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BQ77915: OCD trip at 70mV threshold

Vi Tran
Prodigy 70 points
Part Number: BQ77915

Hi team,

We just had a batch of BMS built using BQ7791506. We are experiencing a higher OCD trip level than expected (70mV instead of 50mV across the shunt).

It roughly equivalent to +40% tolerant while the datasheet shows +-20%. Is it acceptable range?, will it cause any other problems?

Thanks, 

VT

  • 0
    TI__Guru 54715 points

    Hi VT,

    That seems unusual, it is more common for some extra resistance in the board or assembly to cause a trip at a lower current.  Check your sense resistor for a shift, and check the shape of the signal applied to be sure a transient signal shape is not affecting the apparent threshold.  With the filter on the sense resistor it is relatively easy to apply a DC voltage through a resistor to the SRN node to check the threshold.  The part should not have a 40% tolerance on OCD.  Swap units between good and bad boards to confirm the issue follows the part. If the behavior is isolated to the part, return through the supply chain for analysis. 

  • 0 in reply to WM5295
    Prodigy 70 points

    Thanks WM5295,

    Yes, I did measured those threshold again and they did trip at around 50mV which is within specs.

    I have another question about "Filtered Fault Detection". In the specs on section 9.3.7, it mentioned that BQ recognises a fault if it accumulates sufficient trigger sample count. Do you know what timing between each samples?, I cannot find anywhere else on the specs that mention about it.

    Thanks,

    VT

  • 0 in reply to Vi Tran
    TI__Guru 54715 points

    Hi VT,

    The faults are checked by 2 comparators are described in the BQ77915 data sheet section 9.3.1, https://www.ti.com/document-viewer/BQ77915/datasheet/GUID-D0B11237-F102-4565-9FFE-61E68BBF35C2#TITLE-SLUSCU0SLUSBZ79290   But the timing of those checks is not described in the data sheet, it is an internal feature.  The scheduling is not detailed in the data sheet.  Checks for various conditions likely varies with the response time of the check.  The one which will be visible is the temperature where samples can be observed on the VTB or TS pins, it is checked for OTC, UTC, OTD, and UTD.