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BQ25886: VBAT_UVLO

Part Number: BQ25886

Good Morning,

I understood that if only the battery is connected to the IC (no VBUS) and it gets deeply discharged (below the VBAT_UVLO threshold of ca.4V) then the BATFET will turn OFF, isolating the battery from the system. This could be an efficient functional protection for battery packs which does not already integrate some UnderVoltage protection.

Therefore, I tried testing such condition on the Evaluation Board. I connected a power supply with a starting voltage value of 7.2V (which emulates the battery) to the BAT input, and a load of 100Ohm to the SYS output. Then I started reducing the voltage, down to the VBAT_UVLO threshold, but the system voltage never dropped down to 0. It seems that the battery (power supply) is always connected to the system, even if battery voltage is very low. This is not ideal, since in such case an additional external protection shall be added to avoid the battery to get deeply discharged.

Why does this occur? Is it due to the BATFET body diode and the external Schottky Diode (reference component D7 of the Evaluation Board) which keep conducting even if BATFET turns off? I don't know the forward voltage of the BATFET body diode, but it seems strange since conduction seems to occur also at very low battery voltages, down to 0.2V.

Do you have any explanation?

Thank you so much. Kind regards.

  • Hi Alessandro,

    The BQ25886 BATFET cannot be fully turned off due to its body diode.  The Schottky diode on EVM provides another path from BAT to SYS. The Schottky diode turn on voltage is likely 0.2-0.3V while the internal body diode is likely closer to 0.6V.  We do not recommend that you use any of our higher power chargers for the primary undervoltage/overdischarge protection.  Instead, we recommend that you use a battery pack with integrated protection circuitry with the charger as a lost resort.

    Regards,

    Jeff