SN74LV165A: Unused Input Floating

Kyle Mehringer

Part Number: SN74LV165A

Hello,

The SN74LV165A datasheet mentions that unused inputs should be pulled high or low. If one of the unused inputs was left floating by mistake, what kind of issues can be expected? Could it be left floating to avoid a board spin?

Thank you,

Kyle

over 2 years ago

0 Clemens Ladisch over 2 years ago

Guru 316470 points

Increased power consumption is very likely; see [FAQ] How does a slow or floating input affect a CMOS device?

But anything can happen; it's possible for an unused input to affect unrelated parts of the device, as in this example where an inverter behaved like a buffer.

0 Kyle Mehringer over 2 years ago in reply to Clemens Ladisch

TI__Intellectual 1510 points

Hello Clemens,

Is there any difference with Schmitt-Trigger inputs?

Thank you,

Kyle

0 Clemens Ladisch over 2 years ago in reply to Kyle Mehringer

Guru 316470 points

Floating Schmitt-trigger inputs do not cause oscillations, but still have a large shoot-through currents.

0 Kyle Mehringer over 2 years ago in reply to Clemens Ladisch

TI__Intellectual 1510 points

We have a board that currently has a floating input. Do you have data for this? We are trying to determine if a board re-spin is needed or not. I've heard that some competitors don't have this note in their datasheets.

0 Clemens Ladisch over 2 years ago in reply to Kyle Mehringer

Guru 316470 points

LV-A inputs should behave in the same way as AHC inputs:

There is no guarantee that the increased current will not affect other parts of the chip.

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SN74LV165A: Unused Input Floating