The following operating description of the LMR51635 refers to Section 7.2 and to the waveforms in Figure 7-1. The LMR51635 is a step-down, synchronous, buck converter with integrated high-side (HS) and low-side (LS) switches (synchronous rectifier). The LMR51635 supplies a regulated output voltage by turning on the high-side and low-side NMOS switches with controlled duty cycle. During high-side switch ON time, the SW pin voltage swings up to approximately V_IN, and the inductor current, i_L, increases with a linear slope of (V_IN – V_OUT) / L. When the high-side switch is turned off by the control logic, the low-side switch is turned on after an anti-shoot-through dead time. Inductor current discharges through the low-side switch with a slope of –V_OUT / L. The control parameter of a buck converter is defined as Duty Cycle D = t_ON / T_SW, where t_ON is the high-side switch ON time and T_SW is the switching period. The converter control loop maintains a constant output voltage by adjusting the duty cycle D. In an ideal buck converter, where losses are ignored, D is proportional to the output voltage and inversely proportional to the input voltage: D = V_OUT / V_IN.

Figure 7-1 SW Node and Inductor Current Waveforms in Continuous Conduction Mode (CCM)

The LMR51635 employs fixed-frequency peak-current mode control. A voltage feedback loop is used to get accurate DC voltage regulation by adjusting the peak-current command based on voltage offset. The peak inductor current is sensed from the high-side switch and compared to the peak current threshold to control the ON time of the high-side switch. The voltage feedback loop is internally compensated, which allows for fewer external components, making designing easy and providing stable operation when using a variety of output capacitors. The converter operates with fixed switching frequency at normal load conditions. During light-load condition, the LMR51635 operates in PFM mode to maintain high efficiency (PFM version).