Datasheet

LTC3407A-2
8
3407a2f
Input Capacitor (C
IN
) Selection
In continuous mode, the input current of the converter is
a square wave with a duty cycle of approximately V
OUT
/
V
IN
. To prevent large voltage transients, a low equivalent
series resistance (ESR) input capacitor sized for the maxi-
mum RMS current must be used. The maximum RMS
capacitor current is given by:
II
VVV
V
RMS MAX
OUT IN OUT
IN
≈
(– )
where the maximum average output current I
MAX
equals
the peak current minus half the peak-to-peak ripple cur-
rent, I
MAX
= I
LIM
– ΔI
L
/2.
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst-case is commonly used
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
IN
for high frequency decoupling, when not
using an all ceramic capacitor solution.
Table 1. Representative Surface Mount Inductors
MANU- MAX DC
FACTURER PART NUMBER VALUE CURRENT DCR HEIGHT
Taiyo Yuden CB2016T2R2M 2.2μH 510mA 0.13Ω 1.6mm
CB2012T2R2M 2.2μH 530mA 0.33Ω 1.25mm
CB2016T3R3M 3.3μH 410mA 0.27Ω 1.6mm
Panasonic ELT5KT4R7M 4.7μH 950mA 0.2Ω 1.2mm
Sumida CDRH2D18/LD 4.7μH 630mA 0.086Ω 2mm
Murata LQH32CN4R7M23 4.7μH 450mA 0.2Ω 2mm
Taiyo Yuden NR30102R2M 2.2μH 1100mA 0.1Ω 1mm
NR30104R7M 4.7μH 750mA 0.19Ω 1mm
FDK FDKMIPF2520D 4.7μH 1100mA 0.11Ω 1mm
FDKMIPF2520D 3.3μH 1200mA 0.1Ω 1mm
FDKMIPF2520D 2.2μH 1300mA 0.08Ω 1mm
TDK VLF3010AT4R7- 4.7μH 700mA 0.28Ω 1mm
MR70
VLF3010AT3R3- 3.3μH 870mA 0.17Ω 1mm
MR87
VLF3010AT2R2- 2.2μH 1000mA 0.12Ω 1mm
M1R0
APPLICATIO S I FOR ATIO
WUU
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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
L
decreases with
higher inductance and increases with higher V
IN
or V
OUT
:
Δ =
⎛
⎝
⎜
⎞
⎠
⎟
I
V
fL
V
V
L
OUT
O
OUT
IN
•
•–1
Accepting larger values of ΔI
L
allows the use of low
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
L
= 0.3 • I
LIM
, where I
LIM
is the peak switch current limit.
The largest ripple current ΔI
L
occurs at the maximum
input voltage. To guarantee that the ripple current stays
below a specified maximum, the inductor value should be
chosen according to the following equation:
L
V
fI
V
V
OUT
OL
OUT
IN MAX
=
Δ
⎛
⎝
⎜
⎞
⎠
⎟
•
•–
()
1
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.