LTC3410
APPLICATIO S I FOR ATIO
V
IN
2.7V
TO 5.5V
4.7µH
C
IN
4.7µF
CER
V
IN
RUN
V
FB
GND
232k
464k
3410 F01
SW
LTC3410
10pF
Figure 1. High Efficiency Step-Down Converter
The basic LTC3410 application circuit is shown in Figure 1.
External component selection is driven by the load require-
ment and begins with the selection of L followed by C
IN
and
C
OUT
.
Inductor Selection
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
ripple currents. Higher V
IN
or V
OUT
also increases the ripple
current as shown in equation 1. A reasonable starting point
for setting ripple current is
∆I
L
= 120mA (40% of 300mA).
∆
I
L
=
⎛
V
⎞
1
V
OUT
⎜
1
−
OUT
⎟
(
f
)(
L
)
⎝
V
IN
⎠
(1)
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.
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
∆I
L
) 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.
8
U
Inductor Core Selection
V
OUT
1.2V
C
OUT
4.7µF
CER
W
U U
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
are small and don’t 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 vs size require-
ments and any radiated field/EMI requirements than on
what the LTC3410 requires to operate. Table 1 shows some
typical surface mount inductors that work well in
LTC3410 applications.
Table 1. Representative Surface Mount Inductors
MANUFACTURER PART NUMBER
Taiyo Yuden
CB2016T2R2M
CB2012T2R2M
LBC2016T3R3M
ELT5KT4R7M
CDRH2D18/LD
NR30102R2M
NR30104R7M
FDKMIPF2520D
FDKMIPF2520D
FDKMIPF2520D
MAX DC
VALUE CURRENT DCR HEIGHT
2.2µH
2.2µH
3.3µH
4.7µH
4.7µH
2.2µH
4.7µH
4.7µH
3.3µH
2.2µH
510mA
530mA
410mA
950mA
450mA
0.13Ω 1.6mm
0.33Ω 1.25mm
0.27Ω 1.6mm
0.2Ω 1.2mm
0.2Ω
2mm
Panasonic
Sumida
Murata
Taiyo Yuden
FDK
630mA 0.086Ω 2mm
1100mA 0.1Ω 1mm
750mA 0.19Ω 1mm
1100mA 0.11Ω 1mm
1200mA 0.1Ω 1mm
1300mA 0.08Ω 1mm
LQH32CN4R7M23 4.7µH
C
IN
and C
OUT
Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle V
OUT
/V
IN
. 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:
C
IN
required I
RMS
≅
I
OMAX
[
V
OUT
(
V
IN
−
V
OUT
)
]
1/ 2
V
IN
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst-case condition is com-
monly used for design because even significant deviations
do not offer much relief. Note that the capacitor
manufacturer’s ripple current ratings are often based on
3410fb
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