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bq2022A / filtering capacitor on SDQ pin

S.Satoshi
Guru 21675 points
Other Parts Discussed in Thread: BQ2022A

Hi All,

Our customer has evaluated bq2022A and they are considering to add capacitor on SDQ pin for filtering. However there is fear that the capacitor will impact to communicate. So can you suggest the target capacitance that will not impact to communicate?

Best Regards,

Sonoki / Japan Disty

  • 0
    TI__Genius 13700 points

    Hello Sonoki,

    The best answer that I can provide is to look at a similar case of how the RC characteristics of a data line influence how it is driven and what are the acceptable pull-up and series resistor values for various bus capacitance.

    A thorough document that discusses this topic can be found here:

    http://www.edn.com/design/analog/4371297/Design-calculations-for-robust-I2C-communications

    You need to account for the time it will take for the BQ2022A to indicate a "1" using the pull-up resistance and filter cap and bus cap to determine your rise time to ensure that it will fit within the settling time of 15us. This is simply looking at the pull-up value selected and determining how long it will take for the it to pull the capacitance voltage above Vh threshold.

    The fall time can be figured in a similar way by looking at the filter capacitance and bus capacitance and any series resistance that you have between the BQ2022A and the filter cap. You will also need to account for the influence of the current that is sunk from the pull-up resistor.

    The current sinking capabilities of the BQ2022A are 4mA.

    This fall time will also have to fit within the 15us settling time for the BQ2022A.

    The changes that the pull up resistor, series resistor, and bus capacitance bring into the selecting a filter capacitor change the recommendation greatly.

    Less than 100pF should be acceptable for both rising and falling cases. I would recommend double checking for your application parameters.