A 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 conditions of, light-load, output overvoltage, and pre-bias start-up conditions. The LM25149 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 mode, the low-side MOSFET is switched off when reverse current flow is detected by sensing the SW voltage using a zero-cross comparator. The benefit of this configuration is lower power loss during light-load conditions; the disadvantage of diode emulation mode is slower light-load transient response.

Diode emulation is configured with the PFM/SYNC pin. To enable diode emulation and thus achieve low-I_Q current at light loads, connect PFM/SYNC to VDDA. If FPWM with continuous conduction mode (CCM) operation is desired, tie PFM/SYNC to AGND. Note that diode emulation is automatically engaged to prevent reverse current flow during a prebias start-up. A gradual change from DCM to CCM operation provides monotonic start-up performance.

To synchronize the LM25149 to an external source, apply a logic-level clock to the PFM/SYNC pin. The LM25149 can be synchronized to ±20% of the programmed frequency up to a maximum of 2.5 MHz. If there is an RT resistor and a synchronization signal, the LM25149 ignores the RT resistor and synchronizes to the external clock. Under low-V_IN conditions when the minimum off time is reached, the synchronization signal is ignored, allowing the switching frequency to be reduce to maintain output voltage regulation.