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BQ76940: Getting started with 12S design

Erik Anderson41
Intellectual 915 points
Part Number: BQ76940
Other Parts Discussed in Thread: BQ76200, BQ76920

Hi, I'm getting started with a new battery management design.  I am getting through a lot of the datasheet material and starting the schematic.  I have a few questions.  Here are a few details about my design:

1.  We only need to monitor the cells and do balancing.  I plan to use LTC7001 high side FET driver, so I'm planning not to use the CHRG/DISCH pins. I plan to use a dedicated microcontroller to read the status of the cells and determine when to break the circuit from charging.  Is this okay?

2.  I don't need the current sensing, is there any issue with leaving the sense pins floating?  Or should I tie them to ground?

3.  We plan to balance with only 20mA current, so I'm not planning to add external transitors.  With 4.22V cell, I selected 105ohm resistors because there will be two of them in the discharge path, correct?  So, 4.22V / (2*105) = 20mA.  Ok?

4.  I am mostly following figure 15 in the datasheet.  I am confused by the little boxes near the cells that contain "VC10x" and "VC5x" respectively.  Does this mean I should connect these nodes to the VC10x and VC5x pins?  I think so, but not completely sure.

5.  Could you quickly check the schematic so far?  I found it sort of painful to follow the connections from figure 15, because I only have 12 cells.  I think its correct, but wanted to check because some of the VC pins are shorted and it looks different from figure 15.

Thank you,

Erik2664.schematic.pdf 

  • 0
    TI__Guru* 79563 points

    Hi Erik,

    I highly recommend this video to help with the schematic creation if you haven't seen this already: https://training.ti.com/how-create-schematic-bq76920-bq76930-and-bq76940  The documentation can be a little overwhelming, so the video will hopefully help guide you to the best examples and documents for each topic.

    Answers to your questions:

    1. I do not know the LTC7001 very well, but I think this approach will work. If the BQ76940 is not controlling the FETs, the microcontroller will need to read the status registers to appropriately respond to fault protections. Alternatively, there is a BQ76200 high-side FET driver device that is designed to connect to the CHG/DSG pins of the BQ76940.

    2. This is okay. You will not be able to detect short-circuit or over-current faults, but you should tie these to GND if they are not being used. 

    3. The BQ76940 can only go up to 5mA with internal balancing. The BQ76920 (5S version) can go up to 50mA. You will need external balancing for 20mA. See datasheet table 8.1

    4. VC10X and VC5X are nodes in the drawing. The video goes into detail on how to connect these.

    5. The connections look correct according to the datasheet table. 

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Intellectual 915 points

    This is great, just what I needed, I will watch the video and make sure I use external FET balancing.