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BQ76940: xready Alert causing FETs to turn off

Lon Schneider
Prodigy 200 points
Part Number: BQ76940
Other Parts Discussed in Thread: BQ78350-R1

We occasionally find a condition in which the FETs turn off, with no Safety Status bit or other flag to indicate the reason.  Logging found a PF ALERT for AFE_XRDY coinciding with the FETs turning off. What is causing this and what can be done to prevent it?

System has bq78350-R1 and bq76940

  • 0
    TI__Guru* 81035 points

    Hi Lon,

    The BQ76940 has three die internally. The I2C pins connect to the lower die, and each die communicates with the die above it. An XReady fault indicates that the internal communication has been disrupted. This can be due to loss of power on one of the upper die or system noise causing corruption on the internal communication. If the internal watchdog function detects 5 or more unsuccessful transactions (out of 8) in a 125ms time period, then it triggers an XReady fault.

    Is there anything in your system that would cause an increase in noise on the BQ76940 pins at the time where the XReady fault is observed?

    Regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 200 points

    Hi Matt,

    We are examining the layout to see whether any noise inducing traces may be running near the AFE especially  near inputs to the upper sections. Can you indicate which ports may be especially susceptible to noise such that die to die communication could be compromised?  

  • 0 in reply to Lon Schneider
    TI__Guru* 81035 points

    Hi Lon,

    Noise into the device through power or inputs can be a concern. VC5X, VC10X, and BAT are the most sensitive. The large caps on these pins may be difficult to get close to the IC and may not have the best frequency response. Check if the charger gets noisy at CV mode or with light load. 

    Best regards,

    Matt