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TCA9548A: Outputs connected to unpowered I2C devices

JB63
Prodigy 230 points
Part Number: TCA9548A

Using the TCA9548A with master at 3.3V and VCC of chip also at 3.3V.  One of the output ports is connected to some other I2C devices that are powered OFF.  When we select this port, it hangs the bus.  

Can the TCA9548A be used in this manner, or will the pass FET structure within the device cause back powering of the unpowered I2C devices?

Thank you,

John

  • 0
    TI__Guru 73651 points
    Hey JB63,

    "When we select this port, it hangs the bus. "
    I'm unsure what you are saying here.
    What I think you are saying is the I2C lines on channel X is powered off/referenced to zero and then you turn on channel X. The question here is if the channel is referenced to GND or is acting as a floating high-Z.

    "will the pass FET structure within the device cause back powering of the unpowered I2C devices?"
    If channel X is referenced to GND because the pull ups are tied to VccX which is capable of sinking current then the main SDA/SCL lines would see the voltage between it's main pull ups and the VccX pull ups. This forms a voltage divider and likely latches the bus. I don't think this case would occur though but depends on the power supply....

    If channel X is a floating high Z, then it may be possible for the pull up resistors on the main I2C devices to allow current to flow through them, through the pass FET and into the pull up resistors on VccX into the I2C devices which would see the voltage of 3.3V but will be current limited to the two pull up resistor values in series..... I think in this condition it MAY be possible to accidentally back power the device.

    It would probably be best if you try to keep the channel turned off if the devices are being powered down.

    -Bobby