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TCA9545A Vcc voltage selection

Antony Lin
Genius 13480 points
Other Parts Discussed in Thread: TCA9545A, TCA9546A, PCA9546A, TCA9548A

To whom it may concern,

Considering Vdpum=1.8V and Vdpu1~Vdpu4 are either 3.3V or 1.8V, what's the requirement for Vcc voltage?  Based on the datasheet that Vpass should be less than 1.8V in this case, does that mean Vcc shouls be less than 2.7V?  Is there any other constraint for this design?

Thanks,

Antony

  • 0
    TI__Mastermind 30260 points

    Antony,

    Using Figure 17 (and the description on page 18 of the TCA9545A datasheet), the VCC power supply voltage (independent variable on the x-axis) determines the Vpass voltage (dependent variable on the y-axis). 

    In your example, you want Vpass to be lower than 1.8 V, so you can use the graph and find out the maximum value of VCC that will result in an acceptable Vpass voltage.

    I have attached a sketch which shows VCC needs to be less than 3 V, but of course this sketch is approximate.

    Assuming you don't have a 2.5 V voltage supply, I would simply use VCC = 1.8 V. The lowest voltage available will always work.

  • 0 in reply to Brian Berner
    Prodigy 10 points
    I have a similar question about selecting Vcc from Vpass.

    I thought I understood the explanation in the datasheet, and the answer above.

    However, if I look at the example in Figure 15 (page 16 in the TCA9546A datasheet), which shows that the Vdpum, and the Vdpu of the slave channels can be as low as 1.65v, but it also shows the Vcc at 3.3v. Is that correct value for Vcc for that example? If so, then I am not understanding Vpass, and really need your help.

    Thanks.
  • 0 in reply to Walt Amante24
    TI__Mastermind 30260 points
    Walt,

    You are correct that VCC = 3.3V will not be applicable if one of the VDPUx voltages is 1.65V

    The PCA9546A version of this part only operated down to 2.3 V, which meant I2C pins operating at a bus voltage of 2.5 V would be tolerant up to 2.7 V guaranteed.

    In the old example, VCC = 3.3V would always work.

    For the TCA9546A, the example does not always hold true. This will be revised on the D revision of the TCA9548A datasheet, and then funnel down to the other TCA954xA datasheets as well.