The following operating description of the LMR3650x refers to the Functional Block Diagram and to the waveforms in Figure 8-10. The LMR3650x has two behaviors while lightly loaded, AUTO mode and FPWM mode. Regardless of the light load operation configuration, the converter operates in CCM when the load current is greater than half the inductor ripple current.

In CCM, the LMR3650x supplies a regulated output voltage by turning on the internal high-side (HS) and low-side (LS) switches with varying duty cycle (D). During the HS switch on-time, the SW pin voltage, V_SW, swings up to approximately V_IN, and the inductor current, i_L, increases with a linear slope. The HS switch is turned off by the control logic. During the HS switch off-time, t_OFF, the LS switch is turned on. Inductor current discharges through the LS switch, which forces the V_SW to swing below ground by the voltage drop across the LS switch. The converter loop adjusts the duty cycle to maintain a constant output voltage. D is defined by the on-time of the HS switch over the switching period:

In an ideal buck converter where losses are ignored, D is proportional to the output voltage and inversely proportional to the input voltage:

Figure 8-10 SW Voltage and Inductor Current Waveforms in Continuous Conduction Mode (CCM)