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Looking through the internet, I came across following circuit. enter image description here

MCU GPIO generates 0V or 3.3V Level.

VDD may be 5V or 30V. I would like to use the same circuit for both purposes, which is to supply the load with VDD with certain source resistance when MCU GPIO is ON.

My question is that

1) Why did they use voltage divider on the NMOS side? If PMOS can tolerate VGS difference of VDD, can I remove the resistor in the middle?

2) What is the purpose of capacitor between Gate and Drain of the PMOS?

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  • \$\begingroup\$ You should probably use the circuit editor next time, good question though. Make sure you specify a voltage range and try and narrow down the circuit parameters as much as possible \$\endgroup\$
    – Voltage Spike
    Commented Feb 17, 2016 at 4:14
  • \$\begingroup\$ Please use the circuit editor that is built into this site (or, if you're on an android device 'quick copper' works well for me) to make more easily read circuits. Unfortunately, from your hand-drawn diagram I can't tell between resistors & inductors, which would make a rather enormous difference in the functioning of the circuit. ;) \$\endgroup\$ Commented Feb 17, 2016 at 4:25
  • \$\begingroup\$ You also have a circle on the second gate which would imply that it is inverting, I haven't seen any discrete mosfets with an inverted gate that you can buy. \$\endgroup\$
    – Voltage Spike
    Commented Feb 17, 2016 at 4:26

2 Answers 2

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A GPIO high will produce a drain voltage of ~0 at the first FET. Without the divider, this produces essentially VDD between gate and source of the second FET. A VDD of 30 volts greatly exceeds the maximum gate voltage, typically 20 volts. With a 2:1 divider, a VDD of 30 volts produces a gate voltage of 20 volts, while a 6 volt VDD will produce about 4 volts.

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The voltage divider is probably because the 2nd mosfet is an nmos, you have to have the gate voltage higher than the drain for it to turn on, the resistor acts as a divider to accomplish this. A Pmos would be much more sutable for this application. PMOS's make great high side drivers. The resistor may also be used for protection, mosfet gates should never be connected to a low impedance source as the inrush current can destroy the gate. Always use a resistor or other means to increase the impedance.

I can't speak for the purpose of the capacitor, but it will slow the turn on time of the mosfet which is not desirable.

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