The basic relationship of inductance, voltage, and change in current is the starting point for any switching regulator. As noted by the equation here, a change in current will develop a large voltage across the inductor. The amount of inductance for a switching regulator in general is determined by output voltage, input voltage (max), switching frequency (IC and EMI), and maximum ripple current. The output of a switching regulator is basically an LC filter. The inductor is used to store and release energy during the switching phase, and the cap is used to smooth out the voltage. In a switching regulator, the current in the inductor has a triangular waveform; as current goes up, it is charging the input capacitor. During the next phase, the cap is the main source of power. The current charging and discharging generates ripple voltage based on the ESR and ESL of the capacitor and along with the switching frequency of the devices. The typical switching frequency for these regulators ranges between 30 kHz and 1 Mhz. The most common switching frequencies range from 300 to 500 kHz, and as the switching frequency increases, the inductor size is reduced. The current from a switching regulator has AC components that will flow through the output capacitor. The output capacitor ESR defines the ripple voltage generated by the AC component of the current. The choice of inductance controls the ripple current; smaller ripple current means smaller ripple voltage. This relationship makes the choice of inductor size crucial.