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Topic: How to work the external power supply management ? (Read 2309 times) previous topic - next topic


Jun 07, 2011, 11:51 am Last Edit: Jun 07, 2011, 12:31 pm by Nassarane Reason: 1

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

   ---> http://fr.wikiversity.org/wiki/Amplificateur_op%C3%A9rationnel/Comparateur

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…




but a never negative value !

It is relative to the source of the FET.


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.



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?


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