Power Supply Circuit
Nano uses two Step-Down converters based on the EUP3406 IC (U2, U3); these converters take the battery voltage and supply a constant output voltage of 1.8V and 3.3V as shown in the following picture. Step Down converters are ideal for mobile applications because it works with a wide range of voltages, in our case, the EUP3406 is able to supply the output voltage even when the battery voltage decrease at 2.5V, property that cannot happen with a linear regulator.
The EUP3406 let a wide range of output voltage, which can be fixed using different resistor values of R3,R6 and R10,R11 to obtain 3.3V and 1.8V respectively. The supply of these converters and BL8555 lineal regulator, can come directly from the battery or USB voltage supply (VBUS). in addittion, The EUP3406 allows disable the output voltage, if the pin 1 (EN) is in a high logic value, the circuit provides at its output the desired voltage (normal operation of the device); if this pin has a low logic value the output of the converters will be 0V, ensuring that the internal Nano circuitry won't function, avoiding power consuption and holding up the battery charge (energy save). PWEN is the denied PW_ON_N (Q2 Ver.) signal, which in turn is a processor output used to control the voltage of the main supply: A low logic state output indicates ON and a high impedance output OFF.