7.4.1 Forced Pulse Width Modulation (FPWM) Mode

The Synchronous Rectifier section gives a brief introduction to the LM5160 diode emulation feature. The FPWM pin allows the power supply designer to select either CCM or DCM operation at light loads. When FPWM is connected to ground or left floating (FPWM = 0), a pulse-skipping mode and a zero-cross current detector circuit are enabled. The zero-cross detector turns off the low-side FET when the inductor current falls to zero (I_ZX, see Electrical Characteristics). This feature allows the LM5160 regulator to operate in DCM mode at light loads. In the DCM state, the switching frequency decreases with lighter loads.

If FPWM is pulled high (FPWM connected to VCC), the LM5160 operates in CCM even at light loads. This option allows the synchronous rectifier FET to conduct until the start of the next high-side switch cycle. The inductor current drops to zero and then reverse direction (negative direction through inductor), passing from drain to source of the low-side FET. The current flows continuously until the FB comparator initiates another high-side switch on-time. CCM operation reduces efficiency at light load but improves the transient response to step load changes and provides nearly constant switching frequency.