Time (s)

Discontinuous Conduction Mode (DCM)

Time (s)

Time (s)

Time (s)

Time (s)

Discontinuous Conduction Mode (DCM)

DCM - CCM Boundary

Continuous Conduction Mode (CCM)

Continuous Conduction Mode (CCM)

LM21305

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SNVS639F – DECEMBER 2009 – REVISED MARCH 2013

(1)

where D is the duty cycle of the high-side switch, equal to the high-side switch on-time divided by the switching

period. For more details, please refer to the CALCULATING THE DUTY CYCLE section in the Design Guide

provided later. Several diagrams are shown in Figure 22 illustrating continuous conduction mode (CCM),

Discontinuous Conduction Mode (DCM), and the boundary condition. It can be seen that in DCM, whenever the

inductor current reaches zero, the SW node will become high impedance. Ringing will occur on this pin as a

result of the LC tank circuit formed by the inductor and the effective parasitic capacitance at the switch node. At

very light loads, usually below 100 mA, several pulses may be skipped in between switching cycles, effectively

reducing the switching frequency and further improving light-load efficiency.

Figure 22. CCM And DCM Operation

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