The ATmega328P Xplained Mini kit can be powered by a USB or an external voltage input VIN. The default power source is 5.0V from a USB. The USB port is protected with a 500 mA PTC resettable fuse.
The ATmega328P is powered from the 5.0V USB voltage by default.
The figure below shows the possible kit power supply connections.
The input voltage select jumper (J300, 3-pin header footprint) can be soldered in to select between power from the USB port or the VIN pin on the Arduino power header footprint. By default, the selector is bypassed with a 0Ω resistor (R300) to connect the USB voltage to the on-board 3.3V/150 mA regulator and target voltage select header.
The target voltage select jumper (J301, 3-pin header footprint) can be soldered in to select between the on-board 3.3V regulator or the voltage from the input voltage select jumper (J300). The selector is bypassed with a 0Ω resistor (R301) to connect the kit input voltage to the ATmega328P and mEDBG.