LTC3547B_15 Datasheet, PDF (8/16 Pages) Linear Technology – Dual Monolithic 300mA Synchronous Step-Down Regulator

LTC3547B
APPLICATIO S I FOR ATIO
A general LTC3547B 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:
I_L
=
V_OUT
f_O • L
•


1

V_OUT
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).
V_IN
2.5V TO 5.5V
C1
RUN2 V_IN RUN1
V_OUT2
L2
C_F2
LTC3547B
SW2
SW1
L1
C_F1
V_OUT1
C_OUT2 ^R4
V_FB2
V_FB1
GND
R3
R2
R1
Figure 1. LTC3547B General Schematic
C_OUT1
3547b F01
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 LTC3547B requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3547B applications.
Table 1. Representative Surface Mount Inductors
MANU-
MAX DC
FACTURER PART NUMBER VALUE CURRENT DCR HEIGHT
Taiyo Yuden CB2016T2R2M
CB2012T2R2M
CB2016T3R3M
2.2μH
2.2μH
3.3μH
510mA
530mA
410mA
0.13Ω 1.6mm
0.33Ω 1.25mm
0.27Ω 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 1100mA 0.1Ω 1mm
4.7μH 750mA 0.19Ω 1mm
FDK
FDKMIPF2520D 4.7μH 1100mA 0.11Ω 1mm
FDKMIPF2520D 3.3μH 1200mA 0.1Ω 1mm
FDKMIPF2520D 2.2μH 1300mA 0.08Ω 1mm
TDK
VLF3010AT4R7- 4.7μH 700mA 0.24Ω 1mm
MR70
VLF3010AT3R3- 3.3μH 870mA 0.17Ω 1mm
MR87
VLF3010AT2R2- 2.2μH 1000mA 0.12Ω 1mm
M1RD
3547bfb
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