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BQ76940: BMS Communication error

Adithya K
Prodigy 60 points
Part Number: BQ76940

I recently got a PCB assembled with bq76940. I successfully ran i2c scanner and got the address 0x08.

I'm using 9 resistors(I shorted the rest according to the datasheet) instead of cells as of now. 

I supplied 38V to bat+. When I power up the IC, the voltage drops across the resistors are not the same. Some are below 3V while most of them are above 4V. Also, I'm getting an output of 2.2V at REGOUT. 

I'm also getting a BMS communication error every time I try to check the voltage readings.

Please help me by giving more information on the same.

  • 0
    TI__Expert 6085 points
    check the version of the Bq and for the correct REGOUT voltage. check the cell connection. There needs to be a minimum of 3cells/group.
    monitor the ALERT pin. try to increase the stack voltage.
  • 0 in reply to Vish Nadarajah
    Prodigy 60 points
    What do you mean 3 cells/group?
    And what is the maximum stack voltage that can be applied?(The datasheet gives more than one value)
  • 0 in reply to Adithya K
    Prodigy 60 points
    I shorted the pins according to the datasheet (3 cells/group)
  • 0 in reply to Vish Nadarajah
    Prodigy 60 points
    Sir,
    What is the significance of 10 and 10b pins for the cells? The PCB doesn't have 2 seperate pins for 10 and 10b(same goes for 5 and 5b). How do I go about in making the connections with the PCB?

    Also, this board has a regout of 3.3V. I'm getting 2.2V. Is there any particular reason this might occur?

    Regards,
    Adithya
  • 0 in reply to Adithya K
    TI__Guru 54715 points

    Hi Adithya,

    The bq76940 has cells in 3 "groups" each monitored by a different circuit inside the device.  The connection from cell 10 must go to the device power pin, the sense pin for the middle cell group, and the sense pin on the upper cell group.  The same type connection is used at cell 5.  Look carefully at the schematics in the data sheet, at the EVM schematic, or perhaps more simply figure 16 of the bq769x0 design considerations apnote. Figure 8 and the related description indicate to reduce parasitic resistances in your board and interconnect.

    A low REGOUT voltage might be from too much current from the regulator (overload) or too little voltage to the REGSRC pin.With the bq76940 you likely have the source follower circuit to limit the voltage to the REGSRC.  Notice in the data sheet specificaitons that the current drops with increasing load.  The 20 mA is likely a good maximum current to expect from the part, you might check your load condition.  See figures 14 and 15 and the related discussion in the design considerations apnote,  be sure the resistors are small enough for the current used and placed to avoid excessive voltage loss from the load current through these resistors.  REGSRC also provide power for the DSG and CHG drivers so keeping an adequate voltage is important.