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BQ76940: BQ76940 issue analysis

Lin Zhao
Intellectual 1230 points
Part Number: BQ76940

Hello, our BMS is applied to electric bicycles. The battery is a lithium iron phosphate 15-string battery pack. The BQ76940 and the microcontroller read the unit voltage collected by AFE through IIC communication.
Recently, a problem occurred in the verification after the data was read at the production end, and the MCU displayed the data on the host computer with an initial fixed value of 3.2V. And after the fault occurs, will not disappear even power off again.

After the software simulation we found that the verification failed after the data was read, the MCU displayed the data on the host computer with an initial fixed value of 3.2V. Check the code of the verification part and the read-out cell voltage is about 0.002V, which is actually 3.0V. Please help us analyze what causes this abnormality in AFE chips?

  • 0
    TI__Guru 54715 points

    Hi Lin,

    The usual problem with dormant register values is the occurrence of the XREADY fault due to some power loss in the part.  Check the status register 0x00.

    If the problem is on the bottom cell group the part won't ACK

    If the problem is with the midddle group there will be no cell voltage update, after power cycle the registers will read 0

    If the problem is with the top group the bottom group cells will show a voltage but the middle and top will have a dormant value. 

    Check the design for the presence of the power filter components.  Rf should be 1k typically for the BQ76940 and no series diode. Cf is typically about 10uF, in the schematic clips above C57, C58 appear to be Cf for the bottom group, but connection is not included.  Check that the diodes from the top cell input of the group to the group power are present.

    Check the voltage on the CAPn capacitors after boot, these should be 3.3V.

    Check the presence of a thermistor on TS2 and TS3 or pull down resistor to the local reference.  A trapped voltage on TS2 or TS3 from floating inputs will prevent proper boot and cause an XREADY fault with the 0 V voltages reported.