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BQ76940: Summary of BQ76940 issues and two unsolved issues

Duoduo Cheng
Expert 3080 points
Part Number: BQ76940

Two issues unresolved:

1. When discharging, the first battery voltage is sampled uncorrectly.

The above picture is my schematics. When not charging or not discharging, the first cell voltage sampling accuracy is within 5mv, but as long as it is discharged or charged, there are more than 100 mV voltages across the capacitor C48 and the first cell voltage.There are 100mv across fuse and there is no use to change the fuse. Why?

2. There are two times that the front-end sampling is normal, such as C40, C43 terminal voltage is the same as battery actual voltage, but the voltage that BQ76940 read  is only 40 mv, this case only happen to the last two batteries.  power-on has no effect on this issue, is the chip entering an error state?

  • 0
    TI__Guru 54715 points
    Hi Duoduo,
    1. The connection to B- is not shown on the schematic section and the location of GND. Typically GND is connected at the battery side of the sense resistor. With a large resistance to the battery negative, the C0 voltage can go negative with respect to GND and while VC0 can go negative the cell 1 can not measure the negative voltage. So cell 1 reads a lower voltage than the cell.
    Typically there is no current through C0 net unless balancing. If the voltage of C0 goes low enough that VC0 starts to conduct inside the IC, R96 should have the voltage drop rather than the fuse. It sounds like there may be some unexpected current in the fuse, possibly a parallel path with the battery negative. Check connections for battery- and currents in the C0 net.

    2. Seeing a voltage like 40 mV is common on an EVM when the inputs are shorted to reduce cell count. The GUI will apply the device offset to the 0V from the ADC and show some low voltage. In a battery it is unexpected. If there were a short at C40 or C43 or the pins on the IC the cell in question may report the low voltage but the adjacent cell would show a very high voltage. Missing pullup on the SDA line or bus contention with another host hold the line low resulting in a 0V reading which the host applies the offset to, but this would be very unusual to have it synchronous with the reading of the cell voltages.
    If C40 and C43 measure the proper cell voltage but the part reports 40 mV, check the solder at the IC pins and the voltage on the pins. It is difficult due to the find pitch. The part has no known operating mode to skip measurements or fail to report, if the I2C operation is correct the part must be defective.