Datasheet

Time (s)
Discontinuous Conduction Mode (DCM)
I
Peak
Time (s)
Time (s)
Time (s)
Time (s)
Discontinuous Conduction Mode (DCM)
DCM - CCM Boundary
Continuous Conduction Mode (CCM)
Continuous Conduction Mode (CCM)
Switchnode Voltage Switchnode VoltageInductor CurrentInductor CurrentInductor Current
V
IN
I
AVERAGE
I
AVERAGE
V
IN
I
BOUNDARY
=
2Lf
S
V
OUT
(1 -D)
LM21305
www.ti.com 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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