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?



Thanks

 

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.

 

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),


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.