LTC3615 Datasheet, PDF (17/32 Pages) Linear Technology – Dual 4MHz, 3A Synchronous Step-Down DC/DC Converter

LTC3615
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 Selection
In continuous mode, the source current of the top P-chan-
nel 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_
This formula has a maximum at V_IN = 2V_OUT, where I_RMS =
I_OUT/2. This simple worst-case condition is commonly used
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
INDUCTANCE DCR
MAX
DIMENSIONS
(µH)
(mΩ) CURRENT (A)
(mm)
Vishay IHLP-2020BZ-01
0.33
7.6
25
5.18 × 5.49
0.47
8.9
21
5.18 × 5.49
0.68
11.2
15
5.18 × 5.49
1
18.9
16
5.18 × 5.49
Toko DE3518C Series
0.22
8
24
4.3 × 4.7
Sumida CDMC6D28 Series
0.3
3.2
15.4
6.7 × 7.25
0.47
4.2
13.6
6.7 × 7.25
0.68
5.4
11.3
6.7 × 7.25
1
8.8
8.8
6.7 × 7.25
NEC/Tokin MPLC0730L Series
0.47
4.5
16.6
6.9 × 7.7
0.75
7.5
12.2
6.9 × 7.7
1.0
9.0
10.6
6.9 × 7.7
Coilcraft DO1813H Series
0.33
4
10
8.9 × 6.1
0.56
10
7.7
8.9 × 6.1
Coilcraft SLC7530 Series
0.27
0.1
14
7.5 × 6.7
0.35
0.1
11
7.5 × 6.7
0.4
0.1
8
7.5 × 6.7
HEIGHT
(mm)
2
2
2
2
2
3
3
3
3
3.0
3.0
3.0
5
5
3
3
3
3615f
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