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Original question:

PCA9544A: PCA9544A and PCA9545A

PCA9544A: Vpass setting

Shunsuke Nagata
Genius 13010 points
Part Number: PCA9544A
Other Parts Discussed in Thread: TCA9544A

Hi,

I would like to use PCA9544A for below system:

- Master : 3.3V I2C
- Slave : 1.8V I2C

Below information shows Vpass  = 1.9V when Vcc = 3.3V

But the Figure 12 shows Vpass = 25V when Vcc = 3.3V in typical.
Which information is correct?

Anyway, Can PCA9544A meet above condition which 3.3V Master and 1.8V Slave?

Regards,
Nagata.

  • 0
    TI__Guru 71790 points

    Nagata-san,

    I would recommend using TCA9544A and setting the Vcc to 1.8V instead. Doing this would resolve the issue.

    Otherwise, using figure 12 from what you referenced, you would need to set Vcc of PCA9544A to about 2.5V or lower (2.3V lowest Vcc recommended on this device) to ensure it would work for the maximum case (this is the worst case).

    -Bobby

  • 0 in reply to BOBBY
    TI__Genius 13010 points

    Bobby -san,

    Is your suggestion as below?

    - Master : 3.3V I2C
    - Slave : 1.8V I2C
    - VCC : 2.3V~2.5V

    Regards,
    Nagata.

  • 0 in reply to Shunsuke Nagata
    TI__Guru 71790 points

    My suggestion is actually to use the TCA version since it uses a newer process technology, better PoR circuit, and wider Vcc support compared to the PCA. Using the TCA would allow for you to use the 1.8V rail instead of trying to find a 2.3V~2.5V rail.

    If you have to use the PCA device for some reason then yes. What you have posted is correct.

    -Bobby

  • 0 in reply to BOBBY
    TI__Genius 13010 points

    Bobby -san,

    Are your suggestion : 

    TCA9544A
    -Master :3.3V
    -Slave :1.8V
    - VCC:1.8V

    I'm confusing because TCA9544A datasheet shows below. Typical Vpass support the 0.8V when Vcc = 1.8V. Is it possible to support 1.8V slave when Vcc=1.8V?

    Regards,
    Nagata.

  • 0 in reply to Shunsuke Nagata
    TI__Guru 71790 points

    The VPass spec in the datasheet shows the voltage at 100uA and provides the Vth value of the pass FET under those conditions. The difference for what you circled is 1.8V-0.8V = 1.2V =Vth of pass FET

    Using figure 16 in the datasheet:

    This means that if the lowest voltage on the I2C bus is 1.8V (your V pass in the graph) then Vcc MUST be about 2.6V or less in order to support 1.8V
    This means you can use Vcc = 1.8V as a result. Using Vcc= 1.8V means you can support up to 5.5V on the secondary channels.

    This section in the datasheet also walks through an example of this:

    -Bobby