LTC3407-3 データシートの表示（PDF） - Linear Technology
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LTC3407-3
Linear Technology
LTC3407-3 Datasheet PDF : 16 Pages
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LTC3407-3
APPLICATIO S I FOR ATIO
A general LTC3407-3 application circuit is shown in
Figure 2. External component selection is driven by the
load requirement, and begins with the selection of the
inductor L. Once the inductor is chosen, CIN and COUT can
be selected.
Inductor Selection
Although the inductor does not influence the operating
frequency, the inductor value has a direct effect on ripple
current. The inductor ripple current ∆IL decreases with
higher inductance and increases with higher VIN or VOUT:
∆IL
=
VOUT
fO • L
•
⎛
⎝⎜1–
VOUT
VIN
⎞
⎠⎟
Accepting larger values of ∆IL allows the use of low
inductances, but results in higher output voltage ripple,
greater core losses, and lower output current capability.
A reasonable starting point for setting ripple current is
∆IL = 0.3 • ILIM, where ILIM is the peak switch current limit.
The largest ripple current ∆IL occurs at the maximum
input voltage. To guarantee that the ripple current stays
below a specified maximum, the inductor value should be
chosen according to the following equation:
L
=
VOUT
fO • ∆IL
⎛
• ⎝⎜1–
VOUT ⎞
VIN(MAX) ⎠⎟
The inductor value will also have an effect on Burst Mode
operation. The transition from low current operation
begins when the peak inductor current falls below a level
set by the burst clamp. Lower inductor values result in
higher ripple current which causes this to occur at lower
load currents. This causes a dip in efficiency in the upper
range of low current operation. In Burst Mode operation,
lower inductance values will cause the burst frequency to
increase.
Inductor Core Selection
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 mate-
rials are small and don’t radiate much energy, but gener-
ally cost more than powdered iron core inductors with
similar electrical characterisitics. The choice of which
style inductor to use often depends more on the price vs
size requirements and any radiated field/EMI require-
ments than on what the LTC3407-3 requires to operate.
8
Table 1 shows some typical surface mount inductors that
work well in LTC3407-3 applications.
Table 1. Representative Surface Mount Inductors
PART
NUMBER
VALUE DCR
MAX DC
SIZE
(µH) (Ω MAX) CURRENT (A) W × L × H (mm3)
Sumida
2.2
CDRH3D16
3.3
4.7
0.075
0.110
0.162
1.20
3.8 × 3.8 × 1.8
1.10
0.90
Sumida
1.5
CDRH2D11
2.2
0.068
0.170
0.900
0.780
3.2 × 3.2 × 1.2
Sumida
2.2
0.116
CMD4D11
3.3
0.174
0.950
0.770
4.4 × 5.8 × 1.2
Murata
LQH32CN
1.0
0.060
2.2
0.097
1.00
2.5 × 3.2 × 2.0
0.79
Toko
D312F
2.2
0.060
3.3
0.260
1.08
2.5 × 3.2 × 2.0
0.92
Panasonic
3.3
0.17
ELT5KT
4.7
0.20
1.00
4.5 × 5.4 × 1.2
0.95
Input Capacitor (CIN) Selection
In continuous mode, the input current of the converter is
a square wave with a duty cycle of approximately VOUT/
VIN. To prevent large voltage transients, a low equivalent
series resistance (ESR) input capacitor sized for the maxi-
mum RMS current must be used. The maximum RMS
capacitor current is given by:
IRMS ≈IMAX
VOUT(VIN – VOUT )
VIN
where the maximum average output current IMAX equals
the peak current minus half the peak-to-peak ripple cur-
rent, IMAX = ILIM – ∆IL/2.
This formula has a maximum at VIN = 2VOUT, where IRMS
= IOUT/2. This simple worst-case is commonly used to
design because even significant deviations do not offer
much relief. Note that capacitor manufacturer’s ripple
current ratings are often based on only 2000 hours life-
time. This makes it advisable to further derate the capaci-
tor, or choose a capacitor rated at a higher temperature
than required. Several capacitors may also be paralleled to
meet the size or height requirements of the design. An
additional 0.1µF to 1µF ceramic capacitor is also recom-
mended on VIN for high frequency decoupling, when not
using an all ceramic capacitor solution.
34073fa
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