Datasheet

LTC3547
8
3547fa
APPLICATIO S I FOR ATIO
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A general LTC3547 application circuit is shown in
Figure 1. External component selection is driven by the
load requirement, and begins with the selection of the
inductor L. Once the inductor is chosen, C
IN
and C
OUT
can be selected.
Inductor Selection
Although the inductor does not infl uence the operat-
ing frequency, the inductor value has a direct effect on
ripple current. The inductor ripple current ΔI
L
decreases
with higher inductance and increases with higher V
IN
or V
OUT
:
∆I
V
fL
V
V
L
OUT
O
OUT
IN
=−
⎛
⎝
⎜
⎞
⎠
⎟
•
•1
(1)
Accepting larger values of ΔI
L
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 40% of the maximum output load current. So, for a
300mA regulator, ΔI
L
= 120mA (40% of 300mA).
The inductor value will also have an effect on Burst Mode
operation. The transition to low current operation begins
when the peak inductor current falls below a level set by
the internal burst clamp. Lower inductor values result in
higher ripple current which causes the transition to occur
at lower load currents. This causes a dip in effi ciency in
the upper range of low current operation. Furthermore,
lower inductance values will cause the bursts to occur
with increased frequency.
Inductor Core Selection
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 do not radiate much energy, but
generally cost more than powdered iron core inductors
with similar electrical characteristics. The choice of which
style inductor to use often depends more on the price vs
size requirements, and any radiated fi eld/EMI requirements,
than on what the LTC3547 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3547 applications.
Figure 1. LTC3547 General Schematic
C
F2
C
F1
V
IN
2.5V TO 5.5V
V
OUT2
V
OUT1
3547 F01
R3 R1
R4
L2 L1
R2
C
OUT2
C1
C
OUT1
V
IN
RUN2 RUN1
LTC3547
V
FB2
SW2
SW1
V
FB1
GND
Table 1. Representative Surface Mount Inductors
MANU-
FACTURER PART NUMBER VALUE
MAX DC
CURRENT DCR HEIGHT
Taiyo Yuden CB2016T2R2M
CB2012T2R2M
CB2016T3R3M
2.2µH
2.2µH
3.3µH
510mA
530mA
410mA
0.13Ω
0.33Ω
0.27Ω
1.6mm
1.25mm
1.6mm
Panasonic ELT5KT4R7M 4.7µH 950mA 0.2Ω 1.2mm
Sumida CDRH2D18/LD 4.7µH 630mA 0.086Ω 2mm
Murata
LQH32CN4R7M23 4.7µH 450mA 0.2Ω 2mm
Taiyo Yuden NR30102R2M
NR30104R7M
2.2µH
4.7µH
1100mA
750mA
0.1Ω
0.19Ω
1mm
1mm
FDK FDKMIPF2520D
FDKMIPF2520D
FDKMIPF2520D
4.7µH
3.3µH
2.2µH
1100mA
1200mA
1300mA
0.11Ω
0.1Ω
0.08Ω
1mm
1mm
1mm
TDK VLF3010AT4R7-
MR70
VLF3010AT3R3-
MR87
VLF3010AT2R2-
M1RD
4.7µH
3.3µH
2.2µH
700mA
870mA
1000mA
0.24Ω
0.17Ω
0.12Ω
1mm
1mm
1mm