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TCA9545A: I2C transactions when TCA9545A is not powered

Jordan Smith
Prodigy 205 points
Part Number: TCA9545A
Other Parts Discussed in Thread: TCA4311A

Hi

We are using the TCA9545A in the following configuration (see image below), with the TCA9545A and master I2C controller powered from VCC_1, and a device on an output port powered from VCC_2 (and pulled-up to VCC_2). We have a special case where we need to program the device before VCC_1 is present, which means the TCA9545A is not powered on but there will be I2C transactions happening on it's output pins.

When VCC_1 on the TCA9545A is not present (or below UVLO threshold), are the I2C output ports high-impedance, or could there be some conduction path that would cause issues with this approach?

We are trying to understand if we need an I2C isolator (e.g. TCA4311A or similar) to separate the I2C buses when the TCA9545A is not powered.

Thanks

  • +1
    TI__Guru 58120 points

    Hi Jordan,

    Jordan Smith said:
    When VCC_1 on the TCA9545A is not present (or below UVLO threshold), are the I2C output ports high-impedance, or could there be some conduction path that would cause issues with this approach?

    Our device is Hi-Z when there is no power on it's Vcc (Vcc_1 in your diagram) supplied. You should have no issues from our device when you use your programmer to program the device on the secondary channels (assuming you aren't operating the device outside the absolute max spec's). 

    Jordan Smith said:
    We are trying to understand if we need an I2C isolator (e.g. TCA4311A or similar) to separate the I2C buses when the TCA9545A is not powered.

    No, you don't need any kind of I2C buffer or isolator for this kind of application.

    If you'd like, we can do a schematic review of the TCA9545A to verify everything looks okay from a device standpoint.

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 205 points

    Thanks for the quick response Bobby, that's very helpful.