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BQ77216: How is fault latching enabled/disabled?

Timothy Alberg
Prodigy 140 points
Part Number: BQ77216

Hello,

Table 6-1 of the BQ77216 datasheet shows that fault latching is disabled for all parts in the family.  Section 9.3.7 indicates that fault latching can be enabled, however, it does not go into detail how it becomes enabled. 

We have a battery pack using the BQ7721600PWR that intermittently asserts DOUT and COUT during cell insertion with freshly charged cells.  Once asserted, DOUT and COUT will not recover unless the pack is disassembled and then reassembled.  We would like to rule out fault latching as a possiblity.  Could someone explain how fault latching becomes enabled?

Thank you.

  • 0
    TI__Mastermind 20980 points

    Hi Timothy,

    The fault latching option is factory programmed and is not able to be altered by the customer.

    What voltage are you measuring on the COUT and DOUT pins?

    Does this happen with other fault states?

    Regards,

    Max Verboncoeur

  • 0 in reply to Maxwell Verboncoeur
    Prodigy 140 points

    Hello Max,

    Thank you for confirming how the latching option is set. 

    We measured approximately 0.207V on both COUT and DOUT.  We are using a fixed 10k pull-down resistor on TS and we verified that each Vn pin had cell voltages on them so we rulled out Overtemperature and Open Wire as possible faults. 

    We think it is unlikely we are in the oscillator fault state, we observed the BQ77216 polling the TS and VN pins at rate of ~26ms (during the fault with both COUT and DOUT asserted) and ~128ms when no faults are present on a functional battery pack. 

    As we are using freshly charged cells in the pack assembly, we would not expect undervoltage trips to occur or persist if they did occur.

    We understand that some overvoltage trips may occur, but this would not be an issue for us as long as only COUT is asserted. 

    We are connecting 10 cells to the BQ77216 and have V10 thru V16 shorted together.

    If an undervoltage and overvoltage fault were to occur, how would the BQ77216 react?

    Assuming the BQ77216 sets both COUT and DOUT under this condition, does the BQ77216 need all faults to clear before de-asserting both COUT and DOUT?

    Thank you.

    Kind Regards,

    -Tim

  • 0 in reply to Timothy Alberg
    TI__Mastermind 20980 points

    Hi Tim,

    Could you provide your schematic so that we may better assist you in solving this issue?

    Also, are you measuring the cell voltages after the fault has occurred? Or are you measuring as the fault is occurring?

    A possible scenario that could cause this issue is one cell is over-volting and another cell is under-volting. In such a scenario, the device would activate both COUT and DOUT and neither would deactivate until all cells have recovered.

    Another thing I'd like to confirm, are you experiencing this on only one device or multiple devices?

    Regards,

    Max Verboncoeur

  • 0 in reply to Maxwell Verboncoeur
    Prodigy 140 points

    Hello Max,

    We can provide the schematic however we can not share it publicly.  Do you have a secure and/or private way of sharing it?

    We are measureing COUT and DOUT after the fault has occured.

    It seems likely that the scenario you described is what is occuring.  We suspect that some cells are tripping UV and some are tripping OV simultaneously as the pack is being assembled.  We modified a pack to add a series switch between the most positive cell in the pack and VDD for the BQ77216.  Toggling the switch, effectively performing a POR of the BQ77216, clears the faults and allows the pack to operate.  We still have an OV fault after the reset, however with DOUT being de-asserted we can pull current from the pack and power our electronics, which is acceptable for us.

    This problem occurs on all devices when using freshly charged cells in the pack assembly (~4.2V to 4.25V per cell).  When using lightly used cells (~4V to 4.1V per cell) we have not observed the issue.

    Would it be possible to update the datasheet to mention how the BQ77216 reacts to and recovers from multi-fault scenarios?

    Thank you,

    Kind Regards,

    -Tim

  • 0 in reply to Timothy Alberg
    TI__Mastermind 20980 points

    Hi Tim,

    We could continue this through email so that you may share your schematic, however that may no longer be necessary.

    The cell voltages you've provided, were these measurements taken at the BQ77216 pins during a fault?

    If not, could you measure the cell voltages during one of these faults as close to the pins as possible to see if there is indeed a persistent OV fault?

    The OV threshold on the BQ7721600 is 4.325V with a +/-10mV tolerance, and the hysteresis is 100mV. What I expect you may find is that powering on the device causes a brief voltage spike that exceeds the 4.325V threshold. This then relaxes to ~4.25V, however that is still within the hysteresis range of the protection (4.225V +/-10mV), so the protection still triggers. Section 9.3.1 of the datasheet explains this more in depth.

    I expect this is the issue since you do not see this behavior with cells that have a slightly lower voltage.

    Regards,

    Max Verboncoeur