LTC3616 Datasheet, PDF (15/28 Pages) Linear Technology – 6A, 4MHz Monolithic Synchronous Step-Down DC/DC Converter

LTC3616
APPLICATIONS INFORMATION
Ferrite designs have very low core losses and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates “hard,” meaning that inductance
collapses abruptly when the peak design current is ex-
ceeded. This results in an abrupt increase in inductor
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 field/EMI requirements. Table 1 shows
some typical surface mount inductors that work well in
LTC3616 applications.
Input Capacitor (C_IN) Selection
In continuous mode, the source current of the top P-chan-
nel 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. Several capacitors may also be paralleled to meet
size or height requirements in the design.
Table 1. Representative Surface Mount Inductors
INDUCTANCE DCR
MAX
DIMENSIONS
(μH)
(mΩ) CURRENT (A)
(mm)
Vishay IHLP-2525CZ-01
0.10
1.5
60
6.5 × 6.9
0.15
1.9
52
6.5 × 6.9
0.20
2.4
41
6.5 × 6.9
0.22
2.5
40
6.5 × 6.9
0.33
3.5
30
6.5 × 6.9
0.47
4
26
6.5 × 6.9
Sumida CDMC6D28 Series
0.2
2.5
21.7
7.25 × 4.4
0.3
3.2
15.4
7.25 × 4.4
0.47
4.2
13.6
7.25 × 4.4
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
Wurth Electronik WE-HC744312 Series
0.25
2.5
18
7 × 7.7
0.47
3.4
16
7 × 7.7
Coilcraft SLC7530 Series
0.100
0.123
20
7.5 × 6.7
0.188
0.100
21
7.5 × 6.7
0.272
0.100
14
7.5 × 6.7
0.350
0.100
11
7.5 × 6.7
0.400
0.100
8
7.5 × 6.7
HEIGHT
(mm)
3
3
3
3
3
3
3
3
3
3.0
3.0
3.0
3.8
3.8
3
3
3
3
3
3616f
15