LTC3547
APPLICATIO S I FOR ATIO
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 influence 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
:
⎛
⎞
V
V
∆I
L
=
OUT
•
⎜
1
−
OUT
⎟
f
O
• L
⎝
V
IN
⎠
(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 efficiency in
the upper range of low current operation. Furthermore,
lower inductance values will cause the bursts to occur
with increased frequency.
V
IN
2.5V TO 5.5V
C1
RUN2 V
IN
RUN1
L2
V
OUT2
C
F2
LTC3547
SW2
SW1
L1
C
F1
V
OUT1
C
OUT2
R4
Figure 1. LTC3547 General Schematic
3547fa
8
U
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 field/EMI requirements,
than on what the LTC3547 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3547 applications.
Table 1. Representative Surface Mount Inductors
MANU-
FACTURER
Taiyo Yuden
PART NUMBER
CB2016T2R2M
CB2012T2R2M
CB2016T3R3M
ELT5KT4R7M
CDRH2D18/LD
LQH32CN4R7M23
NR30102R2M
NR30104R7M
FDKMIPF2520D
FDKMIPF2520D
FDKMIPF2520D
VLF3010AT4R7-
MR70
VLF3010AT3R3-
MR87
VLF3010AT2R2-
M1RD
MAX DC
VALUE CURRENT
2.2µH
2.2µH
3.3µH
4.7µH
4.7µH
4.7µH
2.2µH
4.7µH
4.7µH
3.3µH
2.2µH
4.7µH
3.3µH
2.2µH
510mA
530mA
410mA
950mA
630mA
450mA
1100mA
750mA
1100mA
1200mA
1300mA
700mA
870mA
1000mA
DCR
HEIGHT
0.13Ω 1.6mm
0.33Ω 1.25mm
0.27Ω 1.6mm
0.2Ω
0.086Ω
0.2Ω
0.1Ω
0.19Ω
0.11Ω
0.1Ω
0.08Ω
0.24Ω
0.17Ω
0.12Ω
1.2mm
2mm
2mm
1mm
1mm
1mm
1mm
1mm
1mm
1mm
1mm
Panasonic
Sumida
Murata
Taiyo Yuden
FDK
TDK
V
FB2
R3
GND
V
FB1
R1
R2
C
OUT1
3547 F01
W
U
U
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
