LTC3615 LINER [Linear Technology], LTC3615 Datasheet - Page 17

• Manufacturer Part Number: LTC3615
• Description: Dual 4MHz, 3A Synchronous Step-Down DC/DC Converter
• Manufacturer: LINER [Linear Technology]

applicaTions inForMaTion
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.
Toroid 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
LTC3615 applications.
Input Capacitor C
In continuous mode, the source current of the top P-chan-
nel MOSFET is a square wave of duty cycle V
prevent large voltage transients, a low ESR capacitor sized
for the maximum RMS current must be used for C
The maximum RMS capacitor current is given by:
I
RMS
=
I
OUT MAX
(
IN
)
•
Selection
V
V
OUT
IN
•



V
V
OUT
IN
– 1



OUT
/V
IN
IN
.
. To
This formula has a maximum at V
I
for design because even significant 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 derate the capacitor,
or choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to meet
size or height requirements in the design.
Table 1. Representative Surface Mount Inductors
Vishay IHLP-2020BZ-01
Toko DE3518C Series
Sumida CDMC6D28 Series
NEC/Tokin MPLC0730L Series
Coilcraft DO1813H Series
Coilcraft SLC7530 Series
INDUCTANCE
OUT
(µH)
0.33
0.47
0.68
0.22
0.47
0.68
0.47
0.75
0.33
0.56
0.27
0.35
/2. This simple worst-case condition is commonly used
0.3
1.0
0.4
1
1
(mΩ)
DCR
11.2
18.9
7.6
8.9
3.2
4.2
5.4
8.8
4.5
7.5
9.0
0.1
0.1
0.1
10
8
4
CURRENT (A)
MAX
15.4
13.6
11.3
16.6
12.2
10.6
8.8
7.7
25
21
15
16
24
10
14
11
8
IN
DIMENSIONS
5.18 × 5.49
5.18 × 5.49
5.18 × 5.49
5.18 × 5.49
6.7 × 7.25
6.7 × 7.25
6.7 × 7.25
6.7 × 7.25
= 2V
4.3 × 4.7
6.9 × 7.7
6.9 × 7.7
6.9 × 7.7
8.9 × 6.1
8.9 × 6.1
7.5 × 6.7
7.5 × 6.7
7.5 × 6.7
(mm)
OUT
LTC3615
, where I
HEIGHT

(mm)
3.0
3.0
3.0
RMS
2
2
2
2
2
3
3
3
3
5
5
3
3
3
3615f
=