What's the function of this transistor on Arduino Mega?

Question

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.

Answer 1

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.