ltc3406ab-2 Linear Technology Corporation, ltc3406ab-2 Datasheet - Page 8

• Manufacturer Part Number: ltc3406ab-2
• Description: 2.25mhz, 600ma Synchronous Step-down Regulator In Thinsot
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTC3406AB-2
The basic LTC3406AB-2 application circuit is shown on
the front page. External component selection is driven by
the load requirement and begins with the selection of L
followed by C
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
rent as shown in equation 1. A reasonable starting point for
setting ripple current is ΔI
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 effi ciency, choose a low
DC-resistance inductor.
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 don’t 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 fi eld/EMI requirements
than on what the LTC3406AB-2 requires to operate. Table
1 shows some typical surface mount inductors that work
well in LTC3406AB-2 applications.
8
Δ =
I
L
( )( )
f L
1
IN
V
and C
OUT
IN
or V
⎛
⎜
⎝
OUT
1
−
OUT
.
V
L
V
OUT
= 240mA (40% of 600mA).
IN
also increases the ripple cur-
⎞
⎟
⎠
(1)
Table 1. Representative Surface Mount Inductors
PART
NUMBER
Sumida
CDRH3D16
Sumida
CMD4D06
Panasonic
ELT5KT
Murata
LQH32CN
C
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle V
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum RMS
capacitor current is given by:
This formula has a maximum at V
I
commonly used for design because even signifi cant de-
viations 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 tem-
perature than required. Always consult the manufacturer
if there is any question.
The selection of C
series resistance (ESR).
RMS
IN
C
and C
IN
= I
required I
OUT
OUT
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
/2. This simple worst-case condition is
Selection
RMS
OUT
(Ω MAX)
0.043
0.075
0.110
0.162
0.116
0.174
0.216
0.060
0.097
0.150
DCR
0.17
0.20
≅
is driven by the required effective
I
OMAX
CURRENT (A)
MAX DC
[
0.950
0.770
0.750
OUT
V
1.55
1.20
1.10
0.90
1.00
0.95
1.00
0.79
0.65
OUT
/V
(
IN
IN
V
IN
. To prevent large
= 2V
V
W × L × H (mm
−
IN
3.8 × 3.8 × 1.8
3.5 × 4.3 × 0.8
4.5 × 5.4 × 1.2
2.5 × 3.2 × 2.0
V
OUT
OUT
SIZE
, where
)
]
1 2 /
3406ab2f
3
)