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Original question:

BQ25895: how can we tell CPU when battery voltage falls to a threshold?

BQ25895: BQ25895 Does not turn off SYS when battery is below SYS_MIN

Kevin Kreger
Expert 1020 points
Part Number: BQ25895

As per REG03 we expect SYS to turn off when the battery drops below SYS_MIN (BTE, we keep default setting of 3.5V).

However, when we tested by draining the battery we found that the battery was still connected (SYS voltage was the battery's 'loaded' voltage) of 2.8V,
When the battery was disconnected and the voltage measured it was 3.3V

We expected SYS to turn off when SYS_MIN was reached. Of course, this is only a concern when we are NOT charging
the battery, otherwise the battery charging use case is shown in the data sheet in Fig. 10. 

If SYS does not turn off to stop battery depletion using this then how is it properly done?

Thanks in advance,

Kevin Kreger

  • 0
    TI__Guru**** 155056 points

    HI Kevin,

    SYS_MIN is the minimum system voltage that the buck converter maintains after the battery is discharged or removed to keep the system operating.  If you want to shut down the buck converter, you can use the HIZ bit in REG00b7.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Expert 1020 points

    Hi Jeff,

    Thanks for the super prompt reply and easy answer!

    We are prototyping this BQ for our new design and noted some gaps in the data sheet. If you would forward 
    these notes to whomever is in charge of the next revision then it will help others:

    1. The data sheet makes references to PSEL which is not an I/O line on this BQ (it looks like a transcription error from an older BQ data sheet)
    2. It is not clear that the PMID output follows the voltage of the input supply. We expected to be ~5V output even with a 12V supply, but it was 12V
    This is pretty major omission that deserves clarity in the data sheet and we needed to redo our design as a result. In fact there is
    an outright error w.r.t. PMID which shows a voltage in boost mode VOTG_RE_RANGE is 4.55-5.55V in the table at bottom of page 9. As well,
    it says boost mode over-voltage threshold is 6V.  It looks like more transcription errors from a BQ that was only designed for 5V USB.
    Thanks Jeff!
    Best,
    Kevin