8.3.2 Light Load Operation

DCM operation is employed in the LM43600 when the inductor current valley reaches zero. The LM43600 will be in DCM when load current is less than half of the peak-to-peak inductor current ripple in CCM. In DCM, the LS switch is turned off when the inductor current reaches zero. Switching loss is reduced by turning off the LS FET at zero current and the conduction loss is lowered by not allowing negative current conduction. Power conversion efficiency is higher in DCM than CCM under the same conditions.

In DCM, the HS switch ON time will reduce with lower load current. When either the minimum HS switch ON time (T_ON-MIN) or the minimum peak inductor current (I_PEAK-MIN) is reached, the switching frequency will decrease to maintain regulation. At this point, the LM43600 operates in PFM. In PFM, switching frequency is decreased by the control loop when load current reduces to maintain output voltage regulation. Switching loss is further reduced in PFM operation due to less frequent switching actions. Figure 34 shows an example of switching frequency decreases with decreased load current.

Figure 34. Switching Frequency Decreases with Lower Load Current in PFM Operation
V_OUT = 5 V F_S = 1 MHz

In PFM operation, a small positive DC offset is required at the output voltage to activate the PFM detector. The lower the frequency in PFM, the more DC offset is needed at V_OUT. Please refer to the Typical Characteristics for typical DC offset at very light load. If the DC offset on V_OUT is not acceptable for a given application, a static load at output is recommended to reduce or eliminate the offset. Lowering values of the feedback divider R_FBT and R_FBB can also serve as a static load. In conditions with low V_IN and/or high frequency, the LM43600 may not enter PFM mode if the output voltage cannot be charged up to provide the trigger to activate the PFM detector. Once the LM43600 is operating in PFM mode at higher V_IN, it will remain in PFM operation when V_IN is reduced.