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BQ40Z50-R2: CFETF False Triggering

Alex Fields1
Prodigy 125 points
Part Number: BQ40Z50-R2
Other Parts Discussed in Thread: BQSTUDIO

We have an application that the CFETF permanent failure is falsely triggering. Our pack is removable thus the PRES pin is toggled often to send the pack to sleep.

I do not see any current paths that would cause the current across the low side sense resistor.

Our sense resistor is a 0.5mOhm resistor and our CFETF parameters are 10mA for 5 seconds. I am assuming this is caused by noise in the sensing, as 10mA is a very low voltage to measure across the sense resistor we have. Is this a known issue based on the thresholds we picked?

  • 0
    TI__Guru** 105760 points

    Hello Alex,

    Our recommended sense resistor value is between 1mOhm and 3mOhm so your resistor is out of the recommended spec, which can cause issues with current measurement accuracy and noise getting on the current sense wires.

    Sincerely,

    Wyatt Keller

  • 0
    Prodigy 125 points

    Wyatt,

    Thank you. A couple followup questions:

    1. you mention noise getting on the current sense wires. This would have to be very low frequency noise that lasts > 5 seconds. Does that seem reasonable?

    2. If it is current measurement accuracy, what is the suggested accuracy for things like this with a 0.5mOhm resistor? +/- 100mA? +/- 50mA?

    3. If I were to increase the threshold from 10mA, is there a suggested threshold to increase to?

    4. Is there a suggested deadband to increase to (right now we have 3mA)

  • 0 in reply to Alex Fields1
    TI__Genius 10370 points

    Alex,

    1) Are you able to see current flowing via the bqStudio [current] register? In order for CFET to trigger you would need SBS.current to be seen > 10mA for 10Sec. Yes this would need to be a constant noise you can measure with an external DMM on the senseR

    2) The ADC is +/- 100mV 

    3) There is no official recommendation. Ideally when the CHGFET is off you should see 0 Current. I would focus on see if the noise is real via en external DMM.

    4) a normal deadband of 3mA should be acceptable. 

    Please just leave the pack ideal talking with bqStudio too see what the current register reports as to see if you can see anything abnormal. 

    Thanks,

    Eric Vos