The LM5160 step-down switching regulator employs a control principle based on a comparator and a one-shot timer, with the output voltage feedback (FB) compared to the voltage at the soft-start (SS) pin (VSS). If the FB voltage is below VSS, the internal buck switch is turned on for a conduction time determined by the input voltage and the one-shot programming resistor (RON). Following the on-time, the buck switch must stay off for the off-time forced by the minimum off-time one-shot. The buck switch remains off until the FB voltage falls below the SS voltage again, when it turns back on for another on-time interval.
During a rapid start-up or when the load current increases suddenly, the regulator operates with minimum off-time per cycle. When regulating the output in steady-state operation, the off-time automatically adjusts to produce the SW voltage duty cycle required for output voltage regulation.
When in regulation, the LM5160 operates in continuous conduction mode at heavy load currents. If FPWM is connected to ground or left floating, the regulator operates in discontinuous conduction mode at light load with the synchronous rectifier FET in diode emulation. With sufficient load, the LM5160 operates in continuous conduction mode with the inductor current never reaching zero during the off-time of the high-side FET. In this mode the operating frequency remains relatively constant with load and line variations. The minimum load current for continuous conduction mode is one-half the inductor’s ripple current amplitude. The operating frequency is programmed by the resistor connected from VIN to RON and can be calculated from Equation 1 with RON expressed in Ohms.
In discontinuous conduction mode, current through the inductor ramps up from zero to a peak value during the on-time, then ramps back to zero before the end of the off-time. The next on-time interval starts when the voltage at FB falls below VSS. When the inductor current is zero during the high-side FET off-time, the load current is supplied by the output capacitor. In this mode, the operating switching frequency is lower than the continuous conduction mode switching frequency and the frequency varies with load. Discontinuous conduction mode maintains conversion efficiency at light loads because the switching losses reduce with the decrease in load and frequency.
The output voltage is set by two external resistors (RFB1, RFB2). Calculate the regulated output voltage from Equation 2.