Diode emulation prevents reverse current through the inductor which requires a lower frequency needed to regulate given a fixed peak inductor current. Diode emulation also limits ripple current as frequency is reduced. With a fixed peak current, as output current is reduced to zero, frequency must be reduced to near zero to maintain regulation.

In auto mode, the low-side device is turned off after SW node current is near zero. As a result, after output current is less than half of what inductor ripple is in CCM, the part operates in DCM which is equivalent to the statement that diode emulation is active.

Figure 8-11 PFM Operation

The LMR3650x has a minimum peak inductor current setting (see I_PEAK-MIN in Section 7.5) while in auto mode. After current is reduced to a low value with fixed input voltage, on-time is constant. Regulation is then achieved by adjusting frequency. This mode of operation is called PFM mode regulation.