Clamping Diodes - Any disadvantages form continuously clamp down?
If you look into the datasheet you linked, on page 5, \$V_{BR}\$ (breakdown voltage) is the "clamp-down voltage" where the diode starts to conduct. For the 3.3V diode, it is at 6.4V typ. One obvious conclusion is, ESD diodes are quite imprecise and may not be good for tight overvoltage protection. So your worry is not whether it will be continuously clamping down the voltage, but rather will it clamp down at a voltage level where you consider as overvoltage.

Following is one example (which may or may not be what you want) of how to use the ESD diode :







simulate this circuit &ndash; Schematic created using CircuitLab

New edit in response to comment:

Look in the datasheet you linked, on Page 8 Fig 8, at \$V_{RWM}\$ (e.g. 3.3V), only \$I_{RM}\$ leakage current of around 1uA flows through the diode. Therefore for a signal up to 3.3V, very very little loading comes from the ESD diode and the signal is essentially unaffected.

At \$V_{BR}\$ break down voltage (e.g. 6.4V), the current through the diode takes a sharp turn upward. And it is this sharp up turn in current sinking capability that would try to clamp the voltage near the break down voltage.

With the voltage clamped at around 6.4V-8V, that is high enough to damage typical 3.3V logic, so a resistor can be added to limit the current into the 3.3V logic pin. But when adding such a series resistor, loading and conditions of the signal must be considered and it may not be functionally acceptable.

Also, for typical digital logic, an unidirectional diode gives better protection because it provides added protection for negative going glitches.