7.3.9 Synchronous Rectifier

The LM5161-Q1 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 LM5161-Q1 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 the FPWM pin is grounded or left floating, an internal ripple injection circuit is enabled. With the internal ripple injection enabled, the typical external feedback ripple injection circuit is no longer required. This feature reduces the component count in the buck applications. For more details see Forced Pulse Width Modulation (FPWM) Mode.

When the FPWM pin 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 the SW pin and through the low-side FET to ground. In a buck regulator application, large negative current will only flow from V_OUT to the SW pin if V_OUT is lifted above the output regulation set-point.