7.3.1 Control Circuit

The LM5161-Q1 step-down switching regulator employs a control principle based on a comparator and a one-shot ON-timer, with the output voltage feedback (FB) compared to the voltage at the Soft-Start (SS) pin (V_SS). If the FB voltage is below V_SS, the internal buck switch is turned on for a time period determined by the input voltage and one-shot programming resistor (R_ON). Following the ON-time, the buck switch must remain off for the minimum OFF-time forced by the minimum OFF-time one-shot. The buck switch remains off until the FB voltage falls below V_SS again, when it turns on for another ON-time one-shot period.

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 pin duty cycle required for output voltage regulation.

When in regulation, the LM5161-Q1 operates in continuous conduction mode at heavy load currents. If the FPWM pin is connected to ground or left floating, the regulator operates in discontinuous conduction mode at light load with the synchronous rectifier FET emulating a diode. With sufficient load, the LM5161-Q1 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 (in Hz) is programmed by the RON pin resistor and can be calculated from Equation 1 with R_ON expressed in ohms.

Equation 1.

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 period starts when the voltage at FB falls below V_SS. 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. The discontinuous conduction mode maintains higher conversion efficiency at light loads since the switching losses decrease with the decrease in load and frequency.

The output voltage is set by two external resistors ( R_FB1, R_FB2). The regulated output voltage is calculated from Equation 2, where V_REF = 2 V (typical) is the feedback reference voltage.

Equation 2.