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I have usually seen Power & Ground Clamp diodes in CMOS structure as per attached figure. They are protection diodes. But I have few doubts :-

1.can someone explain in what situations they are actually working. 2.how are they protecting from high voltage. 3.What kind of diodes are used. 4.Are they present in other logic families as well?CMOS IO Buffer Structure

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    \$\begingroup\$ As far as I remember, MOSFETs always have such diodes, and they work by discharging static charges above +0.6v+Vdd/GND -0.6v on power supply rails. There should be same diodes at the inputs of the element. \$\endgroup\$
    – ilkhd
    Jul 10, 2015 at 4:43

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1-2) They are here to protect the device from electrostatic discharge (ESD), by channeling the charge to power rails. ESD can be understood as a very high voltage, very high impedance source. By connecting that source to VCC/Gnd with a diode (very low impedance) the ESD source is depleted from its charge without harming the device.

3) AFAIK, just normal silicon diode

4) Yes, but in a somewhat different version. In LS-TTL there is a (schottky) diode from input to ground, to limit the negative voltage excursions (they can happen depending on input line L/C characteristics). BJTs are not that sensitive to ESD (they have lower input impedance) but they don't like negative voltages above a few volts

These clamping diodes are also useful in that when you need a level translation from high voltage output to low voltage input, you usually can just use a single resistor (in line) that will limit the current, and let the clamping diodes do the voltage translation.

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In addition to Nicolas' answer, you will normally find these on the input stage as well.

What type of diode is used can be assessed from the 'absolute maximum ratings' table in the datasheet. If Vin is:

V+ + 0.3V then the devices are schottky types

V+ + 0.5V then the devices are PN junction diodes

These diodes can give rise to unusual effects: I have had a situation where the main power pin (V+) on a large programmable device became disconnected, and the device was actually being powered through the ESD protection diodes while at least one input was high; of course, as soon as all inputs went low, the device became un-programmed.

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