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SN74AVC16T245: SN74AVC16T245 input unused pins

Nanfang Liao
Intellectual 2085 points
Part Number: SN74AVC16T245

Hello Team,

My customer used our SN74AVC16T245, for some input unused pins, they are float, customer doesn't connect these pins to GND or VCC. They only connect output pins to VCC. Can you help to check if there is risk? Because Huawei's schedule is very urgent, and they can't modify their board. Can them put serial resistors before these unused pins to reduce the risk？ Thanks

Regards,

Nanfang

  • 0
    TI__Guru* 97046 points

    Hello Nanfang,

    There is definitely risk in leaving CMOS inputs floating.  Footnote 3 of the Recommended Operating Conditions table in the datasheet specifies that: All unused data inputs of the device must be held at VCCI or GND to ensure proper device operation. Refer to the TI application report, Implications of Slow or Floating CMOS Inputs, SCBA004.

    The linked application report describes all of the associated risks with leaving a CMOS input floating.

    There is an added risk since the output is forced to either VCCI or GND in your case.  If the input floats to the opposite value, the output will be short circuited, and there will be excessive current, possibly resulting in damage to the device.

    We cannot guarantee operation under the described conditions.

    Adding a serial resistance to an open input would do nothing, however connecting a large pull-up or pull-down resistor would fix the problem entirely.  A large serial resistance could be added to the outputs to limit current, but then they could also be disconnected for the same effect.