Datasheet

LTC3633A-2/LTC3633A-3
13
3633a23f
APPLICATIONS INFORMATION
A reasonable starting point is to choose a ripple current
that is about 40% of I
OUT(MAX)
. Note that the largest ripple
current occurs at the highest PV
IN
. Exceeding 60% of
I
OUT(MAX)
is not recommended. To guarantee that ripple
current does not exceed a specifi ed maximum, the induc-
tance should be chosen according to:
L =
V
OUT
f•ΔI
L(MAX)
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
1–
V
OUT
V
IN(MAX)
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
Once the value for L is known, the type of inductor must
be selected. Actual core loss is independent of core size
for a fi xed inductor value, but is very dependent on the
inductance selected. As the inductance increases, core
losses decrease. Unfortunately, increased inductance
requires more turns of wire, leading to increased DCR
and copper loss.
Ferrite designs exhibit very low core loss and are pre-
ferred at high switching frequencies, so design goals
can concentrate on copper loss and preventing satura-
tion. Ferrite core material saturates “hard”, which means
that inductance collapses abruptly when the peak design
current is exceeded. This results in an abrupt increase in
inductor ripple current, so it is important to ensure that
the core will not saturate.
Different core materials and shapes will change the size/cur-
rent and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and don’t radiate much energy, but generally cost
more than powdered iron core inductors with similar
characteristics. The choice of which style inductor to use
mainly depends on the price versus size requirements
and any radiated fi eld/EMI requirements. Table 1 gives a
sampling of available surface mount inductors.
Table 1. Inductor Selection Table
INDUCTANCE
(μH)
DCR
(mΩ)
MAX
CURRENT
(A)
DIMENSIONS
(mm)
HEIGHT
(mm)
Würth Electronik WE-HC 744312 Series
0.25
0.47
0.72
1.0
1.5
2.5
3.4
7.5
9.5
10.5
18
16
12
11
9
7 × 7.7 3.8
Vishay IHLP-2020BZ-01 Series
0.22
0.33
0.47
0.68
1
5.2
8.2
8.8
12.4
20
15
12
11.5
10
7
5.2 × 5.5 2
Toko FDV0620 Series
0.20
0.47
1.0
4.5
8.3
18.3
12.4
9.0
5.7
7 × 7.7 2.0
Coilcraft D01813H Series
0.33
0.56
1.2
4
10
17
10
7.7
5.3
6 × 8.9 5.0
TDK RLF7030 Series
1.0
1.5
8.8
9.6
6.4
6.1
6.9 × 7.3 3.2
C
IN
and C
OUT
Selection
The input capacitance, C
IN
, is needed to fi lter the trapezoi-
dal wave current at the drain of the top power MOSFET.
To prevent large voltage transients from occurring, a low
ESR input capacitor sized for the maximum RMS current is
recommended. The maximum RMS current is given by:
I
RMS
=I
OUT(MAX)
V
OUT
V
IN
− V
OUT
()
V
IN
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
≅ I
OUT
/2. This simple worst case condition is com-
monly used for design because even signifi cant deviations
do not offer much relief. Note that ripple current ratings
from capacitor manufacturers are often based on only
2000 hours of life which makes it advisable to further de-
rate the capacitor, or choose a capacitor rated at a higher
temperature than required.