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TCA9543A: Level shifting from 3.3V master to 5V slave devices

Joel Dungan1
Prodigy 20 points
Part Number: TCA9543A

Hello,

I am trying to use the TCA9543a to level shift between a 3.3V master I2C bus and peripherals operating at 5V. My setup is almost identical to a previous post: https://e2e.ti.com/support/interface/f/138/p/646316/2374630, with the exception that I have used 3.3kOhm pull-up resistors.

I have found that (when enabled) the SD0/SC0 and SD1/SC1 lines are held at about 2.25V. This value is right on the edge of the logic threshold for my slave devices operating at 5V and causes intermittent communication issues. I can sweep my 5V supply down to 3.3V, but the values on the slave lines remain at 2.25V. When the 5V supply is lowered, communication is flawless, but changing the power rail is not ideal. I can’t easily test the effect of raising the 3.3V supply.

Is this expected behavior? Could setting Vcc on the TCA9543a to 5V fix this issue? Are there any other debugging suggestions?

Thank you,

Joel Dungan

  • 0
    TI__Guru 72161 points
    Hey Joel,

    Are you able to probe the SD0/SCL0 and SD1/SC1 pins on an o-scope? Do they hold high at 5V when the channels are disabled?

    One possible explanation on why you are seeing 2.25V is because you do not have pull up resistors on your channels.

    Also, just for sanity check here, can you repost your schematic so I can be double sure everything looks okay. You may have been looking at this for too long that you missed something, having a second pair of eyes to check wouldn't hurt.

    (Having the device run at 3.3V Vcc on TCA9543A should be no problem at all)

    Thanks,
    -Bobby
  • 0 in reply to BOBBY
    Prodigy 20 points
    Hi Bobby,

    Thank you for the quick reply! You got me on the right track. The pull-up resistor bank didn't quite fit the pad footprint so some of them were floating. I fixed the solderwork, and everything is running smoothly.

    Best regards,
    Joel