I would like to know, how work the operational amplifier connected (LM358D) to the P-Channel MOSFET transistor (FDN304V)
This part of Arduino board is used to connect the regulator (5V) to USB VCC, when external power supply is connected on the Arduino board.
The LM358D is a voltage comparator :
CMP : on pin 3 of LM358D (+ input)
+3V3 : on pin 2 of LM358D (- input)
GATE_CMD : pin 1 of LM358D (output)
With CMP = (VIN / 2)
CMP > +3V3 : GATE_CMD = +5V
CMP < +3V3 : GATE_CMD = 0V
The P-Channel MOSFET transistor (FDN304V) needs a negative Gate Threshold Voltage.
But the LM538D output connected on the Gate of this transistor (GATE_CMD) is between 0V to +5V but a never negative value !
I don’t know how it is work…