SN74AVC4T774: Level shifter floating pin behaviour

Maher Dyayn

Part Number: SN74AVC4T774

Tool/software:

Hi,

I'm using the 74AVC4T774 level shifter IC in by PCB. My VCCA voltage is 3.3V and VCCB voltage is 1.2V and the direction is set from input B --> A. I wanted to know what the expected behaviour is at the output A if my signal B is left floating? from the datasheet it seem like there is no built-in pull-up/down by the level shifter.

Sincerely,

2 months ago

0 Clemens Ladisch 1 month ago

Guru 318050 points

The output will be low or high or oscillating. You will also get shoot-through currents; see [FAQ] How does a slow or floating input affect a CMOS device?

0 Joshua Salinas 1 month ago

TI__Mastermind 25620 points

Hello Manher,

Clemens is correct, when the input is left floating, the output status will be unknown. We recommend either pulling-up or pulling-down unused input pins.

Regards,

Josh

0 Maher Dyayn 1 month ago in reply to Joshua Salinas

Prodigy 20 points

Hi Josh and Clemens,

thanks for feedback and sorry for my late response. if my VCCB voltage is 1.2V and I leave one of the input B itself as floating, then what voltage would this input B itself go to? would it be high / low or perhaps charge to VCCB maybe? the output A is pulled high to 3.3V

sincerely

0 Joshua Salinas 1 month ago in reply to Maher Dyayn

TI__Mastermind 25620 points

Hello Maher,

Because your inputs are not pull-up high or pull-down low, your inputs will be in an indeterminable state.

Regards,

Josh

0 Clemens Ladisch 1 month ago in reply to Maher Dyayn

Guru 318050 points

"Floating" means exactly this; any weak influence (leakage currents, electrical noise) can affect it, and the input pin's voltage is likely to float around.

The output drives strongly high or low, so the pull-up will not affect this.

0 Maher Dyayn 1 month ago in reply to Clemens Ladisch

Prodigy 20 points

Hi Josh and Clemens,

Is it possible that this floating input B charges to the same voltage as VCCB due to internal leakage in the chip? that is what I'm noticing in my case and just want to confirm if the leakage could be coming from the chip itself

0 Joshua Salinas 1 month ago in reply to Maher Dyayn

TI__Mastermind 25620 points

Hello Maher,

This is possible but as Clemens noted, it could be influenced by other factors like electrical noise.

Given this uncertainty, that's the reason why we always recommended forcing the inputs into a known logic state.

Regards,

Josh

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SN74AVC4T774: Level shifter floating pin behaviour