LTC3547 Datasheet, PDF (8/16 Pages) Linear Technology – Dual Monolithic 300mA Synchronous Step-Down Regulator

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:
∆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).
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.
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-
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
V_IN
2.5V TO 5.5V
V_OUT2
C1
RUN2 V_IN RUN1
L2
LTC3547
L1
SW2
SW1
C_F2
C_F1
C_OUT2 ^R4
V_FB2
V_FB1
GND
R3
R2
R1
Figure 1. LTC3547 General Schematic
V_OUT1
C_OUT1
3547 F01
3547fa
8