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| 型号： | LM3668SD-3.3/NOPB |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 2.05 A DUAL SWITCHING CONTROLLER, 2500 kHz SWITCHING FREQ-MAX, DSO12 |
| 封装： | ROHS COMPLIANT, LLP-12 |
| 文件页数： | 4/20页 |
| 文件大小： | 2497K |
| 代理商： | LM3668SD-3.3/NOPB |
L : min inductor value including worst case tolerances
(30% drop can be considered)
f : minimum switching frequency
V
OUT: output voltage
D: Duty Cycle for CCM Operation
V
OUT : Output Voltage
V
IN: Input Voltage
Example using above equations:
V
IN = 2.8V to 4V
V
OUT = 3.3V
I
OUT = 500mA
L = 2.2H
F = 2MHz
Buck: I
SAT = 567mA
Boost: I
SAT = 638mA
As a result the inductor should be selected according to the
highest of the two I
SAT values.
A more conservative and recommended approach is to
choose an inductor that has a saturation current rating greater
than the maximum current limit of 2.05A.
A 2.2 H inductor with a saturation current rating of at least
2.05A is recommended for most applications.The inductor’s
resistance should be less than 100m
for good efficiency. For
low-cost applications, an unshielded bobbin inductor could be
considered. For noise critical applications, a toroidal or shield-
ed-bobbin inductor should be used. A good practice is to lay
out the board with overlapping footprints of both types for de-
sign flexibility. This allows substitution of a low-noise shielded
inductor, in the event that noise from low-cost bobbin model
is unacceptable.
Suggest Inductors and Suppliers
Model
Vendor
Dimension
s
LxWxH
(mm)
D.C.R
(max)
I
SAT
LPS4012-
222L
Coilcraft
4 x 4 x 1.2
100 m
2.1A
LPS4018-
222L
Coilcraft
4 x 4 x 1.8
70 m
2.5A
1098AS-2
R0M (2F)
TOKO
3 x 2.8x 1.2
67 m
1.8A
( lower
current
application
s)
Input Capacitor Selection
A ceramic input capacitor of at least 10 F, 6.3V is sufficient
for most applications. Place the input capacitor as close as
possible to the PV
IN pin of the device. A larger value may be
used for improved input voltage filtering. Use X7R or X5R
types; do not use Y5V. DC bias characteristics of ceramic ca-
pacitors must be considered when selecting case sizes like
0805 or 0603. The input filter capacitor supplies current to
the PFET switch of the LM3668 in the first half of each cycle
and reduces voltage ripple imposed on the input power
source. A ceramic capacitor’s low ESR provides the best
noise filtering of the input voltage spikes due to this rapidly
changing current.
Output Capacitor Selection
A ceramic output capacitor of 22F, 6.3V is sufficient for most
applications. Multilayer ceramic capacitors such as X5R or
X7R with low ESR is a good choice for this as well. These
capacitors provide an ideal balance between small size, cost,
reliability and performance. Do not use Y5V ceramic capaci-
tors as they have poor dielectric performance over tempera-
ture and poor voltage characteristic for a given value.
Extra attention is required if a smaller case size capacitor is
used in the application. Smaller case size capacitors typically
have less capacitance for a given bias voltage as compared
to a larger case size capacitor with the same bias voltage.
Please contact the capacitor manufacturer for detail informa-
tion regarding capacitance verses case size. Table 1 lists
several capacitor suppliers.
The output filter capacitor smoothes out current flow from the
inductor to the load, helps maintain a steady output voltage
during transient load changes and reduces output voltage
ripple. These capacitors must be selected with sufficient ca-
pacitance and sufficiently low ESR to perform these functions.
Note that the output voltage ripple is dependent on the induc-
tor current ripple and the equivalent series resistance of the
output capacitor (R
ESR).
The R
ESR is frequency dependent (as well as temperature
dependent); make sure the value used for calculations is at
the switching frequency of the part.
TABLE 1. Suggested Capacitors and Suppliers
Model
Type
Vendor
Voltage Rating
Case Size
Inch (mm)
10 F for C
IN
GRM21BR60J106K
Ceramic, X5R
Murata
6.3V
0805 (2012)
JMK212BJ106K
Ceramic, X5R
Taiyo-Yuden
6.3V
0805 (2012)
C2012X5R0J106K
Ceramic, X5R
TDK
6.3V
0805 (2012)
22 F for C
OUT
JMK212BJ226MG
Ceramic, X5R
Taiyo-Yuden
6.3V
0805 (2012)
www.national.com
12
LM3668
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