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Original question:

TCA9803: Configuring multiple slave boards with TCA9803 devices on all boards

TCA9803: Bus capacitance vs number of slave devices

Sunney Kumar
Prodigy 60 points
Part Number: TCA9803

Hi TI team,

We are using TCA9803 as I2C buffer and it has to drive 5 slave devices. (Please see below sketch)

Will it be an issue to drive 5 slaves devices like this?

Clock used in this case would be 400kHz.


If approximate 10-13pF capacitance is considered for each slave device (SCL or SDA) including parasitic, it adds up to 50-55 pF. for 5 slave devices.

The rise time calculation as per formula given in the datasheet yields:

Datasheet of TCA9803 also shows similar correlation. 

  • +1
    Guru 316220 points

    You have to include the trace capacitance, which is about 1 pF/cm.

    Anyway, the TCA9803 will work fine with such a small bus.

  • +1
    TI__Mastermind 46613 points

    Hi Sunney.

    Clemens is correct here. Estimate about 1pF/cm or 3pF/Inch of tracing on the PCB in additional to the 50-55pF for the 5 slave devices. 

    Keep in mind that the curve trace you are looking at is for TCA9803 at 3.6V. Your system shows 3.3V for the supply, which will further steepen the slope of the line. 3.3V is more capable of driving larger loads for the same rise time than a 9803 working at 3.6V. 

    How did you add up to 70pF?

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Prodigy 60 points

    Thank you, Clemens and Tyler, for quick response.

    Tyler, 70 pF is capacitance located on graph for approximate rise time of 45ns. In graph, by mistake I wrote 55ns instead of 45ns.

    Regards,

    Sunney