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Topic: FDN340P Drawn incorrectly on schematic? (Read 2 times) previous topic - next topic

Lydecker

Hi there,

I was just looking at the Leonardo schematic: http://arduino.cc/en/uploads/Main/arduino-leonardo-schematic_3b.pdf

Is is just me, or is the FDN340P at the bottom of the schematic drawn the wrong way around? Should the Source of this P-Channel MOSFET not be connected to the USB's 5V? In the schematic they have connected the Drain to 5V from the USB instead.

It might just be me, but would love clarification on this!

Thanks

CrossRoads

How does it compare to the Duemilanove, Uno, Mega, and other boards with auto power switching?
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

Lydecker

That was one of the first things I checked. All of the boards you mention have it drawn the same way - which I why I thought it couldn't be an error. However that goes against the operation of a P-channel MOSFET as a switch - which is how it is being used here.

Argh!!! Someone please clarify!

Thanks

vasquo

Quote
However that goes against the operation of a P-channel MOSFET as a switch


Why? If gate is low, then switch is ON.  So 5Volts will be coming from VUSB.

If a DC jack is plugged in (and VIN will now be some value) and if VIN is more than 3.3V, then mosfet switches off (disconnects from VUSB). 5Volts will now be coming regulator IC1.

CrossRoads

http://www.fairchildsemi.com/ds/FD/FDN340P.pdf

Here's the best way to view it - with a MOSFET, when the Gate is active, current can flow from D to S, or S to D, and when it's not the two sides are isolated.
So with the comparator driving the P-channel, when the Gate is High (Vin/2 >3.3V) the device is off, the two sides are isolated and 5V comes from the 5V regulator.
When the Gate is Low (Vin/2 < 3.3V) the device is On and the USB is allowed to flow thru as the 5V source.
As a switch that's acceptable operation. As a linear amplifier it probably wouldn't be the preferred mode of operation.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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