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Original question:

TMUX1072: I have a question about the relationship between Powered-off Protection and UVLO.

TMUX1072: Treatment for no connect pin

Shunsuke Nagata
Genius 12240 points
Part Number: TMUX1072

Hi,

Could you please let me know the treatment for the pins which would not use?

My customer is interesting in TMUX1072 but he will use 1-ch. This mean is SEL2, NC2, NO2 and COM2 are not used.

For NC2, NO2 and COM2, Should user set them open or pull-down with 50ohm or connect to GND?

For COM2, Should use leave open or short to GND?

Could you please give me advice?

Regards,
Nagata.

  • 0
    TI__Expert 5660 points

    Hi Nagata,

    Thank you for your question, this happens to be a Frequently Asked Question about signal switches & muxes.

    Please watch this TI Precision Labs video "Can I leave unused pins floating?". This video addresses what to do with several types of unused pins, such as control pins, signal pins, and more.

    You can also find this information in the TMUX1072 datasheet on page 17, highlighted below:

    To summarize, logic pins should always have a defined state. Because TMUX1072 has internal pull-down resistors on input logic pins, no external resistors are required on the logic pins (SEL2 may be left unconnected, or "floating"). Because this is a high-speed device, TI recommends that unused signal pins (NC2, NO2, and COM2) must be connected to ground through a 50ohm resistor to prevent signal reflections back into the device.

    Best regards,
    Kate

  • 0 in reply to Kate Dickson
    TI__Genius 12240 points

    Kate,

    Thank you for your support. But could you please break-down about below your comment? 

    > Because this is a high-speed device, TI recommends that unused signal pins (NC2, NO2, and COM2) must be connected to ground through a 50ohm resistor to prevent signal reflections back into the device.

    I would like to reduce the BOM. So, I don't want to use the 50ohm resistor in NC2, NO2 and COM2 if possible. What will be happen if the pins are open or short to GND?

    Once NC2, NO2 and COM2 are opened, it looks like there are no noise source. Then, user doesn't need to care "signal reflections back"...  

    Regards,
    Nagata.

  • 0 in reply to Shunsuke Nagata
    TI__Expert 5660 points

    Hi Nagata,

    Thanks for the follow-up. What is the frequency of the signal you are passing through the switch? As discussed in the video linked above, if your signal is not high-frequency, you may choose not to terminate the unused signal pins with a resistor.

    As stated in the datasheet, TMUX1072 can be properly operated without any external components. For high-speed applications, TI recommends terminating any unused signal pins to GND through a 50ohm resistor. If these pins are not terminated, this may create an "antenna", causing noise at the "ON" switch via unwanted channel-to-channel crosstalk.

    Best regards,
    Kate

  • 0 in reply to Kate Dickson
    TI__Genius 12240 points

    Kate -san,

    Thank you for your advice.

    Is it possible to avoid signal reflection back without external resister (50 ohm) If connect NC1 - NC2, NO1 - NO2 and SEL1-SEL2?

    In above use case, all of NCx and NOx are used. Could you please let me know your advice?

    Regards,
    Nagata.

  • 0 in reply to Shunsuke Nagata
    TI__Expert 5660 points

    Hi Nagata-san,

    Because the logic input pins have internal pull-down resistors, SEL2 does not need to have any external connections. So, I do not recommend shorting SEL2 to SEL1. 

    I do not recommend shorting NC1-NC2, NO1-NO2, COM1-COM2 as the propagation delays of each channel in the switch may be different (output-to-output skew) and this will interfere with the intended signal.

    Unused signal pins of TMUX1072 may be left unconnected if you choose to not design with external resistors.

    Best regards,
    Kate