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TCA9545A: Asking for the TCA9545A pull up resistor setting

SHH
Mastermind 31446 points
Part Number: TCA9545A

Hi Team,
Customer uses TCA9545A on the I2C as expander.
Master and slave side are 3.3V.


From the d/s figure 19, the min Rp is 900ohm for pull up 3.3V case.

Please provide your comment fpr the pull up resistor on master(1.2k) and slave side(2.2k). Do you have any concern for this design?

customer is worried the 1.2k and 2.2k are parallel relation. so 1.2k//2.2k resistor value is smaller than 900ohm.

Thanks,
SHH

  • 0
    TI__Mastermind 21585 points
    Hello SHH,
    Your customer is correct to be concerned. The rise time is not affected by the pull up resistance, BUT because this is a pass FET architecture you need to be somewhat concerned about when the pass FET is turned on (during LOW). For example, if the master pulls LOW on the bus, then it is pulling low on all the pull up resistors on all the buses that are connected together, so it could affect the VOL. We have other posts on this type of stuff on E2E which you try to use as a research tool. Lot of great examples of questions and answers.
    -Francis Houde
  • 0 in reply to fhoude
    TI__Mastermind 31446 points
    Hi Francis,
    Customer need the suggested resistors value today to do the gerber out.
    What is the suggested resistor value?
    Which example posts that you can provide to us to study?



    Thanks,
    SHH
  • 0 in reply to SHH
    TI__Mastermind 21585 points

    Hello SHH,

    Here is a post that covers your question about how to calculate the appropriate pull up resistor value in an I2C switch.  

    e2e.ti.com/.../597750 pull up resistors on switch#pi239031350=1

    I simply typed in "How to calculate pull up resistor in switch" and there you go.  Lots of posts on how to do it and how the switch works. 

    You could have gotten your answer sooner if you had done a E2E search.  Let me know if you still have questions or if anything is unclear. 

    -Francis Houde