LTC3601 Linear Technology, LTC3601 Datasheet - Page 11

• Manufacturer Part Number: LTC3601
• Description: 15V Monolithic Synchronous Step-Down Regulator
• Manufacturer: Linear Technology

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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 induc-
tor. 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.
New designs for surface mount inductors are available
from Toko, Vishay, NEC/Tokin, Cooper, Coilcraft, TDK and
Würth Electronik. Table 1 gives a sampling of available
surface mount inductors.
Table 1. Inductor Selection Table
Würth Electronik WE-PD2 Typ MS Series
Vishay IHLP-2020BZ-01 Series
Toko DE3518C Series
Sumida CDRH2D18/HP Series
Cooper SD18 Series
Coilcraft LPS4018 Series
TDK VLS252012 Series
APPLICATIONS INFORMATION
INDUCTANCE
(μH)
0.56
0.82
0.47
0.68
0.56
0.56
0.82
0.47
0.82
0.56
0.47
1.2
1.7
2.2
2.2
1.2
1.7
1.2
1.5
2.2
2.2
1.5
2.2
1.1
1
1
1
(mΩ)
DCR
29.4
34.5
39.8
12.4
50.1
20.1
24.7
126
155
9.5
8.8
14
21
27
36
20
24
30
35
33
39
43
30
40
70
56
88
CURRENT
MAX
3.58
3.24
2.97
2.73
2.55
11.5
6.5
5.4
4.8
3.6
4.2
3.3
2.4
2.9
2.5
4.8
2.8
3.3
2.4
(A)
2.1
3.7
2.7
1.8
10
4
7
2
DIMENSIONS
5.2 × 5.8
5.2 × 5.5
3.5 × 3.7
3.2 × 3.2
5.5 × 5.5
2.5 × 2
(mm)
4 × 4
HEIGHT
(mm)
1.8
1.2
1.8
1.7
2
2
2
C
The input capacitance, C
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:
where I
current. This formula has a maximum at V
where I
is commonly used for design because even signifi cant
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
suffi cient 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 volt-
age transients do not pose an overvoltage hazard to the
part.
The selection of C
tive series resistance (ESR) that is required to minimize
voltage ripple and load step transients. The output ripple,
ΔV
The output ripple is highest at maximum input voltage
since ΔI
placed in parallel may be needed to meet the ESR and
RMS current handling requirements. Dry tantalum, special
polymer, aluminum electrolytic, and ceramic capacitors are
all available in surface mount packages. Special polymer
capacitors offer low ESR but have lower capacitance
density than other types. Tantalum capacitors have the
IN
I
OUT
RMS
and C
V
OUT
, is determined by:
RMS
L
OUT(MAX)
=
increases with input voltage. Multiple capacitors
OUT
I
< I
OUT MAX
= I
Selection
(
L
OUT
OUT
8 • f • C
equals the maximum average output
/2. This simple worst-case condition
)
ESR + 1
is primarily determined by the effec-
V
IN
OUT
, is needed to fi lter the trapezoidal
OUT
(
V
V
IN
IN
–
V
OUT
LTC3601
)
IN
= 2V
11
OUT
3601f
,