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SN74CBT3257: What do I do with unused pins of xA and xBx of a 4-channel, 2:1 switch?

Adam Torma
Mastermind 45495 points
Part Number: SN74CBT3257
Other Parts Discussed in Thread: SN74CB3Q3257,

The questions is a recommended treatment of unused xA and xBx pins of SN74CBT3257. Since SN74CBT3257 is an analog switch and those pins are bidirectional actually, I’d like to confirm if there is a recommended way to prevent latch up or problematic behavior according to its internal circuit.

 

The customer uses only the channel #1 and #2. The pins 3A and 4A are treated as open left unconnected and pins 3Bx and 4Bx are pulled down to ground through 10k ohm resistor Could you please let us know your recommendation?

 

Thank you,

Adam

 

  • 0
    TI__Expert 3900 points

    Hi Adam,

    I did a quick E2E search and found the following post for a very similar device.

    Another great point of reference is the switches and multiplexers frequently asked questions:

  • 0 in reply to Dakotah
    TI__Intellectual 940 points

    Hello Adam, Hello Dakotah,

    Thanks for your support. I see. It seems xA and xBx of SN74CBT3257 can be left floating. However, the datasheet of SN74CB3Q3257(very similar but different device) says "If an application requires less than 4 bits, be sure to tie the A side to either high or low on unused channels." as quoted figure below.

    Could you please let me know if SN74CBT3257 has any difference from SN74CB3Q3257 regarding this unsed pin treatment? I'm feeling the customer's treatment, B side pull down is better to prevent any unknown state regardless of S pin state. OE is pulled down to GND(always enebled) in the customer's usage.

    Regards,
    Ikoma

  • 0 in reply to Ikoma
    TI__Intellectual 940 points

    Let me correct the part number. The customer is using SN74CBT3257C. Not SN74CBT3257(non-C version). Sorry.

    Please confirm for SN74CBT3257C if the unsed xA and xBx can be left floating.

    Regards,
    Ikoma

  • 0 in reply to Ikoma
    TI__Expert 3900 points
    Hi Ikoma,

    You are correct in that the SN74CBT3257C and SN74CB3Q3257 are similar devices, with the latter utilize a charge pump. While thhe ability to leave the pins floating is an option. Pulling the B side down to prevent any unknown state seems like the best option here. This will minimize any coupling from the unused channel onto the important channels being used.
  • 0 in reply to Dakotah
    TI__Intellectual 940 points
    Hi Dakotah,

    Understood. Thanks for your support.

    Regards,
    Ikoma
  • 0 in reply to Ikoma
    TI__Expert 3900 points
    No problem! If you have any more questions please feel free to post them on the forum and we would love to help solve your issues!