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參數(shù)資料
| 型號： | LTC3409EDD |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 600mA Low Vin Buck Regulator in 3mm x 3mm DFN |
| 中文描述： | SWITCHING REGULATOR, 3000 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | 3 X 3 MM, PLASTIC, MO-229-WEED, DFN-8 |
| 文件頁數(shù)： | 10/16頁 |
| 文件大小： | 246K |
| 代理商： | LTC3409EDD |
10
LTC3409
3409f
APPLICATIU
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
IN
and C
OUT
.
W
U
U
Inductor Selection
For most applications, the value of the inductor will fall in
the range of 1
μ
H to 10
μ
H. 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
IN
or V
OUT
also increases the ripple
current as shown in Equation 1. A reasonable starting
point for setting ripple current is
I
L
= 240mA (40% of
600mA).
=
I
f LV
V
V
L
OUT
OUT
IN
1
1
–
(1)
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
L
)
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.
Table 1. Representative Surface Mount Inductors
PART
VALUE
NUMBER
(
μ
H)
(
MAX)
Sumida
2.2
CDRH3D18/LD
3.3
Sumida
1.5
CDRH2D11
2.2
Sumida
2.2
CMD4D11
3.3
Murata
1.0
LQH32CN
2.2
Toko
2.2
D312F
3.3
Panasonic
3.3
ELT5KT
4.7
DCR
MAX DC
CURRENT (A)
0.85
0.75
0.90
0.78
0.950
0.770
1.00
0.79
1.08
0.92
1.00
0.95
SIZE
W
×
L
×
H (mm
3
)
3.2
×
3.2
×
2.0
0.041
0.054
0.068
0.170
0.116
0.174
0.060
0.097
0.060
0.260
0.17
0.20
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
C
IN
and C
OUT
Selection
In continuous mode, the source current of the top MOS-
FET is a square wave of duty cycle V
OUT
/V
IN
. To prevent
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:
C
quiredI
I
V
V
V
V
IN
RMS
OUT MAX
(
OUT
IN
OUT
IN
Re
–
)
/
(
)
[
]
1 2
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/2. This simple worst-case condition is commonly
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
OUT
is driven by the required
effective series resistance (ESR). Typically, once the ESR
requirement for C
OUT
has been met, the RMS current
rating generally far exceeds the I
RIPPLE(P-P)
requirement.
The output ripple DV
OUT
is determined by:
=
+
V
I ESR
OUT
OUT
1
8f C
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