About I/O and ESD Protection Diodes

Hi,

Most i/o of digital circuits contain some kind of ESD protection circuit, which frequently consists of clamp diodes shorting the ESD zaps to Vcc or Gnd and limits the maximum i/o voltage of these devices to Vcc + (1 diode voltage drop).

1-I know this will be a silly question but, if applying a voltage higher than Vcc+0.3 can damage pins containing these diodes, how could these diodes protect the i/o from hundred of volts that can be generated by ESD?

2-If an input doesn't contain an ESD protection circuit, what is the thing that limits the maximum voltage that can be safely applied to the input without using any other passive element?

3-Is it safe to apply any voltage to any pin as long as the current through that pin is limited by passive elements below the maximum rating of the devices? Or should voltage and current independently considered?

Thank you in advance...

Hi,
1. The clamping diodes protect against only high voltage transients and not steady voltage. They clamp the inputs to -Vd or Vcc+Vd, which the devices are able to tolerate. Since ESD is of transient in nature, these diodes can carry such currents for the short intervals. Further the build of ESDs itself is prevented by these diodes by clamping the excess voltage during build up so that the current may not reach such high vlaues. Again, the source current of an ESD pulse itself will be limitted due to its path inductance.

2. These limits are as specified in the device data sheets.

3. Many times limiting the current helps to recover from an outside the limit voltage. But in general you need to take care of both voltage and current independently.

Regards,
Laktronics