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BQ76920: Designing 3 Cell BMS with Companion Controller (BQ78350-R1)

Seong Min Lyu
Prodigy 50 points
Part Number: BQ76920
Other Parts Discussed in Thread: BQ24650, , BQ78350-R1, BQ4050, TIDA-00792, BQ76940, TIDA-00255,

I am currently trying to design a system with 3 Li-Ion batteries in series.

One battery is: 3.2V and 3.3Ah, therefore total of 9.6V and 3.3Ah.

Also I am going to use BQ24650 charging IC with the system.

Is there any design examples using 3 Cells, BQ76920, and BQ78350-R1?

Thank you in advance.

  • 0
    TI__Guru 54715 points

    Hi Seong,

    I'm not aware of a design for 3 cells with BQ76920 and BQ78350-R1.  It is somewhat unusual since the BQ4050 offers similar CEDV gauging for 3 cells and is more highly integrated.  The BQ76920 with BQ78350-R1 may be desired of course, one common reason is the design scales to 15 cells with different members of the monitor family.  You might refer to the application note https://www.ti.com/lit/pdf/slua810 figure 2 and short VC2, VC3, and VC4 as shown in the data sheet.  Separate filters on BAT and REGSRC are suitable and may be desired with low voltages of 3 cells.

    For a downloadable schematic starting point  you might download the TIDA-00255 which is the BQ76940 EVM and change to the BQ76920 removing the EVM features and external balancing if not needed.  For a high side switched solution consider TIDA-00792 and make similar adjustments.

  • 0 in reply to WM5295
    Prodigy 50 points

    I currently have BQ76920EVM which consists of BQ78350-R1 and I currently have set it up with 3 Cells with all the unnecessary ports shorted. I am currently trying to modify the register values using the BMS Software that I downloaded from T.I.

    However, I currently am having a hard time understanding how to implement a charging IC with this system.

    Is there a case where charging IC was integrated with this system to build a BMS system with charging feature?

    Or would this be much easier if I use BQ4050? If so is there any design documents available for BQ4050?

    Thank you so much in advance.

  • 0 in reply to WM5295
    Prodigy 50 points

    I currently have BQ76920EVM which consists of BQ78350-R1 and I currently have set it up with 3 Cells with all the unnecessary ports shorted. I am currently trying to modify the register values using the BMS Software that I downloaded from T.I.

    However, I currently am having a hard time understanding how to implement a charging IC with this system.

    Is there a case where charging IC was integrated with this system to build a BMS system with charging feature?

    Or would this be much easier if I use BQ4050? If so is there any design documents available for BQ4050?

    Thank you so much in advance.

  • 0 in reply to Seong Min Lyu
    TI__Guru 54715 points

    Hi Seong,

    It seems you are on the right track.  The technical reference manual for the BQ78350-R1 provides detail on configuration.  For simple operation you will need to get the AFE Cell Map set for the 3 cells (default) and once your configurations are set be sure to set FET_EN.

    Looking at the diagram on page 1 of the  BQ24650 data sheet, the BQ76920 system (EVM) PACK+ would connect to the "Battery Pack" terminal on the right with PACK- connecting to GND.  This is a good time to recognize that since the BQ76920EVM uses low side switching, when the protection FETs are off the battery GND reference will be different from the charger GND reference. It is important to keep this in mind when connecting equipment for test or in the end application, you don't want current bypassing protection switches, altering sense currents, or damaging communication ports.  Use isolation in the interfaces when needed or keep the reference voltages separate.

    In the BQ24650 diagram a thermistor is shown.  It is not shared with the BQ76920 thermistor since its timing is not known and due to the GND reference difference mentioned.

    If you have questions on details of the BQ24650, if you post with that part in the subject you will get the right assistance.