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PCA9545A: PCA9545A Vol

King Tsai
Intellectual 2515 points
Part Number: PCA9545A


Hello, Team

may I know how to determine Vol value ?

base on the pull-up voltage and pull-up resistor?

  • 0
    Guru 318790 points

    The I²C specification requires at most 0.4 V when using at most 3 mA pull-up current, or 0.6 V with 6 mA.

    (The VIL is required to be at least 30% of VCC, which is much higher than 0.4 V. But it is a good idea to have a noise margin.)

  • 0 in reply to Clemens Ladisch
    TI__Intellectual 2515 points

    thank you for the quick reply. 

    so, if I have a 3.3V pull-up voltage with 1k ohm to be the pull-up resistor. 

    I`ll have the current 3.3mA, in this case, the Vol would be 0.4V, correct? 

  • 0 in reply to King Tsai
    Guru 318790 points

    You do not choose VOL; it is set by the I²C specification.

    (3.3 V − 0.4 V) / 3 mA = 967 Ω (which you can see in figure 9-4)

    Please note that the PCA9545A is a passive switch; the pull-up resistors of all enabled channels act in parallel. If you can have two channels enabled, you need about 2 kΩ on each one.

  • 0 in reply to King Tsai
    TI__Guru 53328 points

    Hi King,

    What Clemens is mentioning is correct. 

    I am not sure what graph he mentioned in his previous reply, but do keep in mind his statement about PCA9545 being a passive device. When the switch is on, each channel will see pull-ups in parallel to one another. So if you have 4 channels on at the same time with 1kohm pull-up resistance, you will find yourself at an Req = 1kohm || 1kohm || 1kohm || 1kohm = 250ohm. I would suspect that your i2c bus would have trouble driving a LOW signal with such a strong pull-up resistance. 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    TI__Intellectual 2515 points

    yes, we have trouble driving a low signal right now. 

    you are saying if each channel has their own 1kohm pull-up, when 2 channel on, I will see only 0.5k ohm pull-up for both channel in total, right? 

  • 0 in reply to King Tsai
    Guru 318790 points

    Yes.

    You should size the pull-up resistors so that the total curent of all channels that are enabled at the same time is no more than 3 mA.

  • 0 in reply to King Tsai
    TI__Guru 53328 points

    Hi King,

    Yes, channel that is on will see their respective pull-up resistors combined in parallel resulting in a stronger pull-up resistance to VCC. Please follow Clemens' suggestion.

    Regards,

    Tyler