SN74LVC125A: Pin 12 voltage clamping to Vcc

Johnny Guo

Part Number: SN74LVC125A

Hi, Team,

I have one question about SN74LVC125A need to check with you.

The connection is as below, VCC=3.3V, Pin 12 (Vin) is pulled up to 5V through one resistor. When Vin is floating, the test voltage on Pin 12 is 3.3V.

Is Pin 12 internally clamped to VCC?

Thanks.

Johnny

over 6 years ago

0 Emrys Maier over 6 years ago

TI__Guru* 96591 points

Hi Johnny,
There is no internal clamp diode to Vcc on the SN74LVC125A inputs.

What size resistor is being used? A very large resistor could produce the voltage drop indicated from leakage currents.

0 Johnny Guo over 6 years ago in reply to Emrys Maier

TI__Expert 4910 points

Emrys,

We tried to change different pull-up resistors, but the voltage is still clamped to 3.3V.

Johnny

0 Emrys Maier over 6 years ago in reply to Johnny Guo

TI__Guru* 96591 points

It sounds to me that there is a short somewhere in the design. If there were a clamp diode, it would be clamping at Vcc + 0.7V so ~4V. Since you are seeing exactly 3.3V, it's likely a short between a Vcc trace and the input.

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SN74LVC125A: Pin 12 voltage clamping to Vcc