Regulated +5.0V reference when USB & Jack are plugged in - what is the source?

This is in regards to the Arduino Mega 2560.

I’m close to fully understanding the circuit for powering the Arduino - power jack, USB, and when both are plugged in.

When both are plugged in, the LM358D acting as a comparator gives powering priority to the USB if the jack is < +6.6V. Then, the FDN304V mosfet is turned on, allowing the USB voltage to get regulated to +3.3V via the LP2985-33DBVR. The +5.0V from the USB also now is referenced by “5V” on the schematic.

When both are plugged in but the jack is >= 6.6V, the mosfet stays “closed”, and the voltage from the jack goes to the MC33269ST-5.0T3 regulator for the +5.0V.

In both scenarios though, the MC33269ST-5.0T3 regulates +5.0V (unless it doesn’t for some reason?)

My question is this: which voltage does the +5V reference on the schematic refer to when the USB is powering the board? Fundamentals of electricity may come in to play here, so forgive me.



The comparator output is LOW, so the P channel MOSFET is fully conducting. The MOSFET prevents overvoltage from being backfed to computer’s USB port.

The mosfet is just acting as an almost ideal diode, to stop backflow of the 5volt supply into the computer, in case you power the Mega via V-in or on the DC socket.

The Nano replaces that circuitry with a Schottky diode with a ~ 0.3V forward voltage drop, that's why the Nano's 5V pin is only ~ 4.7V.

There is no "reference" voltage on the Arduino, its 5V power, ie nominally 4.75 to 5.25. A voltage reference conventionally refers to a DC level signal with high accuracy and low noise, used for analog purposes (for instance you could feed a reference voltage into the AREF pin on the ATmega microcontroller), don't expect accuracy or low noise from a digital 5V supply rail, especially if on USB!

Thank you all for the responses.

Final question: When the USB is the primary source of power for the board, is the input voltage from the USB used as the input voltage to the LP2985-33DBVR (U2)?

Or is the output voltage of the MC33269ST-5.0T3 the only input to U2?

Thanks Anthony

USB supply end up on the 5volt rail (via a 500mA polyfuse and that mosfet diode). External supply also end up on the 5volt rail (via the 5volt regulator). The 5volt rail powers everything on the board, including the 3.3volt regulator. Leo..

Interesting, so both 5 volts (when both USB and jack are plugged in) meet at the same node. What would nodal analysis show for the combined voltage? Thanks for helping me out here :)

Below 6.6volt on V-in, the 5volt rail gets current from the source with the highest voltage (regulator and/or USB). Current is shared, because of the internal resistances of parts involved. So the 5volt rail already gets something from an external supply through the 5volt regulator above ~5.5volt on V-in. Sort of a smooth takeover until at 6.6volt the ideal diode/mosfet is turned into a common diode (body diode). Leo..

Thanks Leo,

I decided to get a scope going on the output terminal of the Mosfet, and what you said about a "smooth takeover" of voltage is apparent..the concept makes much more sense now. Cheers, Anthony