SN74LV1T34: VCC=open, Input=3.3V

Shuji Ishiwata

Part Number: SN74LV1T34
Other Parts Discussed in Thread: TXU0101, 5-8-LOGIC-EVM

Hi All,

I have a question about SN74LV1T34.

When the VCC pin is left open and a 3.3V signal is applied to the input, the input voltage back-feeds into the VCC pin, resulting in a voltage of approximately 3.0V.
To prevent this back-flow to the VCC pin, a 47kohm series resistor is placed on the input line.

Is there any issue with installing a 47kohm series resistor on the input?

Best Regards,
Ishiwata

1 month ago

0 Clemens Ladisch 1 month ago

Guru 325520 points

LV-T inputs are overvoltage tolerant and cannot feed into V_CC; that would be possible only from the output pin.

0 Shuji Ishiwata 1 month ago in reply to Clemens Ladisch

Mastermind 8880 points

Hi Clemens,

Thank you for your reply.

A customer reported that when 3.3V is applied to the input terminal while VCC is left open, both VCC and the output show approximately 2.5V.
Could you please verify this using an actual device?

Best Regards,
Ishiwata

0 Joshua Salinas 1 month ago in reply to Shuji Ishiwata

TI__Mastermind 40500 points

Hello Shuji,

The 5V tolerant inputs are mainly there to support down translation (For example VCC = 2.5V while applying a 3.3V input) and it's not meant for floating conditions.

This part does not support VCC Disconnect which forces I/Os into a Hi-Z state when one supply is floating (disconnected). If this is required, then I would recommend looking into the TXU0101.

Regards,

Josh

0 Shuji Ishiwata 1 month ago in reply to Joshua Salinas

Mastermind 8880 points

Hi Josh,

Thank you for your reply.
I would also like to introduce the TXU0101.

When using the SN74LV1T34, is there any issue with placing a 47 kΩ resistor in series with the input terminal to act as a current-limiting resistor?

Best Regards,
Ishiwata

0 Jack Guan 1 month ago in reply to Shuji Ishiwata

TI__Guru 59411 points

Hi Ishiwata-san,

We will get back to you shortly.

Regards,

Jack

0 Joshua Salinas 1 month ago in reply to Shuji Ishiwata

TI__Mastermind 40500 points

Hello Ishiwata-san,

The issue is not necessary the current but the input voltage when VCC is floating. Adding a current limiting resistor won't improve much.

Regards,

Josh

0 Shuji Ishiwata 1 month ago in reply to Joshua Salinas

Mastermind 8880 points

Hi Josh-san, Jack-san,

The customer is using a 47kΩ resistor at the input as a countermeasure; this limits the current and causes the 2.5V Vcc level to drop to approximately 0.5V.
They would like to confirm whether there are any issues with placing a current-limiting resistor on the input terminal of the device.

Best Regards,
Ishiwata

0 Joshua Salinas 29 days ago in reply to Shuji Ishiwata

TI__Mastermind 40500 points

Hello Ishiwata-san,

It seems like adding a series resistor helped but my next concern would be when VCC is active and a signal is driven to the input. If you provide a 3.3V input signal, the part won't detect it. VIH at VCC=3.3V is 1.39V

Regards,

Josh

0 Shuji Ishiwata 26 days ago in reply to Joshua Salinas

Mastermind 8880 points

Hi Josh-san,

Thank you for your reply.

I understand the point regarding the current-limiting resistor.

I have an additional question.
Is there a signal path to the output terminal if 3.3V is applied to the input terminal while the VCC pin is left open?

Best Regards,
Ishiwata

0 Tyler Townsend 26 days ago in reply to Shuji Ishiwata

TI__Guru 62928 points

Hi Ishiwata-san,

We will get a response to you by tomorrow eod 7/7.

Regards,

Tyler

0 Joshua Salinas 25 days ago in reply to Shuji Ishiwata

TI__Mastermind 40500 points

Hello Ishiwata-san,

Since we don't validate open-supply conditions, I cannot guarantee no signal will propagate from the input to the output when VCC is left open.

Regards,

Josh

0 Shuji Ishiwata 24 days ago in reply to Joshua Salinas

Mastermind 8880 points

Hi Josh-san,

Does this mean there is a possibility of a signal path from input terminal A to output terminal B when VCC is not connected?
Can this be verified using the device alone? I do not require a formal guarantee; I simply want to confirm whether such a path is possible.

Best Regards,
Ishiwata

0 Joshua Salinas 24 days ago in reply to Shuji Ishiwata

TI__Mastermind 40500 points

Hello Ishiwata-san,

Yes I can verify this. Please allow until the end of the week to provide results.

Regards,

Josh

0 Shuji Ishiwata 19 days ago in reply to Joshua Salinas

Mastermind 8880 points

Hi Josh-san,

Thank you for your support.

How is the progress？

Best Regards,
Ishiwata

0 Joshua Salinas 19 days ago in reply to Shuji Ishiwata

TI__Mastermind 40500 points

Hello Ishiwata-san,

See images below. I'm using the 5-8-LOGIC-EVM to evaluate SN74LV1T34-DCK.

Test Conditions used:

Pin 1: Floating
Pin 2: 3.3V, 1MHz input signal
Pin 3: GND
Pin 4: Output
Pin 5: Floating

Input Signal: Channel 2 (Red)

Output Signal: Channel 3 (Blue)

You can see there is a slight signal on the output, around 56mV

Regards,

Josh

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SN74LV1T34: VCC=open, Input=3.3V