The LM5160 provides an internal low-side synchronous rectifier N-channel FET. This low-side FET provides a low resistance path for the inductor current when the high-side FET is turned off.
With the FPWM pin connected to ground or left floating, the LM5160 synchronous rectifier operates in diode emulation mode. Diode emulation enables the pulse-skipping during light load conditions. This leads to a reduction in the average switching frequency at light loads. Switching losses and FET gate driver losses, both of which are proportional to switching frequency, are significantly reduced and efficiency is improved. This pulse-skipping mode also reduces the circulating inductor currents and losses associated with a continuous conduction mode (CCM).
When FPWM is pulled high, diode emulation is disabled. The inductor current can flow in either direction through the low-side FET, resulting in CCM operation with nearly constant switching frequency. A negative sink current limit circuit limits the current that can flow into SW and through the low-side FET to ground. In a buck regulator application, large negative current typically only flows from VOUT to SW if VOUT is lifted above the output regulation setpoint.