A fully synchronous buck regulator implemented with a low-side synchronous MOSFET rather than a diode has the capability to sink negative current from the output during light-load, overvoltage, and pre-bias start-up conditions. The LM5143-Q1 provides a diode emulation feature that can be enabled to prevent reverse (drain-to-source) current flow in the low-side MOSFET. When configured for diode emulation (DEM), the low-side MOSFET is switched off when reverse current flow is detected by sensing of the applicable SW voltage using a zero-cross comparator. The benefit of this configuration is lower power loss at light-load conditions; the disadvantage being slower light-load transient response.

The diode emulation feature is configured with the DEMB pin. To enable diode emulation and thus achieve discontinuous conduction mode (DCM) operation at light loads, connect DEMB to AGND. If FPWM or continuous conduction mode (CCM) operation is desired, tie DEMB to VDDA. See Table 8-4. Note that diode emulation is automatically engaged to prevent reverse current flow during a pre-bias start-up in FPWM. A gradual change from DCM to CCM operation provides monotonic start-up performance.