LTC3407-2 Linear Technology, LTC3407-2 Datasheet - Page 8

• Manufacturer Part Number: LTC3407-2
• Description: 2.25MHz Step-Down DC/DC Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3407-2
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 LTC3407-2 requires to operate.
Table 1 shows some typical surface mount inductors that
work well in LTC3407-2 applications.
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.
8
IN
I
OUT/2
. To prevent large voltage transients, a low equivalent
RMS
MAX
. This simple worst-case is commonly used to
≈
I
= I
MAX
LIM
IN
for high frequency decoupling, when not
– ∆I
V
IN
OUT IN
U
) Selection
L
/2.
(
V
V
IN
U
–
V
OUT
IN
)
W
= 2V
OUT
, where I
MAX
U
equals
OUT
RMS
/
Table 1. Representative Surface Mount Inductors
PART
NUMBER
Sumida
CDRH3D16
Sumida
CDRH2D11
Sumida
CMD4D11
Murata
LQH32CN
Toko
D312F
Panasonic
ELT5KT
Output Capacitor (C
The selection of C
minimize voltage ripple and load step transients. Typically,
once the ESR requirement is satisfied, the capacitance is
adequate for filtering. The output ripple (∆V
mined by:
where f = operating frequency, C
and ∆I
is highest at maximum input voltage since ∆I
with input voltage. With ∆I
will be less than 100mV at maximum V
with:
Once the ESR requirements for C
RMS current rating generally far exceeds the I
requirement, except for an all ceramic solution.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the capacitance, ESR or RMS
current handling requirement of the application. Alumi-
num electrolytic, special polymer, ceramic and dry tantulum
capacitors are all available in surface mount packages. The
OS-CON semiconductor dielectric capacitor available from
Sanyo has the lowest ESR(size) product of any aluminum
electrolytic at a somewhat higher price. Special polymer
ESR
∆
V
OUT
L
COUT
= ripple current in the inductor. The output ripple
≈ ∆
< 150mΩ
VALUE
(µH)
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
I ESR
L
⎛
⎜
⎝
OUT
OUT
(Ω MAX)
0.075
0.110
0.162
0.068
0.170
0.116
0.174
0.060
0.097
0.060
0.260
+
DCR
0.17
0.20
is driven by the required ESR to
) Selection
8
f C
O OUT
L
= 0.3 • I
1
CURRENT (A) W × L × H (mm
OUT
MAX DC
www.DataSheet4U.com
0.900
0.780
0.950
0.770
OUT
1.20
1.10
0.90
1.00
0.79
1.08
0.92
1.00
0.95
⎞
⎟
⎠
= output capacitance
LIM
IN
have been met, the
and f
the output ripple
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
O
L
= 2.25MHz
RIPPLE(P-P)
sn34072 34072fs
) is deter-
increases
SIZE
3
)