Datasheet

L =
I
LR
x f
SW
x V
IN
V
OUT
x (V
IN
- V
OUT
)
1.3 x 10
-10
V
IN(MAX)
x 150 ns
R
ON
t
LM3102
www.ti.com SNVS515G –SEPTEMBER 2007–REVISED JANUARY 2012
R
ON
: Equation 2 can be used to select R
ON
if a desired operating frequency is selected. But the minimum value
of R
ON
is determined by the minimum on-time. It can be calculated as follows:
(8)
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 27.
On the other hand, the minimum off-time of 260 ns can limit the maximum duty ratio. Larger R
ON
should be
selected in any application requiring large duty ratio.
Figure 27. Maximum V
IN
for selected R
ON
L: The main parameter affected by the inductor is the amplitude of inductor current ripple (I
LR
). Once I
LR
is
selected, L can be determined by:
where
• V
IN
is the maximum input voltage
• f
SW
is determined from Equation 2 (9)
where and .
If the output current I
OUT
is determined, by assuming that I
OUT
= I
L
, the higher and lower peak of I
LR
can be
determined. Beware that the higher peak of I
LR
should not be larger than the saturation current of the inductor
and current limits of the main and synchronous MOSFETs. Also, the lower peak of I
LR
must be positive if CCM
operation is required.
Figure 28. Inductor selection for V
OUT
= 3.3V
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