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While reading through the 74HC4050 datasheet I came along this input protection circuit.

In order for the mosfet to turn on the gate voltage has to be more positive than the source voltage though they are shorted to GND in the circuit.

What is that transistor doing there and how does it work?

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    \$\begingroup\$ Here's a search term which you can research further: ggNMOS (grounded gate NMOS) \$\endgroup\$
    – SamGibson
    Aug 23, 2021 at 2:51
  • \$\begingroup\$ @SamGibson Thank you!. Do you have any idea on what's D1 doing there? \$\endgroup\$
    – MLuna
    Aug 23, 2021 at 22:49

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TI's AHC/AHCT Designer's Guide describes how this transistor works (in this case, Q6 is part of a larger protection circuit):

AHC ESD protection

Figure 12 shows protective circuits used for advanced high-speed CMOS devices. To meet the requirements outlined previously, the protective circuit is constructed in two stages. The input is first protected by a thyristor consisting of transistors Q2 and Q3. This provides coarse protection. If the input voltage rises above about 15 V, transistor Q1 breaks down and fires the thyristor. The latter then short circuits the high currents. Resistors R1 and R2 have values of only a few ohms. Therefore, the holding current of the thyristor is several tens of mA. When the current is reduced again at the end of the discharge, the thyristor is extinguished. Transistors Q4, Q5, and Q6 operate as fine protection and are intended principally to protect the input from excessive voltages. When there are overvoltages at the input, these transistors are driven into breakdown and limit the voltage, while resistor R3 limits the current.

So the MOSFET never switches on; it allows current to pass when its VDS breakdown voltage is exceeded. It is used like a Zener diode.

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  • \$\begingroup\$ Thank you!. Do you have any idea on what's D1 doing there? \$\endgroup\$
    – MLuna
    Aug 23, 2021 at 22:49
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    \$\begingroup\$ D1 is the body diode of the MOSFET. It was not put there deliberately, but clamping negative voltages is a nice side effect. \$\endgroup\$
    – CL.
    Aug 24, 2021 at 8:19

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