How to work the external power supply management ?

Using Arduino General Electronics
1

Hi,

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.

---> http://arduino.cc/en/uploads/Main/arduino-uno-schematic.pdf

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

---> Amplificateur opérationnel/Comparateur — Wikiversité

The P-Channel MOSFET transistor (FDN304V) needs a negative Gate Threshold Voltage.

---> http://www.adrirobot.it/datasheet/transistor/pdf/FDN304P.pdf

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…

Thanks

Nassa.

2

but a never negative value !

It is relative to the source of the FET.

3

Hi Grumpy Mike,

Ok, I understood !
When the GATE_CMD is on 0V then the Ugs is -5V.
the FET threshold is reach.

Thanks very much.

4

Hi,

I understood the step where there is not external supply, and therefore VG = 0V and VD = 5V from USB. However, at the first instant, also VS should be 0V due to the inexistence of external supply and the MOSFET will not work, with VGS = 0V. How it works then?

Regards,
Antonio