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BQ76940: Calibrating failure with BQ76940 and BQ78350

Zie Hao Kek
Prodigy 60 points
Part Number: BQ76940
Other Parts Discussed in Thread: BQ78350, BQSTUDIO, EV2400

I have a schematic using the bq76940 and bq78350. I am trying to get it working but there are some problems with the calibration. In particular when discharging >1A.

The current sense resistor im using is 10mR across the SRN/SRP on the bq76940, but when I discharge >1A, the whole IC stops working. My bqstudio is no longer able to detect the bq78350 even after I reset everything. Are there any troubleshooting procedures to find out what went wrong?

  • 0
    TI__Guru* 81034 points

    Hi,

    By default, the Over-current protection setting for the BQ76940 is set to 8mV, so a current of 1A across a 10mohm resistor would cause the discharge FET to turn off. If you are using low-side FETs, you may be losing the ground connection between your EV2400 and the BQ78350 device which will cause you to lose communication on the SMBus. 

    You may want to check the REGOUT pin of the BQ76940 which supplies power to the BQ78350 to make sure it is still on at 2.5V. If you still see 2.5V, then the BQ78350 is still being powered and SMBus communication should be possible if the FETs are closed. 

    This other thread may also be helpful to debug: https://e2e.ti.com/support/power-management/f/196/t/649992?BQ78350-R1-reduce-the-voltage-SMBUS-communication-is-disconnected

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 60 points

    Is there any way to change the Over-current protection setting for the BQ76940 through the BQ78350 from 8mV to, lets say 100mV?
    I know that I need to change those values through the PROTECT2 register on the BQ76940, but I cannot find where to change it in the BQ78350 datasheet/TRF.
    I read through the datasheet of the BQ78350 and its' TRF, and the default value for OCD is -6000mA.

  • 0 in reply to Zie Hao Kek
    TI__Guru* 81034 points

    Hi Zie,

    Yes, you can change this setting. In the BQ78350, it is under the parameter name AOLD Threshold. This controls the hardware (AFE) OCD. There is also a software-based OCD parameter which can be set for a different threshold and delay.

    Best regards,

    Matt