Datasheet

0 10 20 30 40
0
5
10
15
20
25
INDUCTANCE (PH)
V
IN
(V)
R
ON
= 25 k:
R
ON
= 50 k:
R
ON
= 100 k:
INDUCTANCE (PH)
0
10 20 30 40
V
IN
(V)
0.0
5.0
10.0
15.0
20.0
25.0
R
ON
= 25 k:
R
ON
= 50 k:
R
ON
= 100 k:
L =
I
OR
x F
S
x V
IN
V
OUT
x (V
IN
- V
OUT
)
LM3100
SNVS421G –JANUARY 2006–REVISED APRIL 2013 www.ti.com
I
OR(MAX1)
= 2 x I
O(min)
(9)
or
I
OR(MAX2)
= 2 x (1.9A - I
O(max)
) (10)
The lesser of the two ripple amplitudes calculated above is then used in the following equation:
(11)
where V
IN
is the maximum input voltage and Fs is determined from Equation 1. This provides a value for L. The
next larger standard value should be used. L should be rated for the I
PK
current level shown in Figure 21.
Figure 23. Inductor Selector for V
OUT
= 3.3V
Figure 24. Inductor Selector for V
OUT
= 0.8V
C
VCC
: The capacitor on the V
CC
output provides not only noise filtering and stability, but also prevents false
triggering of the V
CC
UVLO at the buck switch on/off transitions. For this reason, C
VCC
should be no smaller than
680 nF for stability, and should be a good quality, low ESR, ceramic capacitor.
C
O
and C
O3
: C
O
should generally be no smaller than 10 µF. Experimentation is usually necessary to determine
the minimum value for C
O
, as the nature of the load may require a larger value. A load which creates significant
transients requires a larger value for C
O
than a fixed load.
C
O3
is a small value ceramic capacitor to further suppress high frequency noise at V
OUT
. A 47nF is
recommended, located close to the LM3100.
C
IN
and C
IN3
: C
IN
’s purpose is to supply most of the switch current during the on-time, and limit the voltage ripple
at V
IN
, assume the voltage source feeding V
IN
has an output impedance greater than zero. If the source’s
dynamic impedance is high (effectively a current source), C
IN
supplies the average input current, but not the
ripple current.
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