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TCA9548A: Device powered OFF

JB63
Prodigy 230 points
Part Number: TCA9548A

The TCA9548 is powered OFF, but one of the SDA and SCL output pairs is connected to another active I2C bus that has 4.7K pullups to a 5V supply.

1.) Is there a problem with loading the active bus, back powering the device or a signal appearing on the input or any of the other output channels? 

2.) Do you have details on the Pass-FET architecture that might help to explain this?

Thank you

  • 0
    Guru 243820 points

    As shown in the recommended operating conditions table, all SCL/SDA pins allow voltages higher than VCC.

    When the source/drain voltage is at or higher than the gate voltage, then the pass FET is off, and no current can flow through it. (Also, there is no ESD protection diode from the I/O pin to VCC.)

  • 0 in reply to Clemens Ladisch
    Prodigy 230 points

    Clemens,

    Thank you for the response. 

    Since there are no protection diodes on the outputs, then that is not a source of back powering. Great!

    I'm guessing that the Pass FET is a P-CH type with the Source connected to the Master input and the Drain connected to the Output.  At the output I will have +5V pulled up through 4.7K.  Since the TCA9548 is powered OFF, that would leave the Pass FET with 0V at the Gate and 0V at the Source (Vgs=0V). 

    If it is a PCH FET, then Vgs = 0V would turn it OFF and therefore my +5V activity on the output would be isolated from the input.

    Is my thinking correct on this?

    Thank you,

    John

     

  • 0 in reply to JB63
    Guru 243820 points

    The pass transistor is an N-channel MOSFET, with the gate connected to VCC (if the switch is enabled) or GND (switch disabled), the body connected to GND, and source/drain connected to the I/Os. Due to the separate body connection, source and drain are completely symmetrical.

    When powered off, the gate should be at GND, so the FET is always off.