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BQ25731: Behavior Without Battery Connected

Tristan Zinnschlag
Prodigy 10 points
Part Number: BQ25731
Other Parts Discussed in Thread: BQ76922, , BQ25730

Tool/software:

Good day,

I would like to include a charging circuit for 4S Li-Ion batteries in our new powerboard design. For this purpose I had the BQ25731 as the charging IC and the BQ76922 as the monitor and protection IC in mind. However, I am not entirely sure if the BQ25731 would still output a voltage when the battery pack is removed (e.g., triggered by the undervoltage protection of the BQ76922) so that the microcontroller (and the system itself) can still be debugged via USB-C PD cable in case of a faulty or removed battery.
In other words, can the BQ25731 even start the buck/boost converter automaticly after POR with only 5V at the input (i.e., before a higher voltage has been negotiated with the USB-C PD charger) without a connected battery? The datasheet mentions that VREGN regulates to 6V, which wouldn’t be possible in this scenario.

If I understood the alternative charging IC with power path (BQ25730) correctly, it would have a default charging current of 0A after restarting (i.e., if an input voltage is applied to the USB port while the battery is missing), which would not be suitable for the planned setup. Otherwise, I would need to place an LDO on both the V_IN and V_SYS sides to supply the battery management MCU, which I would prefer to avoid. Is this the same for the BQ25731?

Thank you in advance!




  • +1
    TI__Mastermind 33445 points

    HI,Tristan,

    Tristan Zinnschlag said:
    In other words, can the BQ25731 even start the buck/boost converter automaticly after POR with only 5V at the input (i.e., before a higher voltage has been negotiated with the USB-C PD charger) without a connected battery?

    BQ25731 will switch even if there is no battery connected. It charges the on-board VBAT caps as a small "battery" by default, you can draw current from the VSYS rail. The VSYS voltage is max charge voltage.  If charge is inhibited, BQ25731 could not hold the VBAT, and stops switching. 

    If you need a VSYS rail even if the VBAT=0, choose BQ25730. Otherwise, BQ25731. 

    Regards,

    Tiger

  • 0 in reply to Tiger Zhou
    Prodigy 10 points

    Thank you for your answer. I think the BQ25731 is the better choice since I only need a small current from the VSYS rail as a backup for the MCU in case the battery fails, so I can read the status of the individual cells from the gauge/protection IC.