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BQ77216: BQ77216 Open wire operation

drouin david
Prodigy 20 points
Part Number: BQ77216

Hi team,

I want to use the BQ7721602 circuit with 13 cells.

I would like to understand how the Open Wire detection works, taking the example on V5.

In case of open wire on V5, the voltage on V5 will decrease to reach about the voltage of V4. Is this correct?

The VOW threshold is -200mV. Does this mean that the fault will be triggered when V5 = V4+200mV, or V5=V4-200mV?

Thanks

  • 0
    TI__Mastermind 42060 points

    Hello Drouin,

    In case of open wire on V5, the voltage on V5 will decrease to reach about the voltage of V4. Is this correct?
    Yes, the voltage on V5 would collapse down to V4.

    The open-wire fault trigger would occur when V5 is less than V4 - (-200mV), which is V5 < V4 + 200mV, as mentioned in Section 9.3.2 Open Wire Fault Detection of the datasheet.

    Given than in an open-wire scenario V5 will collapse to V4, the threshold has a 200-mV difference, the open-wire protection feature will trigger after the pre-programmed delay time. It essentially measures the differential voltage difference between V5 and V4, if it is less than 200-mV (which in this case it would be ~0-V), the protection is triggered.

    To help understanding, in a normal scenario where all cells are connected properly (and assuming a 3.7-V per cell), V5 would be 18.5-V and V4 would be 14.8-V. The differential voltage of cell 5 would be V5 - V4, which is 18.5-V - 14.8-V = 3.7-V, this is more than 200-mV so everything is working as intended.

    In a V5 open-wire situation, V5 = V4 = 14.8-V. So, 14.8-V - 14.8-V = 0-V < 200-mV, triggering open-wire protection.

    I hope this helped! Slight smile

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 20 points

    Hello

    Thank you for your answer.

    For this point it is clear to me.
    But I have others questions.

    First:
    If a Vn input is not used, it must be shunted (0V voltage).
    Its voltage being lower than VUV_MIN (Uv Detection Disabled Threshold = 500mV nominal), this input is inhibited.

    The Open Wire VOW detection threshold is 200mV.On this shunted input, the voltage is therefore below the threshold. Is an Open Wire fault being reported? Or is the Open Wire detection mechanism inhibited?

    Second:

    For my application, the VSS supply of the circuit is made by a cable connected to the negative polarity of the V1 cell. This cable is also used to supply the entire circuit board. A voltage drop of several mV (20-30mV) will be present on the cable. This voltage drop will affect the accuracy of the OV and UV thresholds of the V1 cell. Is this correct?
    If so, is it possible to inhibit V1 (shunted to VSS), and use channels V2 to V14? Is there a risk of triggering the Open wire fault of V1?

    Regards

    D. Drouin

  • +1 in reply to drouin david
    TI__Mastermind 42060 points

    Hello Drouin,

    Yes, open-wire protection of a shorted cell is inhibited when shorted, UV or OV from a shorted cell is also disabled. This is one of our FAQ: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/972586/faq-bq77216-how-do-i-reduce-the-cell-count.

    On the second, yes I'd be concerned that this would cause measurement errors in V1, as you'd want Vss to have a 0-V potential. If there's a 20 to 30-mV drop on the negative terminal of V1, it would be referenced to a higher voltage than intended. The FAQ above mentions that cell 1 should not be shorted, you should short from the top to bottom, so this is not advisable. I would recommend you possible get a thicker cable of sorts that has a lower resistance to lower the voltage drop. Make sure to make Vss stable across the board.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 20 points

    Thank you for your answers.

    Best Regards