Datasheet

LTC3601
11
3601fc
For more information www.linear.com/LTC3601
APPLICATIONS INFORMATION
Once the value for L is known the type of inductor must
be selected. Actual core loss is independent of core size
for a fixed inductor value but is very dependent on the
inductance selected. As the inductance increases, core loss
decreases. Unfortunately, increased inductance requires
more turns of wire leading to increased 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 saturation.
Ferrite core materials saturate “hard,” meaning the induc-
tance collapses abruptly when the peak design current is
exceeded. This collapse will result in an abrupt increase
in inductor ripple current, so it is important to ensure the
core will not 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 materi-
als 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 field/EMI requirements. New designs for sur-
face mount inductors are available from Toko, Vishay, NEC/
Tokin, Cooper, Coilcraft, TDK and Wurth Electronik. Table
1 gives a sampling of available surface mount inductors.
C
IN
and C
OUT
Selection
The input capacitance, C
IN
, is needed to filter 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
where I
OUT(MAX)
equals the maximum average output
current. 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 cur-
rent 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.
Several capacitors may be paralleled to meet the require-
ments of the design. For low input voltage applications
sufficient bulk input capacitance is needed to minimize
transient effects during output load changes. Even though
the LTC3601 design includes an overvoltage protection
circuit, care must always be taken to ensure input voltage
transients do not pose an overvoltage hazard to the part.
The selection of C
OUT
is primarily determined by the effec-
tive series resistance (ESR) that is required to minimize
Table 1. Inductor Selection Table
INDUCTANCE
(µH)
DCR
(mΩ)
MAX
CURRENT (A)
DIMENSIONS
(mm)
HEIGHT
(mm)
Würth Electronik WE-PD2 Typ MS Series
0.56
0.82
1.2
1.7
2.2
9.5
14
21
27
36
6.5
5.4
4.8
4
3.6
5.2 × 5.8 2
Vishay IHLP-2020BZ-01 Series
0.47
0.68
1
2.2
8.8
12.4
20
50.1
11.5
10
7
4.2
5.2 × 5.5 2
Toko DE3518C Series
0.56
1.2
1.7
24
30
35
3.3
2.4
2.1
3.5 × 3.7 1.8
Sumida CDRH2D18/HP Series
0.56
0.82
1.1
33
39
43
3.7
2.9
2.5
3.2 × 3.2 2
Cooper SD18 Series
0.47
0.82
1.2
1.5
2.2
20.1
24.7
29.4
34.5
39.8
3.58
3.24
2.97
2.73
2.55
5.5 × 5.5 1.8
Coilcraft LPS4018 Series
0.56
1
2.2
30
40
70
4.8
2.8
2.7
4 × 4 1.7
TDK VLS252012 Series
0.47
1
1.5
2.2
56
88
126
155
3.3
2.4
2
1.8
2.5 × 2 1.2
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