The following operation description of the TPS54538 refers to the functional block diagram and to the waveforms in Figure 6-1. The TPS54538 is a synchronous buck converter with integrated high-side (HS) and low-side (LS) MOSFETs (synchronous rectifier). The TPS54538 supplies a regulated output voltage by turning on the HS and LS NMOS switches with controlled duty cycle. During high-side switch on time, the SW pin voltage swings up to approximately VIN, and the inductor current, iL, increases with linear slope (VIN – VOUT) / L. When the HS switch is turned off by the control logic, the LS switch is turned on after an anti-shoot–through dead time. Inductor current discharges through the low-side switch with a slope of –VOUT / 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 = VOUT / VIN.

Figure 6-1 SW Node and Inductor Current Waveforms in Continuous Conduction Mode (CCM)

The TPS54538 employs the 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 HS switch and compared to the peak current threshold to control the on time of the HS switch. The voltage feedback loop is internally compensated, which allows for fewer external components, makes design easy, and provides stable operation with almost any combination of output capacitors.