LTC3407A-2 Linear Technology, LTC3407A-2 Datasheet - Page 8

• Manufacturer Part Number: LTC3407A-2
• Description: Dual Synchronous 800mA2.25MHz Step-Down DC/DC Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3407A-2
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 ΔI
higher inductance and increases with higher V
Accepting larger values of ΔI
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
ΔI
The largest ripple current ΔI
input voltage. To guarantee that the ripple current stays
below a specified maximum, the inductor value should be
chosen according to the following equation:
The inductor value will also have an effect on Burst Mode
operation. The transition from low current operation be-
gins when the peak inductor current falls below a level set
by the burst clamp. Lower inductor values result in higher
ripple current which causes this transition to occur at
lower load currents. This causes 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.
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 characterisitics. The choice of which
style inductor to use often depends more on the price vs
size requirements and any radiated field/EMI require-
ments than on what the LTC3407A-2 requires to operate.
Table 1 shows some typical surface mount inductors that
work well in LTC3407A-2 applications.
8
L
L
= 0.3 • I
Δ =
=
I
L
f
O
V
OUT
•
V
f
O
Δ
LIM
OUT
•
I
L
L
, where I
•
•
⎛
⎜
⎝
⎛
⎝ ⎜
1
– 1
–
U
V
V
IN MAX
LIM
V
V
OUT
IN
OUT
(
U
is the peak switch current limit.
⎞
⎠ ⎟
)
L
⎞
⎟
⎠
L
occurs at the maximum
allows the use of low
W
L
decreases with
IN
U
or V
OUT
:
Input Capacitor (C
In continuous mode, the input current of the converter is
a square wave with a duty cycle of approximately V
V
series resistance (ESR) input capacitor sized for the maxi-
mum RMS current must be used. The maximum RMS
capacitor current is given by:
where the maximum average output current I
the peak current minus half the peak-to-peak ripple cur-
rent, I
This formula has a maximum at V
I
to design because even significant deviations do not offer
much relief. Note that capacitor manufacturer’s ripple
current ratings are often based on only 2000 hours life-
time. This makes it advisable to further derate the capaci-
tor, or choose a capacitor rated at a higher temperature
than required. Several capacitors may also be paralleled to
meet the size or height requirements of the design. An
additional 0.1μF to 1μF ceramic capacitor is also recom-
mended on V
using an all ceramic capacitor solution.
Table 1. Representative Surface Mount Inductors
FACTURER
Taiyo Yuden CB2016T2R2M
Panasonic
Sumida
Murata
Taiyo Yuden NR30102R2M
FDK
TDK
RMS
IN
MANU-
I
. To prevent large voltage transients, a low equivalent
RMS
= I
MAX
OUT
≈
I
MAX
= I
CB2012T2R2M
CB2016T3R3M
ELT5KT4R7M
CDRH2D18/LD
LQH32CN4R7M23 4.7μH
NR30104R7M
FDKMIPF2520D
FDKMIPF2520D
FDKMIPF2520D
VLF3010AT4R7-
MR70
VLF3010AT3R3-
MR87
VLF3010AT2R2-
M1R0
/2. This simple worst-case is commonly used
PART NUMBER
LIM
IN
for high frequency decoupling, when not
– ΔI
V
OUT
IN
) Selection
L
/2.
(
V
V
IN
IN
VALUE CURRENT
2.2μH
2.2μH
3.3μH
4.7μH
4.7μH
2.2μH
4.7μH
4.7μH
3.3μH
2.2μH
4.7μH
3.3μH
2.2μH
–
V
OUT
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MAX DC
1100mA
1100mA
1200mA
1300mA
1000mA
510mA
530mA
410mA
950mA
630mA
450mA
750mA
700mA
870mA
)
IN
= 2V
0.086Ω
0.13Ω
0.33Ω 1.25mm
0.27Ω
0.19Ω
0.11Ω
0.08Ω
0.28Ω
0.17Ω
0.12Ω
0.2Ω
0.2Ω
0.1Ω
0.1Ω
DCR
OUT
MAX
, where
HEIGHT
1.6mm
1.6mm
1.2mm
equals
2mm
2mm
1mm
1mm
1mm
1mm
1mm
1mm
1mm
1mm
3407a2f
OUT
/