LTC3547 LINER [Linear Technology], LTC3547 Datasheet - Page 8

• Manufacturer Part Number: LTC3547
• Description: Dual Monolithic 300mA Synchronous Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC3547
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
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
with higher inductance and increases with higher V
or V
Accepting larger values of ΔI
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
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.
8
∆I
OUT
L
=
:
V
f
O
OUT
•
L
• 1
⎛
⎝ ⎜
U
−
L
= 120mA (40% of 300mA).
V
V
OUT
IN
U
V
OUT2
⎞
⎠ ⎟
2.5V TO 5.5V
L
allows the use of low
V
W
C
IN
OUT2
C
R4
Figure 1. LTC3547 General Schematic
F2
IN
L
C1
decreases
U
and C
L2
R3
OUT
RUN2
(1)
SW2
V
FB2
IN
LTC3547
GND
V
IN
Table 1. Representative Surface Mount Inductors
Taiyo Yuden CB2016T2R2M
Panasonic
Sumida
Murata
Taiyo Yuden NR30102R2M
FDK
TDK
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.
RUN1
FACTURER
SW1
V
FB1
MANU-
L1
R1
CB2012T2R2M
CB2016T3R3M
ELT5KT4R7M
CDRH2D18/LD
LQH32CN4R7M23
NR30104R7M
FDKMIPF2520D
FDKMIPF2520D
FDKMIPF2520D
VLF3010AT4R7-
MR70
VLF3010AT3R3-
MR87
VLF3010AT2R2-
M1RD
PART NUMBER
C
R2
F1
3547 F01
VALUE
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
C
V
OUT1
OUT1
CURRENT
MAX DC
1100mA
1100mA
1200mA
1300mA
1000mA
510mA
530mA
410mA
950mA
630mA
450mA
750mA
700mA
870mA
0.086Ω
0.13Ω
0.33Ω
0.27Ω
0.19Ω
0.11Ω
0.08Ω
0.24Ω
0.17Ω
0.12Ω
0.2Ω
0.2Ω
0.1Ω
0.1Ω
DCR
1.25mm
HEIGHT
1.6mm
1.6mm
1.2mm
2mm
2mm
1mm
1mm
1mm
1mm
1mm
1mm
1mm
1mm
3547fa