Datasheet

LMZ23603
www.ti.com SNVS711I –MARCH 2011–REVISED OCTOBER 2013
DISCONTINUOUS CONDUCTION AND CONTINUOUS CONDUCTION MODES
At light load the regulator will operate in discontinuous conduction mode (DCM). With load currents above the
critical conduction point, it will operate in continuous conduction mode (CCM). In CCM, current flows through the
inductor through the entire switching cycle and never falls to zero during the off-time. When operating in DCM,
inductor current is maintained to an average value equaling Iout. Inductor current exhibits normal behavior for the
emulated current mode control method used. Output voltage ripple typically increases during this mode of
operation.
Following is a comparison pair of waveforms of the showing both CCM (upper) and DCM operating modes.
Figure 55. CCM and DCM Operating Modes
V
IN
= 12V, V
O
= 3.3V, I
O
= 3A/0.3A 2 μsec/div
The approximate formula for determining the DCM/CCM boundary is as follows:
I
DCB
≊V
O
*(V
IN
–V
O
)/(2*3.3 μH*f
SW(CCM)
*V
IN
) (16)
The inductor internal to the module is 3.3 μH. This value was chosen as a good balance between low and high
input voltage applications. The main parameter affected by the inductor is the amplitude of the inductor ripple
current (I
LR
). I
LR
can be calculated with:
I
LR P-P
=V
O
*(V
IN
- V
O
)/(3.3µH*f
SW
*V
IN
)
where
• V
IN
is the maximum input voltage
• f
SW
is typically 812 kHz (17)
If the output current I
O
is determined by assuming that I
O
= I
L
, the higher and lower peak of I
LR
can be
determined.
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