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bq25890h: Vsys behavior when vbat < 3.5V

Eric Steimle
Expert 2290 points
Part Number: BQ25890H

I'm not clear on what happens to Vsys when the battery voltage drops below 3.5V, the datasheet seems to say it will be "regulated" to 3.5V so does that mean if the battery drops below 3.5V the charger behaves as a boost regulator to step up the voltage at Vsys?  If so how low will it go before it shuts off?

Thank you

  • 0
    TI__Expert 8230 points

    Hello Eric,

    When the adapter is plugged in, the output of SYS pin will be one of 2: if VBAT<MINSYS (in this example it's set to 3.5V), the bq25890H's converter is in buck mode and will regulate the VBUS voltage down to MINSYS (3.5V). As the battery is charged and the battery voltage increases past MINSYS, the SYS voltage begins to track the battery voltage. In this case, VSYS ~= VBAT + ICHG*RDSON of the internal BATFET.

    If the adapter is not plugged in, the device enters supplement mode where SYS tracks the battery voltage as the battery discharges; in this case VSYS = VBAT- ISYS*RDSON of BATFET.

    If OTG is enabled, then the converter boosts the battery voltage and provides a 5V output at the VBUS pin.

    Hope this helps.

  • 0 in reply to Fernando Lopez
    Expert 2290 points
    Okay so then let’s say the battery discharges to 3.0V and there is no adapter plugged in. So in that case Vsys = 3.0V minus the drop across the bat-fet? Or does it shut off under a certain Vbatt voltage.

    Thank you,
    -Eric
  • 0 in reply to Eric Steimle
    TI__Expert 8230 points

    Hello Eric,

    The battery should discharge down to the VBAT_UVLO threshold, typically ~2.3V at which point the BATFET turns off to prevent the battery from being deeply discharged.

    In theory this shouldn't happen, as the protection FETs on the battery pack should have tripped way before the battery reaches such a low voltage. Usually the cell under-voltage protection thresholds of protection FETs is anywhere between 2.5-2.6V.