LTC3410
APPLICATIO S I FOR ATIO
W U U
U
Inductor Core Selection
4.7µH
V
IN
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Tor-
oid or shielded pot cores in ferrite or permalloy materials
aresmallanddon’tradiatemuchenergy, butgenerallycost
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 vs size require-
ments and any radiated field/EMI requirements than on
whattheLTC3410requirestooperate.Table1showssome
typical surface mount inductors that work well in
LTC3410 applications.
V
OUT
2.7V
V
SW
LTC3410
RUN
IN
1.2V
10pF
C
TO 5.5V
IN
C
OUT
4.7µF
4.7µF
CER
CER
V
FB
232k
GND
464k
3410 F01
Figure 1. High Efficiency Step-Down Converter
ThebasicLTC3410applicationcircuitisshowninFigure 1.
Externalcomponentselectionisdrivenbytheloadrequire-
ment and begins with the selection of L followed by CIN and
Table 1. Representative Surface Mount Inductors
MAX DC
COUT
.
MANUFACTURER PART NUMBER
VALUE CURRENT DCR HEIGHT
Inductor Selection
Taiyo Yuden
CB2016T2R2M
CB2012T2R2M
LBC2016T3R3M
2.2µH 510mA 0.13Ω 1.6mm
2.2µH 530mA 0.33Ω 1.25mm
3.3µH 410mA 0.27Ω 1.6mm
For most applications, the value of the inductor will fall in
the range of 2.2µH to 4.7µH. Its value is chosen based on
the desired ripple current. Large value inductors lower
ripple current and small value inductors result in higher
ripplecurrents.HigherVIN orVOUT alsoincreasestheripple
currentasshowninequation1. Areasonablestartingpoint
for setting ripple current is ∆IL = 120mA (40% of 300mA).
Panasonic
Sumida
ELT5KT4R7M
CDRH2D18/LD
4.7µH 950mA 0.2Ω 1.2mm
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
FDKMIPF2520D
FDKMIPF2520D
4.7µH 1100mA 0.11Ω 1mm
3.3µH 1200mA 0.1Ω 1mm
2.2µH 1300mA 0.08Ω 1mm
⎛
⎝
⎞
VOUT
1
∆IL =
VOUT 1−
(1)
⎜
⎟
⎠
f L
( )( )
V
IN
CIN and COUT Selection
The DC current rating of the inductor should be at least
equal to the maximum load current plus half the ripple
current to prevent core saturation. Thus, a 360mA rated
inductor should be enough for most applications (300mA
+ 60mA). For better efficiency, choose a low DC-resistance
inductor.
Incontinuousmode,thesourcecurrentofthetopMOSFET
is a square wave of duty cycle VOUT/VIN. To prevent large
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum
RMS capacitor current is given by:
1/2
The inductor value also has an effect on Burst Mode
operation. The transition to low current operation begins
when the inductor current peaks fall to approximately
100mA. Lower inductor values (higher ∆IL) will cause this
to occur at lower load currents, which can cause a dip in
efficiency in the upper range of low current operation. In
Burst Mode operation, lower inductance values will cause
the burst frequency to increase.
V
V − V
OUT
(
)
]
[
OUT IN
CIN requiredIRMS ≅IOMAX
V
IN
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst-case condition is com-
monlyusedfordesignbecauseevensignificantdeviations
do not offer much relief. Note that the capacitor
manufacturer’s ripple current ratings are often based on
3410fb
8
Linear [ Linear ]
