a page relating to "straight" Arduino. check our ATtami for a cheap-ass clone.
VCC for the ATMEGA chip in the center of the arduino board is supplied by a 5v LDO voltage regulator.
in the following pic schema we can see the power block including and a Schottky-based switch between USB power (Vusb) and Vin.
you have 3 Vout pins on the basic Arduino (uno)
- 5V - regulated from the output of the 7805 regulator or Vusb
- Vin - unregulated, directly from external power supply pin (on nano it's just a Vin pin. no DC jack)
- 3v3 - supplied by the regulator built into the FT232R serial-to-USB chip on the board.
excerpt from the FTDI FT232R data sheet
excerpt from the Arduino nano official schema
switching between Vusb and 5V is done by a comparator/MOSFET switch.
- 5V - regulated from the output of the AMS1117 regulator or Vusb
- Vin - unregulated, directly from external power supply (DC jack)
- 3v3 - supplied by the a dedicated 3v3 regulator LP2985