The circuit in Figure 6-28(a) illustrates the basic configuration for a step-down switching regulator.
When switch S1 is closed, the current in the inductor and the voltage across the capacitor start to build up. The current increases while switch S1 is closed as shown by the inductor waveform in Figure 6-28(b). The peak current in the inductor is dependent on the time S1 is closed (ton).
When S1 opens, the current through the inductor is Ipk. Since the current cannot change instantaneously, the voltage across the inductor inverts, and the blocking diode (DI) is forward biased providing a current path for the discharge of the inductor into the load and filter capacitor.
The inductor current discharges linearly as illustrated in Figure 6-28b.
For the output voltage to remain constant, the net charge delivered to the filter capacitor must be zero. The charge delivered to the capacitor from the inductor must be dissipated in the load. Since the charge developed in the inductor is fixed (constant on-time), the time required for the load to dissipate that charge will vary with the load requirements. It is important to use a filter capacitor with minimal ESR. Note, however, some ripple voltage is required for proper operation of the regulator.
Figure 6-29 shows a positive, step-down configuration both with and without an external pass transistor. Design equations for calculating the external components are included.