LTC3619B_15 Datasheet, PDF (11/20 Pages) Linear Technology – 400mA/800mA Synchronous Step-Down DC/DC with Average Input Current Limit

LTC3619B
Applications Information
A general LTC3619B application circuit is shown in Figure 2.
External component selection is driven by the load require-
ment, 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 operating
frequency, the inductor value has a direct effect on ripple
current. The inductor ripple current DI_L decreases with
higher inductance and increases with higher V_IN or V_OUT:
ΔI_L
=
V_OUT
f_O • L
•


1−
V_OUT
V_IN


(1)
Accepting larger values of DI_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 800mA
regulator, DI_L = 320mA (40% of 800mA).
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
V_OUT2
C1
L2
C_F2
RUN2 V_IN RUN1
PGOOD2 PGOOD1
LTC3619B
SW2
SW1
L1
C_F1
C_OUT2 ^R4
V_FB2
R3 ^RLIM
V_FB1
GND
R2
R1
C_LIM R_LIM
V_OUT1
C_OUT1
3619B F02
Inductor Core Selection
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. To-
roid 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 induc-
tor to use often depends more on the price versus size
requirements, and any radiated field/EMI requirements,
than on what the LTC3619B requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3619B applications.
Table 1. Representative Surface Mount Inductors
MANU-
MAX DC
FACTURER PART NUMBER VALUE CURRENT DCR HEIGHT
Coilcraft
LPS4012-152ML
LPS4012-222ML
LPS4012-332ML
LPS4012-472ML
LPS4018-222ML
LPS4018-332ML
LPS4018-472ML
1.5µH
2.2µH
3.3µH
4.7µH
2.2µH
3.3µH
4.7µH
2200mA
1750mA
1450mA
1450mA
2300mA
2000mA
1800mA
0.070Ω
0.100Ω
0.100Ω
0.170Ω
0.070Ω
0.080Ω
0.125Ω
1.2mm
1.2mm
1.2mm
1.2mm
1.8mm
1.8mm
1.8mm
FDK FDKMIPF2520D 4.7µH 1100mA 0.11Ω 1mm
FDKMIPF2520D 3.3µH 1200mA 0.1Ω 1mm
FDKMIPF2520D 2.2µH 1300mA 0.08Ω 1mm
Murata LQH32CN4R7M23 4.7µH 450mA 0.2Ω 2mm
Panasonic ELT5KT4R7M 4.7µH 950mA 0.2Ω 1.2mm
Sumida
CDRH2D18/LD
CDH38D11SNP-
3R3M
CDH38D11SNP-
2R2M
4.7µH
3.3μH
2.2μH
630mA
1560mA
1900mA
0.086Ω
0.115Ω
0.082Ω
2mm
1.2mm
1.2mm
Taiyo Yuden CB2016T2R2M
CB2012T2R2M
CB2016T3R3M
NR30102R2M
NR30104R7M
2.2µH
2.2µH
3.3µH
2.2µH
4.7µH
510mA
530mA
410mA
1100mA
750mA
0.13Ω
0.33Ω
0.27Ω
0.1Ω
0.19Ω
1.6mm
1.25mm
1.6mm
1mm
1mm
TDK VLF3010AT4R7- 4.7µH 700mA 0.28Ω 1mm
MR70
VLF3010AT3R3- 3.3µH 870mA 0.17Ω 1mm
MR87
VLF3010AT2R2- 2.2µH 1000mA 0.12Ω 1mm
M1R0
VLF4012AT-2R2 2.2µH 1500mA 0.076Ω 1.2mm
M1R5
VLF5012ST-3R3 3.3μH 1700mA 0.095Ω 1.2mm
M1R7
VLF5014ST-2R2 2.2µH 2300mA 0.059Ω 1.4mm
M2R3
Figure 2. LTC3619B General Schematic
3619bfb
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