In Figure 1 the input voltage is shown as an ideal voltage source with infinite current capacity and no resistance. All real voltage sources have a finite current capacity and some finite resistance as shown in Figure 3 so an input capacitor is used to store charge and smooth out the current drawn from the source. When S1 is closed, the input current is the inductor current that we’ve already seen is an increasing ramp as shown in Figure 2. The average value during the time S1 is closed is the same as the output current. When S1 is open, the input current is zero. So the input current to the supply can be approximated by a rectangular pulse of magnitude equal to the load current and duty cycle equal to the duty cycle of S1. The dc component of the input current is the average of this rectangular pulse and is simply S1’s duty cycle times the load current3. The capacitor supplies the input current’s ac component and the RMS value of the input capacitor current is (1)RMSLOADIIDD=− where D is the converter’s duty cycle (S1’s duty cycle). (This expression or variations of it4 shows up without explanation in most switching regulator data sheets and application notes. See the sidebar for the derivation.) Strange as it may seem, the input capacitor is chosen based on its ESR and current rating rather than its capacitance value. As with the output capacitor, low impedance at the switching frequency is what’s important to minimize the input ripple current ripple that the source sees. Since monolithic ceramic capacitors have ESRs on the order of 10 milliohms or less, choosing a capacitance that yields a reactance of about that value or less is a good start. To further minimize input ripple current an inductor can be inserted before the input capacitor.