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BQ77915: queries on device operation during OCD and OCC faults

Mahendra Patel1
Expert 3560 points
Part Number: BQ77915

Hello Team,

My customer is testing their boards with BQ7791506 and has following observations, can you please provide your comments?

1.       OCD1 threshold of the device is 50mV as per datasheet. Sometime device does detect fault but not always. What might be the issue in customer design? What is the tolerance of 50mV OCD1 threshold?

2.       Customer is using 200kΩ resistor on OCDP pin instead of 196kΩ recommended for 45ms/20ms delay. Is this value mismatch causing problem?

3.       As per section 9.3.2.8, device should recover from OCC fault by itself. But in customer design, they have to disconnect the battery and reset the system if OCC occurs, otherwise design doesn’t recover by itself after OCC fault.

Please share your comments.

Thank you!

Regards,

Mahendra Patel 

  • 0
    TI__Expert 7465 points

    Hi,

    To answer your first question, the design should not change the OCD voltage threshold. The Rsns will determine at which current the fault will trigger, but the voltage should remain the same. Please make sure you are measuring directly from the pins. The tolerance of the OCD voltage threshold is +-20%. Also, we recommend using a resistor with 1% tolerance. 

    A 200kOhm resistor should not be a problem. That value can be adjusted to configure the delay. Keep in mind this may cause a slightly longer delay than 196KOhm. 

    The BQ7791506 version of the device is configured to only recover from an OCC fault with load detection. Please check to see if VLD > VLDT and ensure that the condition causing the OCC fault is no longer present.

  • 0 in reply to Shawn Hinkle
    TI__Expert 3560 points

    Hello Shawn,

    VSS connection of BQ99715 and -ve terminal of cell were not connected with each other which caused random OCD threshold in customer design. This is fixed now.

    in  customer design, RGS_CHG = 1Mohm and RLD = 470kohm. Customer has designed the system to detect OCC at 30A. Device is able to detect OCC at 30A and both CHG  and DSG MOSFETs are turning off when OCC fault occurs.

    After this, they remove the charger, but design don't recover. Even after removing the charger, when they connect the load or DMM to battery pack they are not getting any voltage across battery pack because both MOSFETs are off.

    what may be wrong? do they need 3.3Mohm for RGS_CHG instead of 1Mohm?

    can you please explain load detection mechanism in little more detail here? we are not able to understand it from datasheet.

    Regards,

    Mahendra

  • 0 in reply to Mahendra Patel1
    TI__Expert 7465 points

    Hi,

    If load removal is required, which all of our current configurations do, then you should change Rgs_chg to 3MOhm. This is described in table 10-1 in the datasheet. The RLD value is fine. Also, make sure RCHG is 1MOhm. Hopefully, this should help your issue. 

    The device should recover from the OCC fault when VLD > VLDT for they required delay time, tLD_DEG. This means the voltage on the LD pin must exceed ~1.35V for more than ~2.3ms.