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SN74CBT16244: Hot Swap for DIO's - Selcting a Switch

Christian Sommer
Prodigy 30 points
Part Number: SN74CBT16244

For a research project we are looking for a way to ensure hot swapping for our developed Mezzanine Cards. When inserting/changing such a Card the Digital Outputs could be already high while the card connects to the Backplane connector (and with that to GND and Vcc). I found your Digital Bus Switch Selection Guide which shows multiple Switch’s/MUX which are looking quite good to solve this problem (e.g. SN74CBT16244). But how is the behaviour when a Signal is high while powering up these Switch’s? Or for example when the card is inserted and (because of tolerances) the signal pin has connection while the GND pin is still not connected.

Thanks in advance for your help.

  • 0
    TI__Expert 3980 points

    Hi Christian,

    What you are looking for is powered-off protection. The datasheet is not very clear on this, but as you see at II with VCC = 0 and VI = 5.5 V the current is below 10uA. This indicates that even when no voltage is present at VCC, the device does not have high leakage currents which could turn on the switch or damage down the line components. 

    The powered-off protection works up to a voltage of 0.5-0.7 V on VCC, you'll have to make sure, that you discharge any left-over charge in decoupling capacitors, for example with a MOhm resistor parallel to the decoupling. The exact value can be tuned to optimize discharge time and current. 

    To further improve your design, we recommend to pull up notOE via a resistor and pull it down with the control signal. This makes sure, that the switches are always high-Z when not pulled low.

    Let us know if you have more questions on your design.

    Best regards,

    Ambroise

  • 0 in reply to Ambroise Suter
    Prodigy 30 points

    Hi Ambroise

    Thank you for your answer! So, if I understand you right, the whole CBT (and CBTS and CBTD) – Series will be working well for this purpose?!

    For our design we will use some CBT’s (like the SN74CBT16244) which were powered by 5V and are working with 3.3V logic. The notOE is pulled up to VCC and will be pulled down over our control signal. What I didn’t understand is, in our case when the card is inserted (we use some DIN 41612 Type C connectors which have no special/longer power pin’s) the notOE is (maybe) low at the moment the DIO’s are already connected. Is there a series with an OE pin which can be pulled down to GND and then be pulled up over the control signal or doesn’t matter that?

    Thanks again for your help!

    Best regards

    Christian

  • 0 in reply to Christian Sommer
    TI__Expert 3980 points

    Hi Christian,

    you are very welcome! All CBT and CB3 devices have powered-off protection, but not the CBTD. Our App Note on the CBT/CB3 family give more insight to this, see chapter 4.8.

    I see you concern on notOE, unfortunately we don't have a device with OE, where you could pull it high to turn on the switch.

    But as the device itself has powered-off protection built in, you should be safe.

    Best regards,

    Ambroise