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TCA9617A: Working Principle

Lisa Xu
Intellectual 505 points
Part Number: TCA9617A
Other Parts Discussed in Thread: TCA9548A

Dear TI Team:

We have IC-TCA9617A in our design, In datasheet it highlights"allowing two buses of 550pF to be connected in an I2C application."

1. It means A side can meet 400pF standard requirement and B side can meet 550pF bus capacitance. Right?

2. Could you explan How TCA9617A can meet 550pF requirement, which parameters are the main factor to guarantee TCA9617A to meet 550pF?

3. How to set the pull up resistor value at B side, because if the resistor is too low which cause the bus will not be pulled low?

Best Regards

Lisa

  • +1
    TI__Mastermind 47218 points

    Hi Lisa,

    Lisa Xu said:
    1. It means A side can meet 400pF standard requirement and B side can meet 550pF bus capacitance. Right?

    Not quite. A-side and B-side can meet 550pF bus capacitance according to the requirements of the fast mode+ specification in the i2c standard. 

    Lisa Xu said:
    2. Could you explan How TCA9617A can meet 550pF requirement, which parameters are the main factor to guarantee TCA9617A to meet 550pF?

    This is mostly due to the strength of the pull-down drivers inside the 9617A. 

    IOL = 30mA (max) is the key spec. Normal I2C standard mode and fast mode (100kHz and 400kHz respectively) have a spec of IOL = 3mA at 0.4V = VOL. Fast mode+ requires a minimum IOL(min) = 20 mA or more at VOL = 0.4V to help with larger bus capacitance loads. 

    Lisa Xu said:
    3. How to set the pull up resistor value at B side, because if the resistor is too low which cause the bus will not be pulled low?

    If resistor is too "low" which means the resistance value is small indicating a very strong pull-up resistor could push VOL up high enough to be above a VIL. In order to prevent this the pull-up resistor value on B-side needs to be high enough to prevent this. 

    This application note here helps to show the calculation when choosing the correct pull-up resistance. 

    https://www.ti.com/lit/an/slva689/slva689.pdf?ts=1713379057233&ref_url=https%253A%252F%252Fwww.google.com%252F

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Intellectual 505 points

    Hi Tyler:

    Thanks for you explanation.

    For question 1: So for Standard & Fast mode,B side support 550pF,right?

    For question 2:

    2.1 Could you share the meaning of  min:0.1 mA for IOLB? when will we use this value?

    2.2  When I2C read status, the slave device will control the high and low status of the SDA line, and current flows to SDA pin of Slave device,

    but maybe salve cannot support 30mA, so it seems not only master(TCA9617A ast as master role) should support 30mA,

    but also slave device should aslo support 30mA to meet 550pF, Right?

    Maybe My logic is wrong, need your clarify.

    4. Per the material you shared, there are some questions confuse me,need you help.

    4.1. it use IOL for R (min) calculation.usually in IC datasheet shows min ,typ and max value for IOL parameter.

    For example, I2C switch TCA9548A datasheet shows min & typ value for SDA IOL(pic showed below), So we need use min value of IOL for R (min) calculation,right?

    4.2 The material you shared showed pic as below(Page 2, I cut the bus buffer/repeater), there are 3 segment for this I2C bus, L1,L2,L3(marked below),

    In my understanding, if we gurantee the multiplexer just Keep one path on at a time, so the L1+L2<400pF, L1+L3<400pF at standard and fast mode,Right?

    and when L3 on, so the RP calculated by equation below is Parallel equivalent resistance of RP1 & RP3，right？

    Best Regards

    Lisa

  • +1 in reply to Lisa Xu
    Guru 316690 points

    1. Yes.

    2.1. This value is not really useful in practice. You would get a current of 0.1 mA only with very weak pull-up resistors, which would result in very slow rising edges, so you would need to use very low I²C speeds.

    2.2. Yes, all devices on the bus (segment) must be able to sink the current through the pull-up resistors.

    4.1. Yes. But note that these values (0.4 V at 3 mA, 0.6 V at 6 mA) are copied from the I²C specification; you can assume that all standard-conforming devices support them.

    4.2. Correct.

  • 0 in reply to Lisa Xu
    TI__Mastermind 47218 points

    Hi Lisa,

    Please let me know if you have any more questions. 

    Regards,

    Tyler

  • 0 in reply to Clemens Ladisch
    Intellectual 505 points

    Thanks Clemens, Thanks for your sharing.

  • 0 in reply to Tyler Townsend
    Intellectual 505 points

    Thanks Tyler,Thanks for your sharing, You can close this case.