Datasheet

LTC3618
14
3618fc
For more information www.linear.com/LTC3618
The largest ripple current occurs at the highest V
IN
. To
guarantee that the ripple current stays below a specified
maximum, the inductor value should be chosen according
to the following equation:
L =
V
OUT
f
SW
• I
L(MAX)
• 1–
V
OUT
V
IN(MAX)
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. Actual core loss is independent of core size
for fixed inductor value, but it is very dependent on the
inductance selected. As the inductance increases, core
losses decrease. Unfortunately, increased inductance
requires more turns of wire, and therefore, copper losses
will increase.
Ferrite designs have very low core losses and are pre
-
ferred at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
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 and consequent output voltage ripple. Do
not allow a ferrite core to saturate!
Different core materials and shapes will change the size/
current and
price/current relationship of an inductor.
T
oroid or shielded
pot cores in ferrite or permalloy materials
are small and do not 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 field/EMI requirements. Table 1 shows
some typical surface
mount inductors that work well in
LTC3618 applications.
Input Capacitor C
IN
Selection
In continuous mode, the source current of the top P-
channel MOSFET is a square wave of duty cycle V
OUT
/V
IN
.
To prevent large voltage transients, a low ESR capacitor
sized for the maximum RMS current must be used for C
IN
.
The maximum RMS capacitor current is given by:
I
RMS
= I
OUT(MAX)
•
V
OUT
V
IN
•
V
IN
V
OUT
– 1






applicaTions inForMaTion
Table 1. Representative Surface Mount Inductors
INDUCTANCE
(µH)
DCR
(mΩ)
MAX
CURRENT (A)
DIMENSIONS
(mm)
HEIGHT
(mm)
Vishay IHLP-2020BZ-01
0.33
7.6 25 5.18 × 5.49 2
0.47 8.9 21 5.18 × 5.49 2
0.68 11.2 15 5.18 × 5.49 2
1 18.9 16 5.18 × 5.49 2
Toko DE3518C Series
0.22 8 24 4.3 × 4.7 2
Sumida CDMC6D28 Series
0.3 3.2 15.4 6.7 × 7.25 3
0.47 4.2 13.6 6.7 × 7.25 3
0.68 5.4 11.3 6.7 × 7.25 3
1 8.8 8.8 6.7 × 7.25 3
NEC/Tokin MPLC0730L Series
0.47 4.5 16.6 6.9 × 7.7 3.0
0.75 7.5 12.2 6.9 × 7.7 3.0
1.0 9.0 10.6 6.9 × 7.7 3.0
Coilcraft DO1813H Series
0.33 4 10 8.9 × 6.1 5
0.56 10 7.7 8.9 × 6.1 5
Coilcraft SLC7530 Series
0.27 0.1 14 7.5 × 6.7 3
0.35 0.1 11 7.5 × 6.7 3
0.4 0.1 8 7.5 × 6.7 3