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TCA9545A: Unpowered upstream or delayed-powered downstream case

ilhan taygurt
Prodigy 235 points
Part Number: TCA9545A

Dear Responsible;

In my design i am planning to use TCA9545A in a backplane-based design given below.

I wonder if it is allowed to operate the following cases;

  • Unpowered upstream but powered downstream
  • Powered upstream but any or fully powered downstream port(s)

Is there any backdrive issue for both conditions?

Is there any specific design ticks&tips that should be followed?

Thanks in advance

Regards

  • 0
    TI__Guru 53575 points

    Hi Ilhan, 

    I can say that the device is designed to work in the second case that you specify. When TCA9545A and its Master device are powered (upstream devices), they can operate independently of any downstream devices while the relevant channels are disabled. In this configuration, the SDn and SCn channels are high impedance and can be unpowered (no active pull-ups) without affecting Master-side communication. Once a downstream device is connected or powered-on, TCA9545A can relay this change (using the /INTn pin) to the Master so the associated channel can be enabled.

    The case where downstream devices are active when TCA9545A is unpowered is not an intended use-case for the device. Ideally with a hard pull-down on Vcc, there will be no gate voltage generated for the internal pass-FETs and the SDn and SCn channels will be high impedance - thus allowing the downstream devices to use the I2C bus. However, if the device inputs are floating in a powered-off state, there may be some adverse behavior that disrupts downstream communication. TCA9545 will also not receive communication from downstream devices while unpowered and will be unable to switch different channels based on I2C input. 

    Let me know if I understood your question correctly. Let me know if you have any more questions.

    Regards,
    Eric

  • 0 in reply to Eric Schott1
    Prodigy 235 points

    Dear Mr. Schott,

    Thank you for your reply,

    I understood the solution; i have to care second case.

    But what would be case for if un-powered upstream port side powered lets say a few seconds later? I have two concerns;

    • Does this condition casue catastrophic failure of any ICs?
    • If not is it possible to re-cycle with RESET#?

    I would be glad if you demonstrate the problem with modelling internal structure of TCA9545A:)

    Thanks in advance

    Regards

  • 0 in reply to ilhan taygurt
    TI__Guru 53575 points

    Hi Ilhan,

    No, powering the downstream channels while TCA9545A should not cause damage to the device as long as voltage level requirements are met. It is possible that the unpowered device would cause disruption in communication on lines that are active depending on the steady-state of the supply lines. The channel enables use a pass-FET method so the condition of the gate supply (created using Vcc) may cause issues while Vcc floats. The functional block diagram in Section 8.2 of the datasheet illustrates how the device is laid out. If this pin is at a known low state, the channels will likely be high impedance. In any case, the device will resume normal operation once it receives power and is configured by the master. 

    Regards,
    Eric