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BQ76920: false SCD detection

Davide Renzetti
Prodigy 215 points
Part Number: BQ76920
Other Parts Discussed in Thread: BQ76200,

Hello everyone,

in my board, sometimes (I don' t know why and when), the SCD trigger ON! In my board there is 15x0.008ohm resistor in parallel, and when the false SCD triggered ON, it was about 120A and the Bq7692003 read 58.65mV. My SCD setup was: 150mV/400us and OCD: 100mV/160ms. I have a temperature sensor next the shunt and the mosfet. When the SCD triggered ON the temperatures were: 

SHUNT:92°C

Mosfet:77°C

BQ7692003:62°C (sensor inside the Bq7692003)

Ambient: 49°C

I have a picture from oscilloscope, where the Yellow is DSG driver (from Bq76200), the light blue is the gate of discharge mosfet, the purple is the voltage on shunt, and the blue is the the Vpack voltage. Sadly, I the GND of probles of the oscilloscope don't were near the shunt resistor. 

As you can see, the voltage on shunt never reach 150mV.

Some solution? Thank you in advance.

  • 0
    TI__Guru 54715 points

    Hi Davide,

    The part will respond to the voltage at the sense pins SRP and SRN.  Be sure you don't have noise coupling into the traces between the shunt and the BQ76920. While there are large spikes on the sense voltage the filter components should attenuate those before they reach the pins.  Be sure nothing can rectify them to cause a false voltage at the pins.  Check the layout and be sure the signals are picked up at the sense resistor so that there is not a higher signal sent to the sense pins.  Check the filter position, you want it near the IC. 

    You might also check for loss of power on REGSRC or REGOUT.  Loss of REGSRC can result in a current fault when power returns since the current sense is powered by REGOUT.  You might check for transients on REGOUT or coupled into it. 

    If course check that the MCU is not unexpectedly writing the threshold lower. Also be sure the reserved bits in the PROTECT3 register are left at 0 although the reported threshold shift is smaller than your situation.

    Please let us know what you find.

  • 0 in reply to WM5295
    Prodigy 215 points

    Thank you,

    I have the shunt and the filter next to IC and I checked the MCU and PROTEC3 register, without finding problems.

    I will investigate about REGSCR losses. Thank you for your advices. I will keep you uptaded.

    Best Regards,

    Davide