LTC3409 LINER [Linear Technology], LTC3409 Datasheet - Page 10

• Manufacturer Part Number: LTC3409
• Description: 600mA Low Vin Buck Regulator in 3mm x 3mm DFN
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC3409
The basic LTC3409 application circuit is shown on the first
page of this data sheet. External component selection is
driven by the load requirement and begins with the selec-
tion of L followed by C
Inductor Selection
For most applications, the value of the inductor will fall in
the range of 1µH to 10µH. 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
current as shown in Equation 1. A reasonable starting
point for setting ripple current is ∆I
600mA).
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 efficiency, choose a low DC resis-
tance inductor. The inductor value also has an effect on
Burst Mode operation. The transition to low current opera-
tion begins when the inductor current peaks fall to ap-
proximately 200mA. Lower inductor values (higher ∆I
will cause this to occur at lower load currents, which can
cause 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 characteristics. The choice of which style
inductor to use often depends more on the price vs size
requirements and any radiated field/EMI requirements
than on what the LTC3409 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3409 applications.
10
∆ =
I
L
f L
•
1
V
OUT
U
IN
⎛
⎜
⎝
1
or V
–
IN
V
and C
V
U
OUT
OUT
IN
⎞
⎟
⎠
OUT
also increases the ripple
.
W
L
= 240mA (40% of
U
(1)
L
)
Table 1. Representative Surface Mount Inductors
PART
NUMBER
Sumida
CDRH3D18/LD
Sumida
CDRH2D11
Sumida
CMD4D11
Murata
LQH32CN
Toko
D312F
Panasonic
ELT5KT
C
In continuous mode, the source current of the top MOS-
FET is a square wave of duty cycle V
large voltage transients, a low ESR input capacitor sized
for the maximum RMS current must be used. The maxi-
mum RMS capacitor current is given by:
This formula has a maximum at V
= I
used for design because even significant deviations 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 temperature than
required. Always consult the manufacturer if there is any
question. The selection of C
effective series resistance (ESR). Typically, once the ESR
requirement for C
rating generally far exceeds the I
The output ripple DV
IN
C
OUT
∆
IN
and C
V
Re
OUT
/2. This simple worst-case condition is commonly
quired I
OUT
= ∆
VALUE
Selection
(µH)
I ESR
2.2
3.3
1.5
2.2
2.2
3.3
1.0
2.2
2.2
3.3
3.3
4.7
L
⎛
⎜
⎝
RMS
OUT
(Ω MAX)
OUT
≅
0.041
0.054
0.068
0.170
0.116
0.174
0.060
0.097
0.060
0.260
DCR
0.17
0.20
+
has been met, the RMS current
I
OUT MAX
8 • •
is determined by:
(
f C
OUT
1
CURRENT (A) W × L × H (mm
OUT
is driven by the required
MAX DC
)
RIPPLE(P-P)
0.950
0.770
IN
0.85
0.75
0.90
0.78
1.00
0.79
1.08
0.92
1.00
0.95
[
V
OUT
= 2V
⎞
⎟
⎠
OUT
(
/V
OUT
V
IN
IN
V
3.2 × 3.2 × 2.0
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
requirement.
, where I
. To prevent
–
IN
V
SIZE
OUT
)
]
RMS
1 2
3409f
/
3
)