Arduino Automatically Selects Power Source

Thread Starter

waulu

Joined Dec 23, 2016
12
Hello,

I was looking at the schematic of Arduino Uno and the way it selects the power source. I have attached the schematic, the circuit that I have doubts is located on the top part of the sheet.

When both sources (External and USB) are connected, the P-MOSFET will be turned off, but the diode will allow current to flow. Couldn't this situation cause problems? So if 5V_USB_Voltage is a little bit higher than the 5V_Regulator, a current will flow to the 5V_Regulator. On the other hand, there's a drop voltage across the diode, so after the diode, it appears a voltage 5V_USB_Voltage - Diode_Forward_Voltage, let's imagine that's 4.3V. I have attached an image to clarify my doubt. What happens in the situation?

power_selector.png

Thanks
 

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Last edited:

AlbertHall

Joined Jun 4, 2014
8,973
If the external power is connected then IC2 will be regulating that eternal power to the +5V rail. This means that the voltage across the diode will be close to zero so no current will flow.
 

Thread Starter

waulu

Joined Dec 23, 2016
12
Thank you, forget that thing about the 4.4V, I just made a mistake =)

Ok, according to the USB 2.0 specifications, VBUS range from 4.75V to 5.25V, and the IC2 (MC33269) has an output tolerance of 1%, that means 4.95V to 5.05V, worst-case scenario, 5.25 V - 4.95V = 0.3 V.

Now I would like to know how much current could cross the diode. According to the datasheet of the P-MOSFET (FDN340P),
diode_FDN340P.png

Is this the right graph? At 25ºC the current across the diode is less than 0.0001 A but at 125ºC is about 0.002 A.
 

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