Datasheet

L =
I
LR
x f
SW
x V
IN
V
OUT
x (V
IN
- V
OUT
)
LM3103
SNVS523F –SEPTEMBER 2007–REVISED APRIL 2013 www.ti.com
If R
ON
calculated from Equation 2 is smaller than the minimum value determined in Equation 8, a lower frequency
should be selected to re-calculate R
ON
by Equation 2. Alternatively, V
IN(MAX)
can also be limited in order to keep
the frequency unchanged. The relationship of V
IN(MAX)
and R
ON
is shown in Figure 23.
On the other hand, the minimum off-time of 240 ns can limit the maximum duty ratio. This may be significant at
low V
IN
. A larger R
ON
should be selected in any application requiring a large duty ratio.
Figure 23. Maximum V
IN
for selected R
ON
L: The main parameter affected by the inductor is the amplitude of the inductor current ripple (I
LR
), which is
recommended to be greater than 0.3A. Once I
LR
is selected, L can be determined by:
(9)
where V
IN
is the input voltage and f
SW
is determined from Equation 2.
If the output current I
OUT
is known, by assuming that I
OUT
= I
L
, the peak and valley of I
LR
can be determined.
Beware that the peak of I
LR
should not be larger than the saturation current of the inductor and the current rating
of the main and synchronous MOSFETs. Also, the valley of I
LR
must be positive if CCM operation is required.
Figure 24. Inductor selection for V
OUT
= 3.3V
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