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BQ76940: BQ76940 VC5X issue

Bharath Balakrishnan
Prodigy 85 points
Part Number: BQ76940


Hi,

I'm using the AFE chip BQ76940 for a 48V BMS system with the AFE configured as a 13S system. Out of the 45 boards we made, 4 to 5 boards worked fine for the first few hours, later on they developed an error in the measurement of Cell 5 voltage. Below mentioned are the voltages I've observed on the board.

  • REGOUT = 2.52V
  • Voltage of Cell5 before filter resistor R5 = 3.9V (That is the actual cell voltage).
  • Voltage of Cell5 after filter resistor R5 = 3.43V (Hence there is a voltage drop of 0.47V across R5)
  • Voltage drop across the diode connecting the VC5 to VC5X = 0.54V
  • VC5X to CAP2 = 3.29V
  • VC10X to CAP3 = 3.29V
  • Voltage across the resistor (1kOhm)  connecting Cell5 to VC5X = 0.84V

From the last point in the above design, the pin VC5X appears to be sinking around 0.84mA as opposed to the 25uA mentioned in the datasheet.

Kindly let me know how to address this issue. This issue hasn't appeared on the remaining 40 boards I have mentioned.

Thanks and Regards,

Bharath B

  • 0
    TI__Guru 54715 points

    Hi Bharath,

    The lower cell group seems to be drawing abnormal current. Either the part is damaged or there is excess current on the board.   Check the voltage of CAP1 and current from the pin into the capacitor. Current from this pin should normally be 0, the capacitor stabilizes the internal regulator.  Current from the pin into the capacitor or some other feature on the board can cause the effect you describe.  If you are using the same capacitors for CAP1, 2 and 3, the same effect would be expected on those capacitors which you have not noticed, so look for something specific on the routing or contamination in that area.

    Also inspect the VC5X net for any unexpected leakage or load.  Similar to CAPn, the same capacitors are likely used for each section, so something unique to the loading or layout may be suspect.  You might try to lift the VC5X pin to see if current is going into the net or the device pin.

    If the current is not coming out of the part it must be consumed inside due to damage.  Consider any test fixture or transients the part may have been exposed to in test.