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PCA9546A: Partial power compliant

John Wallace1
Mastermind 41080 points

Part Number: PCA9546A
Other Parts Discussed in Thread: TCA9546A

Customer is asking if this device is partial power down compliant.

  • 0
    TI__Guru* 79551 points
    Hey John,

    Can you elaborate on what you mean when you say partial power down? I assume you mean when Vcc is not GND but also below recommended here.

    I would like to also point out this device does have a power on reset errata stated on page 19 of the datasheet (section 10).

    Lastly, we always recommend when choosing our parts to choose the TCA line up if available as it fixes previous bugs, generally is more cost competitive, typically p2p replacement to PCA line up, and generally supports larger Vcc ranges. In this case, TCA9546A.

    Thanks,
    -Bobby
  • 0 in reply to BOBBY
    TI__Mastermind 41080 points
    Hi Bobby,
    If VCC to power the device is different from the SDA/SCL pullup voltages, then we don't want to backfeed the pullup voltage thru the device to VCC. Also, we don't want the outputs to drag down the inputs when the device is powered off. In other words, isolating the inputs and outputs from each other when the device is powered down. Would this be consistent across the PCA/TCA954x families of parts?
    Thanks,
    John
  • 0 in reply to John Wallace1
    TI__Guru* 79551 points
    Hey John,

    Thanks for giving me more background on your concerns.

    "If VCC to power the device is different from the SDA/SCL pullup voltages, then we don't want to backfeed the pullup voltage thru the device to VCC. "
    I have never heard of this happening with our switches before and it should be impossible due to the architecture (pass FET). The connection from SCx to SDA/SCL is done with a pass FET and does not have any internal pull ups or pull downs. The only connection is to the drain and source pins. It should not be possible for current (not including parasitic) to be passed to the channel outputs/Inputs.

    "Also, we don't want the outputs to drag down the inputs when the device is powered off."
    -The device should be high impedance when Vcc is GND. The pass FET's gate needs to see a voltage in order to pass a low but if Vcc is GND then the gate won't turn on (assuming the input voltages are not violated and breaks the drain to source).

    "In other words, isolating the inputs and outputs from each other when the device is powered down. Would this be consistent across the PCA/TCA954x families of parts?"
    -All of our switches, TCA and PCA, use the pass FET architecture so as long as the device starts up properly then a low should not be passed unless the channel is enabled (if violated channels may be turned on during start up and if one side is a logic low it will propagate the lower to the other side). Note that the start up requirements are stricter on PCA due to the POR errata than the TCA. For this reason I would recommend the TCA line up.

    Thanks,
    -Bobby