SN74LVC14A: undershoot at logic input violating Abs. max.

Bertram Brummer

Intellectual 1906 points

Part Number: SN74LVC14A

We use SNx4LVC14A on our design. I’m testing signal integrity on its input signal. Found a note from datasheet. Cycled as below

“The input and output negative-voltage ratings may be exceeded if the input and output current ratings are observed.”

Could you please give a further explanation about this note.

For example, if I have a undershoot lower than -0.5v for 10ns on a static input signal. Does this violate your specification or not?

I guess the puls and the energy is too low to exceed the max. input clamp current of 50mA.

Is there any tool formula how to calculate proof this? WC customer could add a series resistor. What size woudl you suggets?

over 7 years ago

0 Emrys Maier over 7 years ago

TI__Guru* 97046 points

Hi Bertram,
Overshoot/undershoot is very common in logic circuits and very rarely causes any problems, but that's not to say that it can't.

It's not a question of time, but of current. The clamp diode is only guaranteed to support up to 50mA. If the line is driven below 0.5V for 1ns and the driver is strong enough to force 51mA through it, then the diode is no longer guaranteed to function.

If they want to be absolutely certain that this cannot happen, they should add a series resistor with the input that will guarantee less than 50mA with their overshoot value. It should be noted that by adding a series resistor, the overshoot will be reduced. Typically, a value of 33 to 330 ohms is used for dampening overshoot.

Logic

Logic forum

SN74LVC14A: undershoot at logic input violating Abs. max.