Datasheet

'i
L
=
(V
IN
- V
OUT
) x D
L x f
SW
I
DCB
=
(V
IN
- V
OUT
) x D
2 x L x f
SW
LMZ12010
www.ti.com SNVS667G –FEBRUARY 2010–REVISED OCTOBER 2013
Figure 53. Pre-Biased Startup
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). When operating in DCM, inductor
current is maintained to an average value equaling Iout . In DCM the low-side switch will turn off when the
inductor current falls to zero, this causes the inductor current to resonate. Although it is in DCM, the current is
allowed to go slightly negative to charge the bootstrap capacitor.
In CCM, current flows through the inductor through the entire switching cycle and never falls to zero during the
off-time.
Following is a comparison pair of waveforms showing both the CCM (upper) and DCM operating modes.
Figure 54. CCM and DCM Operating Modes
V
IN
= 12V, V
O
= 3.3V, I
O
= 3A/0.3A
The approximate formula for determining the DCM/CCM boundary is as follows:
(17)
The inductor internal to the module is 2.2 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 (Δi
L
). Δi
L
can be calculated with:
(18)
Where V
IN
is the maximum input voltage and f
SW
is typically 359 kHz.
If the output current I
OUT
is determined by assuming that I
OUT
= I
L
, the higher and lower peak of Δi
L
can be
determined.
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