Since the MOSFET’s usefulness has been established, all we can do is to find a way to bring it to conductance, when needed. Since it is a P channel, in order to do so we need to polarize its gate by means of voltage that is a negative one with respect to the source: given that it is a low gate voltage MOSFET, in order obtain the required resistance (RdsON under 100 mΩ), 2.5 volts are enough. The R12 resistor, that negatively polarizes the gate, takes care of it. In contrast, the D4 and D5 Schottky diodes for small signals, along with the R11 resistor (the latter is used in order to avoid that a voltage greater than the tolerable one arrives on the gate, thus it operates as a voltage divider, along with R12), deal with bringing the MOSFET to the interdiction, when a voltage is found at the USB and/or PWRIN inputs.
The chosen voltage reaches U3’s pins 4 and 5 (U3 is the foundation of the SEPIC converter). The converter has been widely described in the dedicated articles, therefore we will only give a brief sketch about it here; the advantage of this diagram lies mainly in the possibility to obtain an output voltage that is greater than the input one, thus adding the advantages of a Buck converter to those of a Boost one, but still using a single switching integrated circuit only.
The voltage output is regulated by the voltage divider (here composed of R13 and R14), that supply a reference voltage of 0.6 volts at the integrated circuit’s FB input, when 5 volts are found at the output.
At the SEPIC converter’s output we therefore find a 5 volts direct current. The green POWER LED operates as a power light, and it may be deactivated by cutting the SMD PWLED bridge, thus reducing the consumption to a minimum (if you want to power Fishino MEGA by means of a battery, which was requested by many Fishino UNO’s users).
The 5 volts voltage exiting the SEPIC enters a further switching converter: this time it is a simple Buck (or step-down) converter that, for efficiency reasons, has been created by means of a synchronous converter, that is to say that it has been supplied with a second internal MOSFET, in the place of the Schottky diode that is commonly used. 3.3 volts exit this converter, and they are needed in order to power the WiFi module and the microSD reader: as anticipated, the available current is about 800 mA – 1 ampere as a total between 5 volts and 3 volts, which enables to power many external modules without the power supply suffering a crisis, as it often happens with Arduino and its linear regulators.
By the way, you will be able to notice the extremely reduced size of the three inductors and of the power supply’s filter capacitors: this has been possible thanks to the high operating frequency of the switching integrated circuits (above one MHz) and to the availability of high capacity ceramic capacitors.
