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BQ25798: Power switch questions

Lillian Liu(SH)
Expert 4620 points
Part Number: BQ25798
Other Parts Discussed in Thread: BQ24133

Tool/software:

Hi team,

Customer try to use BQ25798 to power 3 cells in test board.

Q1:When the external power supply is plugged in and the power is switched to the external power supply under battery power supply (discharge state), will the Vsys pin be powered off? To what extent?

Q2: When the battery power supply is on, can the chip detect the voltage of the battery pack? When the overall voltage is lower than 10.8V (when the voltage of a single cell is lower than 3.6V), the under-power state is output for light indication. 

Q3: BQ25798 is an I2C slave. An I2C master must be used to control the registers, write the maximum charge current, the conversion voltage of pre-charge and constant current charge, and the percentage of the target charge voltage. Otherwise, the default maximum charge current of 1A will be used for charging. If the customer does not have an MCU, do we have any charger recommendations that do not use I2C communication?

Thanks

Lillian

  • 0
    TI__Guru**** 165866 points

    Hi Lillian,

    Regarding 1, no.  The internal BATFET stays on (and is connected to SYS via the FET body diode) when VBUS is not present.  The switchover to converter output instead of battery is in the us range and there will never be more than a diode drop difference between BAT and SYS.

    Regarding 2, the charger doesn't have auto detect/report low battery.  However, host software can periodically use the ADC to read battery voltage and then change a GPIO output.

    Regarding 3, for 3S pack standalone, I recommend BQ24133 for ICHG=2.5A and BQ2417X for ICHG=4A.  These are buck only chargers so VBUS>3x4.2V = 12.6V only.  Also, there are direct power path, meaning they pass the input voltage directly to the system.

    Regards,

    Jeff