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TCA9548A: VOL, IOL, Pull-up resistor

Ella KIM
Expert 4212 points
Part Number: TCA9548A

Hello,

I have a discussion with customer about the whole confusion on VOL, IOL, and pull-up resistor value. Could you help understanding? Thanks!

Per my understanding, VOLmax 0.4V at IOLmin 3mA is specified in I2C bus spec, but TCA9548A would sink 6mA typically.

Then, the proper calculation for Rpu under 3.3Vcc is : Rpu.min =  (3.3V - 0.4V) / 6mA = 3.23kohm ; IOL should not 3mA but 6mA. Is it correct?

For the equation (2), I'm referring to the attachment, I2C bus specification and user manual.

UM10204_I2C-bus spec and user manual.pdf

  • 0
    Guru 316440 points

    The datasheet does not guarantee more than 3 mA. So you should design the circuit for no more than 3 mA. (Use less than 3 mA unless you actually need high speed, or have a high bus capacitance.)

    Please note that the pull-up resistors of all enabled channels act in parallel.

  • 0 in reply to Clemens Ladisch
    TI__Expert 4212 points

    Clemens,

    Thank you for your comment.

    Clemens Ladisch said:
    The datasheet does not guarantee more than 3 mA.

    In datasheet, IOL specifies 3mA minimum and 6mA typical. You can find the capture in my first thread. I thought the device would be able to sink 6mA typically. Is my understanding wrong?

  • +1 in reply to Ella KIM
    TI__Mastermind 26806 points

    Hi Ella,

    Clemens is correct - when designing, you must account for the worst-case. Here, the calculation is to help estimate the minimum allowable pull-up resistance. This is to ensure that the low-level signal is low enough to be properly recognized. Thus, you have to ensure that even the weakest possible situation is accounted for. In this case, the worst-case minimum current sinking capability is 3 mA.

    You are correct that it is typically 6 mA, but designing using 6 mA as the worst-case would be incorrect, as it would not account for natural variation between different devices.

    Best,

    Danny