, etc. are set to 1S values.  So if you have any other battery combination, then these settings will be incorrect.  In particular, this is an issue with SYSOVP, which cannot be changed via I2C register write, but is only set from CELL_BATPRES.  The 1S setting is 5V SYSOVP, so if you are using any other number of cells, this is not going to work.

    Instead, please boot BQ25708 with CELL_BATPRES set to the correct configuration assuming the battery is present and then drive the CELL_BATPRES to ground after the device boots.  (The timing is not tight here.  In fact, I routinely use the EVM with no battery attached without driving CELL_BATPRES to ground, not that this is recommended practice.  My point being that you can perform a read from your uC to verify that the device is powered on and then drive CELL_BATPRES to GND and even if it takes a few seconds, it is not going to be an issue.)

    Regards,

    Steve

  • 0 in reply to Steve Preissig
    Prodigy 30 points
    Okay I think I understand.

    A separate issue that we have is that there are circumstances where we want to terminate charging immediately. Would it be sensible to briefly
    pull CELL_BATPRES down for this purpose?
  • 0 in reply to Julius Schmidt
    TI__Mastermind 25800 points
    Hey Julius,

    This can be done through the CELL_BATPRES pin. However, there is a bit that can achieve the same thing via the SMBus interface.


    Regards,
    Joel H
  • 0 in reply to Julius Schmidt
    TI__Mastermind 25800 points
    Hey Julius,

    This can be done through the CELL_BATPRES pin. However, there is a bit that can achieve the same thing via the SMBus interface through a bit.


    Regards,
    Joel H