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BQ76930: Schematic Design Question / Review

Michael Majdalani
Prodigy 50 points
Part Number: BQ76930
Other Parts Discussed in Thread: BQ76920, BQ76940, TIDA-00449

Hello,

I've been having trouble getting the BQ76930 to boot / give a REGOUT + I2C signals. 

I was wondering if this design is correct or if there are any apparent or glaring issues? The boot signal shown will be connected to a GPIO of a microcontroller. 

 51-0468-SC_02.pdf

  • 0
    TI__Guru* 81034 points

    Hi Michael,

    Sorry to hear about the issue you are seeing. I looked over your schematic and notice a couple of things:

    - REGSRC is connected to your BAT voltage. REGSRC should be between 6V and 25V according to the datasheet. For BQ76920, it is okay to connect it to the same voltage as BAT, but for the BQ76930 or BQ76940, you need to use a source follower circuit to reduce the voltage as shown in the datasheet. 

    - The capacitor C8 should be connected from VC1 to GND. This is not the cause of your issue and many designs work fine with the way you have it connected, but it is a good best practice. It may reduce the accuracy of the bottom cell measurement slightly.

    - It looks like you are missing the input filter components Rf and Cf shown in Figure 13 of the datasheet. These should connect from point 'A' of the figure on the left side to the VC5X pin. 

    This video was released recently to help with the schematic creation process and covers the above items. It is available on youtube and ti.com.

    https://training.ti.com/how-create-schematic-bq76920-bq76930-and-bq76940

    https://www.youtube.com/watch?v=B5dsIc2C_xA

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 50 points

    Is there any benefit to using the source follower circuit rather than a plain voltage divider for REGSRC? Our charger is 30V max capable and it normally runs at about 29.5V. 

    As for the input filter components, I only see that connection on figures that don't use external balancing FETS. I believe R16 and C10 stand for Rf and Cf in my schematic. Do you have a recommendation as to what to add to achieve the same effect?

    Thanks,

    -Mike 

  • 0 in reply to Michael Majdalani
    TI__Guru* 81034 points

    Hi Mike,

    I recommend looking at TIDA-00449 (www.ti.com/.../TIDA-00449) as an example with external FETs. After taking a second look, I realized you are also missing the Rf and Cf going into BAT. 

    There should be a 10uF cap from VC10X to VC5X and a 10uF cap from VC5X to GND. I think you are right about R16, but C10 is not the Cf.

    Regarding a resistor divider for REGSRC, this is generally not recommended. It will pull a constant current from the cells and the voltage into REGSRC will vary with the input current.

    Regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 50 points

    Hey Matt,

    I've added the 10uF cap from VC10X to VC5X and 10uF cap from VC5X to GND. 

    Are C3 and R3 the Cf and Rf going into BAT? 

    I'm going to mark this answered since we will need time for board revving and testing. Thanks a lot for your help!

  • 0 in reply to Michael Majdalani
    TI__Guru* 81034 points

    Hi Michael,

    The Cf for the BAT pin is the 10uF from BAT to VC5X. (I said VC10X before which would make sense for the BQ76940, but the BQ76930 does not have a VC10X pin). The Rf should be between PACK+ and the BAT pin.

    Matt