LTC3406B
APPLICATIO S I FOR ATIO
The basic LTC3406B application circuit is shown in Figure
1. External component selection is driven by the load
requirement 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 1µH 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
= 240mA (40% of 600mA).
∆
I
L
=
( )( )
⎛
V
⎞
V
OUT
⎜
1
−
OUT
⎟
V
IN
⎠
⎝
f L
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 720mA rated
inductor should be enough for most applications (600mA
+ 120mA). For better efficiency, choose a low DC-resis-
tance inductor.
Inductor Core Selection
Different core materials and shapes will change the size/
current and price/current relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy mate-
rials are small and don’t radiate much energy, but gener-
ally 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 LTC3406B requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3406B applications.
8
U
Table 1. Representative Surface Mount Inductors
PART
NUMBER
Sumida
CDRH3D16
VALUE
(µH)
1.5
2.2
3.3
4.7
2.2
3.3
4.7
3.3
4.7
1.0
2.2
4.7
DCR
(Ω MAX)
0.043
0.075
0.110
0.162
0.116
0.174
0.216
0.17
0.20
0.060
0.097
0.150
MAX DC
SIZE
CURRENT (A) W
×
L
×
H (mm
3
)
1.55
1.20
1.10
0.90
0.950
0.770
0.750
1.00
0.95
1.00
0.79
0.65
3.8
×
3.8
×
1.8
Sumida
CMD4D06
Panasonic
ELT5KT
Murata
LQH3C
3.5
×
4.3
×
0.8
4.5
×
5.4
×
1.2
2.5
×
3.2
×
2.0
(1)
W
U U
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
V
IN
)
]
1/ 2
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
2000 hours of life. This makes it advisable to further derate
the capacitor, or choose a capacitor rated at a higher
temperature than required. Always consult the manufac-
turer if there is any question.
The selection of C
OUT
is driven by the required effective
series resistance (ESR).
3406bfa
LINER [ LINEAR TECHNOLOGY ]
