The design example here is a current limited switch, a charger per the USB Battery Charging 1.2 specification. Starting with an input voltage of 5.3V with a 2.5A capability some initial work can begin. According to the specification, V_BUS output should be between 4.75V and 5.25V with a maximum current of 1.5A. Items in the red box confirm that the AP22653 will allow 5.3V_IN and be able to provide 1.5A. Calculations determine that the output voltage is within USB spec by checking voltage drop under worst case with maximum R_DS(ON), maximum thermal dissipation while operating at the maximum output current.