LTC3407EMSE-2 Linear Technology, LTC3407EMSE-2 Datasheet - Page 8

• Manufacturer Part Number: LTC3407EMSE-2
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,CMOS,TSSOP,10PIN,PLASTIC
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3407-3
A general LTC3407-3 application circuit is shown in
Figure 2. External component selection is driven by the
load requirement, and begins with the selection of the
inductor L. Once the inductor is chosen, C
be selected.
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 ∆I
higher inductance and increases with higher V
Accepting larger values of ∆I
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
∆I
The largest ripple current ∆I
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
begins 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 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 LTC3407-3 requires to operate.
8
L
L
∆ =
= 0.3 • I
=
I
L
f
O
V
OUT
•
V
f
O
LIM
∆
OUT
•
I
L
L
, where I
•
•
⎛
⎜
⎝
⎛
⎜
⎝
1
– 1
U
–
V
V
LIM
IN MAX
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
IN
decreases with
and C
IN
U
or V
OUT
OUT
can
:
Table 1 shows some typical surface mount inductors that
work well in LTC3407-3 applications.
Table 1. Representative Surface Mount Inductors
PART
NUMBER
Sumida
CDRH3D16
Sumida
CDRH2D11
Sumida
CMD4D11
Murata
LQH32CN
Toko
D312F
Panasonic
ELT5KT
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
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.
IN
I
OUT
. To prevent large voltage transients, a low equivalent
RMS
MAX
/2. This simple worst-case is commonly used to
≈
I
MAX
= I
VALUE
LIM
(µH)
IN
2.2
3.3
4.7
1.5
2.2
2.2
3.3
1.0
2.2
2.2
3.3
3.3
4.7
for high frequency decoupling, when not
– ∆I
V
OUT
IN
) Selection
(Ω MAX)
L
0.075
0.110
0.162
0.068
0.170
0.116
0.174
0.060
0.097
0.060
0.260
/2.
(
DCR
0.17
0.20
V
V
IN
IN
–
V
OUT
CURRENT (A) W × L × H (mm
MAX DC
0.900
0.780
0.950
0.770
IN
1.20
1.10
0.90
1.00
0.79
1.08
0.92
1.00
0.95
)
= 2V
OUT
3.8 × 3.8 × 1.8
3.2 × 3.2 × 1.2
4.4 × 5.8 × 1.2
2.5 × 3.2 × 2.0
2.5 × 3.2 × 2.0
4.5 × 5.4 × 1.2
, where I
MAX
SIZE
equals
34073fa
OUT
RMS
3
)
/