LTC3612_15 Datasheet, PDF (16/30 Pages) Linear Technology – 3A, 4MHz Monolithic Synchronous Step-Down DC/DC Converter

LTC3612
Applications Information
inductor ripple current and consequently output voltage
ripple. Do not allow a ferrite core to saturate and select
external inductors respecting the temperature range of
the application!
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
LTC3612 applications.
Input Capacitor (C_IN) Selection
In continuous mode, the source current of the top P-
channel 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 at V_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. Generally select the capacitors respecting the
temperature range of the application! Several capacitors
may also be paralleled to meet size or height requirements
in the design.
Output Capacitor (C_OUT ) Selection
The selection of C_OUT is typically driven by the required
ESR to minimize voltage ripple and load step transients
(low ESR ceramic capacitors are discussed in the next
section). Typically, once the ESR requirement is satisfied,
Table 1. Representative Surface Mount Inductors
INDUCTANCE DCR
MAX
DIMENSIONS
(µH)
(mΩ) CURRENT (A)
(mm)
Vishay IHLP-2525AH-01 Series
0.33
7
12
6.7 × 7
0.47
9
11
6.7 × 7
0.68
13
9
6.7 × 7
0.82
15
8
6.7 × 7
1.0
18
7
6.7 × 7
Vishay IHLP-1616BZ-01 Series
0.22
8
24
4.3 × 4.7
0.47
18
11.5
4.3 × 4.7
1.00
37
8.5
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
Cooper HCP0703 Series
0.22
2.8
23
7 × 7.3
0.47
4.2
17
7 × 7.3
0.68
5.5
15
7 × 7.3
0.82
8.0
13
7 × 7.3
1.0
10.0
11
7 × 7.3
1.5
9.6
61
6.9 × 7.3
Würth Elektronik WE-HC744312 Series
0.25
2.5
18
7 × 7.7
0.47
3.4
16
7 × 7.7
0.72
7.5
12
7 × 7.7
1.0
9.5
11
7 × 7.7
1.5
10.5
9
7 × 7.7
Coilcraft DO1813H Series
0.33
4
10
8.9 × 6.1
0.56
10
7.7
8.9 × 6.1
Coilcraft v 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)
1.8
1.8
1.8
1.8
1.8
2
2
2
3
3
3
3
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.2
3.8
3.8
3.8
3.8
3.8
5
5
3
3
3
3612fc
16
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