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BQ76930: SCD and OCD not working (overcurrent discharge)

Eduardo Pinto1
Prodigy 50 points
Part Number: BQ76930

Hello,

I'm designing a BMS board for EBikes batteries (10S4P Lipo battery) based on the BQ7693003DBTR and the BA34z100-G1. Everything seems to be working fine with the exception of the functionalities SCD and OCD. I'm using the bq769X0 Evaluation Software to configure the BQ76930 but with my program the problem is the same! The bits SCD and OCD System Status (0x00) never change even chaning the initial values (0x00) for the registers PROTECTION1 and PROTECTION2.

These are the only thing not working. I had tested piratically everything else with success (temperature sensors, individual cell calibration, UV, OV, cell voltages, etc...). 

I think that the problem could be that during discharge the coulomb meter gives me an negative value. Is this the correct sign? Is because of this that the BQ76930 never detects the SCD or OCD situation? If so, could I simple exchange the two signals cutting some tracks on the PCB?

(The absolute value of the CC seems correct and direct proporcional to the sense resistors I'm using (two 2mohm resistors in paralell).  I attach bellow the current sensor schematic.

I'm using a configuration with different paths for charging and discharging..

  • 0
    TI__Guru 54715 points

    Hi Eduardo,

    A negative sign for discharge current is correct. If the CC reads current and it is reasonably correct, the pins should be connected and OCD should trigger .  As you know, default OCD threshold is 8 mV.  If you cycle power, enable the FETs and apply > 8A you should have an OCD status, remember the tolerance on the trip level. 

    To avoid current you could apply 1V through 1k ohm to the SRN pin at the top of R84.  The voltage divider should give you about 91 mV which will exceed most of the OCD and SCD thresholds.  Adjust the voltage as needed to adjust the simulated current. You should also see the level in the CC.  If the CC shows the "current" but there is no OCD/SCD trip, cycle power to the part and try without register writes. If the CC reads but no OCD or SCD trip, the part must have been damaged in some manner, try a different device.  With the simulated voltage you should be able to find a trip level without register writes, then clear and repeat as needed.

  • 0 in reply to WM5295
    Prodigy 50 points

    Hi,

    I'm really ashamed :-) It was at the end of a long day and I was looking to the monitor and I convinced myself that I was reading an value in mV greater that was tens of mV supposedly greater than  the 8mV or 22mV threshold corresponding to the default values of PROTECTx registers.

    After a good night sleep everything I made new tests  and everything is working as supposed ... That made me remember some episodes from the National Geographic TV show "Mayday" where the airplanes pilots convinces himself of a certain fact... Also rereading the FAQs doc was clear that the negative signal was right!

    Anyway thank you very much for the tips and for the very quick answer!

    King regards,

    Eduardo