LTC3604 Datasheet, PDF (11/24 Pages) Linear Technology – 2.5A, 15V Monolithic Synchronous Step-Down Regulator

LTC3604
APPLICATIONS INFORMATION
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 field/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
INDUCTANCE DCR
MAX
(μH)
(mΩ) CURRENT (A)
Würth Electronik WE-PD2 Typ MS Series
0.27
5.3
8.2
0.56
9.5
6.5
0.82
14
5.4
1.2
21
4.8
1.7
27
4
2.2
36
3.6
Vishay IHLP-2020BZ-01 Series
0.22
5.2
15
0.33
8.2
12
0.47
8.8
11.5
0.68
12.4
10
1
20
7
2.2
50.1
4.2
Toko DE3518C Series
0.56
24
3.3
Sumida CDRH2D18/HP Series
0.36
29
4.6
0.56
33
3.7
0.82
39
2.9
Cooper SD18 Series
0.47
20.1
3.58
0.82
24.7
3.24
1.2
29.4
2.97
1.5
34.5
2.73
Coilcraft LPS4018 Series
0.56
30
4.8
1
40
2.8
2.2
70
2.7
TDK VLS252012 Series
0.47
56
3.3
DIMENSIONS
(mm)
5.2 × 5.8
5.2 × 5.5
3.5 × 3.7
3.2 × 3.2
5.5 × 5.5
4×4
2.5 × 2
HEIGHT
(mm)
2
2
1.8
2
1.8
1.7
1.2
C_IN and C_OUT Selection
The input capacitance, C_IN, is needed to filter the trapezoidal
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:
( ) _{IRMS = IOUT(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 LTC3604 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
voltage ripple and load step transients. The output ripple,
ΔV_OUT, is determined by:
ΔV_OUT
<
ΔI_L
⎛
⎝⎜ ^ESR
+
8
•
f
1
• C_OUT
⎞
⎠⎟
The output ripple is highest at maximum input voltage
since ΔI_L increases with input voltage. Multiple capacitors
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
3604f
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