user7904: I am trying to get information about the transistor on the Arduino Mega board below:

It says "340P" on the transistor. I want be able to know which model transition is this so that I can check its documentation. It also want to know what it is driving or its function on the Arduino board. 

user4143: It's a P-channel MOSFET.  It's job is to act as a "dropless" diode.
The principle is this:

MOSFETs have a built-in diode across them in reverse bias (an effect of the chemistry)
The P-channel MOSFET is connected backwards in series with the incoming USB power.
The internal diode conducts power when there is +5V in to the USB to give power to the board.
The diode imposes a voltage drop, but there is enough voltage still to run the circuitry.
An op-amp compares the incoming voltage from the barrel jack (divided by 2) against 3.3V.  If it's less than 3.3V (6.6V incoming) or not there at all, it turns on the P-channel MOSFET
The MOSFET then short circuits the internal diode removing the voltage drop, giving the full 5V from the USB to the rest of the board.


simulate this circuit – Schematic created using CircuitLab
Note: diode shown is internal to the MOSFET.
The MOSFET is designated T2 in the schematic, and is a FDN340P on the reference design, although the actual model is not that critical, as long as the threshold voltage is above about -3V and it can happily handle 500mA or more.

7904: <p>I am trying to get information about the transistor on the Arduino Mega board below:</p>

<p><a href="https://i.stack.imgur.com/Ti8qo.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Ti8qo.jpg" alt="enter image description here"></a></p>

<p>It says "340P" on the transistor. I want be able to know which model transition is this so that I can check its documentation. It also want to know what it is driving or its function on the Arduino board. </p>
 4143: <p>It's a P-channel MOSFET.  It's job is to act as a "dropless" diode.</p>

<p>The principle is this:</p>

<ul>
<li>MOSFETs have a built-in diode across them in reverse bias (an effect of the chemistry)</li>
<li>The P-channel MOSFET is connected backwards in series with the incoming USB power.</li>
<li>The internal diode conducts power when there is +5V in to the USB to give power to the board.</li>
<li>The diode imposes a voltage drop, but there is enough voltage still to run the circuitry.</li>
<li>An op-amp compares the incoming voltage from the barrel jack (divided by 2) against 3.3V.  If it's less than 3.3V (6.6V incoming) or not there at all, it turns on the P-channel MOSFET</li>
<li>The MOSFET then short circuits the internal diode removing the voltage drop, giving the full 5V from the USB to the rest of the board.</li>
</ul>



<p><img src="https://i.stack.imgur.com/1Ky5G.png" alt="schematic"></p>



<p><sup><a href="/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2f1Ky5G.png">simulate this circuit</a> &ndash; Schematic created using <a href="https://www.circuitlab.com/" rel="noreferrer">CircuitLab</a></sup></p>

<p>Note: diode shown is internal to the MOSFET.</p>

<p>The MOSFET is designated T2 in <a href="https://www.arduino.cc/en/uploads/Main/arduino-mega2560-schematic.pdf" rel="noreferrer">the schematic</a>, and is a FDN340P on the reference design, although the actual model is not that critical, as long as the threshold voltage is above about -3V and it can happily handle 500mA or more.</p>
