ltc3612 Linear Technology Corporation, ltc3612 Datasheet - Page 15

• Manufacturer Part Number: ltc3612
• Description: 3a, 4mhz Monolithic Synchronous Step-down Dc/dc Converter
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
ripple current and consequently output voltage ripple. Do
not allow a ferrite core to saturate!
Different core materials and shapes will change the size/cur-
rent and price/current relationship of an inductor. 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. Table 1 shows
some typical surface mount inductors that work well in
LTC3612 applications.
Input Capacitor (C
In continuous mode, the source current of the top P-chan-
nel MOSFET is a square wave of duty cycle V
prevent large voltage transients, a low ESR capacitor sized
for the maximum RMS current must be used at V
The maximum RMS capacitor current is given by:
This formula has a maximum at V
I
for design because even signifi cant 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.
Output Capacitor (C
The selection of C
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 satisfi ed,
the capacitance is adequate for fi ltering. The output ripple
ΔV
OUT
I
OUT
ΔV
RMS
/2. This simple worst-case condition is commonly used
OUT
is determined by:
=I
≤ ΔI
OUT(MAX)
L
• ESR +
⎛
⎝ ⎜
OUT
IN
•
OUT
) Selection
V
is typically driven by the required
V
OUT
) Selection
IN
8 • f
•
SW
⎛
⎝ ⎜
IN
1
V
• C
V
OUT
= 2V
IN
OUT
– 1
OUT
⎞
⎠ ⎟
⎞
⎠ ⎟
, where I
OUT
/V
IN
RMS
IN
.
. To
=
Table 1. Representative Surface Mount Inductors
Vishay IHLP-2525AH-01 Series
Vishay IHLP-1616BZ-01 Series
Sumida CDMC6D28 Series
NEC/Tokin MPLC0730L Series
Cooper HCP0703 Series
Wurth Electronik WE-HC744312 Series
Coilcraft DO1813H Series
Coilcraft v Series
INDUCTANCE
(μH)
0.33
0.47
0.68
0.82
0.22
0.47
1.00
0.47
0.68
0.47
0.75
0.22
0.47
0.68
0.82
0.25
0.47
0.72
0.33
0.56
0.27
0.35
1.0
0.3
1.0
1.0
1.5
1.0
1.5
0.4
1
(mΩ)
DCR
10.0
10.5
3.2
4.2
5.4
8.8
4.5
7.5
9.0
2.8
4.2
5.5
8.0
9.6
2.5
3.4
7.5
9.5
0.1
0.1
0.1
13
15
18
18
37
10
7
9
8
4
CURRENT (A) DIMENSIONS (mm)
MAX
11.5
15.4
13.6
11.3
16.6
12.2
10.6
8.5
8.8
7.7
12
11
24
23
17
15
13
11
61
18
16
12
11
10
14
11
9
8
7
9
8
6.7 × 7.25
6.7 × 7.25
6.7 × 7.25
6.7 × 7.25
4.3 × 4.7
4.3 × 4.7
4.3 × 4.7
6.9 × 7.7
6.9 × 7.7
6.9 × 7.7
6.9 × 7.3
8.9 × 6.1
8.9 × 6.1
7.5 × 6.7
7.5 × 6.7
7.5 × 6.7
6.7 × 7
6.7 × 7
6.7 × 7
6.7 × 7
6.7 × 7
7 × 7.3
7 × 7.3
7 × 7.3
7 × 7.3
7 × 7.3
7 × 7.7
7 × 7.7
7 × 7.7
7 × 7.7
7 × 7.7
LTC3612
HEIGHT
15
(mm)
1.8
1.8
1.8
1.8
1.8
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
2
2
2
3
3
3
3
5
5
3
3
3
3612f