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TCA9800: whether we can use pull up resistors at B side? what's the risk?

Frank Xiao
Genius 12055 points
Part Number: TCA9800

Hi team,

My customer is using the TCA9800 now. Currently we use the pull up resistors at the vccb side(we put the switches between the B side and pull up resistors). Whether it has risks? What;s the risk? ( the block diagram is below) Thanks.

  • 0
    TI__Guru 53575 points

    Hi Frank,

    The B-side of TCA9800 is NOT to be connected to Vcc through pull-up resistors. This also applies to pull-up resistors that are connected through FET-based switches and level shifters. Such resistors would provide an external current source to these pins which rely on their own internal current source to monitor communication direction. If these pull-up resistors are included, it could prevent the device from relaying communication between sides. 

    For this application, I would recommend flipping TCA9800 so that the B-side is connected to the CPU and the 4.7k-ohm resistor removed. The A-side of TCA9800 is fine to connect to switches and requires pull-up resistors for operation. 

    Let me know if you have any more questions or would like other recommendations for this design.

    Regards,
    Eric

  • 0 in reply to Eric Schott1
    TI__Genius 12055 points

    Hi Eric,

    currently A side is 1.8v, B side is 3.3v. Whether we can change the positions? How we can judge which side use A, which side use B for normal case? Thanks.

  • 0 in reply to Frank Xiao
    TI__Guru 53575 points

    Hi Frank,

    The datasheet states: "The only requirements are that no external source of current (pull-up resistor or current source) be on the B-pins of the TCA980x, and that both VCCA/VCCB values are within the recommended range."

    Because both sides of the device can operate at 1.8v and 3.3v, this should not present an issue if the device sides are flipped. Because the pull-up resistors on the switch side of the device would provide an external source of current, it would not be possible to use the device in its current orientation. The A-side is fine to use in such conditions. By flipping the device so that the B-side is connected to the CPU and the 4.7k-ohm resistor is removed, there will be no external source of current presented to this side of the device - thus satisfying this requirement. 

    Let me know if this makes sense.

    Regards,
    Eric