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參數(shù)資料
| 型號： | LM3352 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Regulated 200 mA Buck-Boost Switched Capacitor DC/DC Converter |
| 中文描述： | 穩(wěn)壓200毫安的Buck - Boost開關(guān)電容DC / DC變換器 |
| 文件頁數(shù)： | 8/11頁 |
| 文件大?。?/td> | 516K |
| 代理商： | LM3352 |
Filter Capacitor Selection
a) CAPACITOR TECHNOLOGIES
The three major technologies of capacitors that can be used
as filter capacitors for LM3352 are: i) tantalum, ii) ceramic
and iii) polymer electrolytic technologies.
i) Tantalum
Tantalum capacitors are widely used in switching regulators.
Tantalum capacitors have the highest CV rating of any tech-
nology; as a result, high values of capacitance can be ob-
tained in relatively small package sizes. It is also possible to
obtain high value tantalum capacitors in very low profile
(
<
1.2 mm) packages. This makes the tantalums attractive
for low-profile, small size applications. Tantalums also pos-
sess very good temperature stability; i.e., the change in the
capacitance value, and impedance over temperature is rela-
tively small. However, the tantalum capacitors have relatively
high ESR values which can lead to higher voltage ripple and
their frequency stability (variation over frequency) is not very
good, especially at high frequencies (
>
1 MHz).
ii) Ceramic
Ceramic capacitors have the lowest ESR of the three tech-
nologies and their frequency stability is exceptionally good.
These characteristics make the ceramics an attractive
choice for low ripple, high frequency applications. However,
the temperature stability of the ceramics is bad, except for
the X7R and X5R dielectric types. High capacitance values
(
>
1
μF)
are
achievable
Taiyo-yuden which are suitable for use with regulators. Ce-
ramics are taller and larger than the tantalums of the same
capacitance value.
from
companies
such
as
iii) Polymer Electrolytic
Polymer electrolytic is a third suitable technology. Polymer
capacitors provide some of the best features of both the ce-
ramic and the tantalum technologies. They provide very low
ESR values while still achieving high capacitance values.
However, their ESR is still higher than the ceramics, and
their capacitance value is lower than the tantalums of the
same size. Polymers offer good frequency stability (compa-
rable to ceramics) and good temperature stability (compa-
rable to tantalums). The Aluminum Polymer Electrolytics of-
fered by Cornell-Dubilier and Panasonic, and the POSCAPs
offered by Sanyo fall under this category.
Table 1 compares the features of the three capacitor tech-
nologies.
TABLE 1. Comparison of Capacitor Technologies
Ceramic
Tantalum
Polymer
Electrolytic
Low
Low
ESR
Relative Height
Lowest
Low for Small Values (
<
10 μF); Taller for
Higher Values
Large
X7R/X5R-Acceptable
Good
Low
Low
Lowest
High
Lowest
Relative Footprint
Temperature Stability
Frequency Stability
V
OUT
Ripple Magnitude
@
<
50 mA
V
OUT
Ripple Magnitude
@
>
100 mA
dv/dt of V
OUT
Ripple
@
All Loads
Small
Good
Acceptable
High
Slightly Higher
High
Largest
Good
Good
Low
Low
Low
b) CAPACITOR SELECTION
i) Output Capacitor (C
OUT
)
The output capacitor C
directly affects the magnitude of
the output ripple voltage so C
should be carefully se-
lected. The graphs titled V
Ripple vs. C
in the Typical
Performance Characteristics section show how the ripple
voltage magnitude is affected by the C
value and the ca-
pacitor technology. These graphs are taken at the gain at
which worst case ripple is observed. In general, the higher
the value of C
, the lower the output ripple magnitude. At
lighter loads, the low ESR ceramics offer a much lower V
ripple than the higher ESR tantalums of the same value. At
higher loads, the ceramics offer a slightly lower V
ripple
magnitude than the tantalums of the same value. However,
the dv/dt of the V
ripple with the ceramics and polymer
electrolytics is much lower than the tantalums under all load
conditions. The tantalums are suggested for very low profile,
small size applications. The ceramics and polymer electrolyt-
ics are a good choice for low ripple, low noise applications
where size is less of a concern.
ii) Input Capacitor (C
IN
)
The input capacitor C
directly affects the magnitude of the
input ripple voltage, and to a lesser degree the V
ripple.
A higher value C
will give a lower V
ripple. To optimize
low input and output ripple as well as size a 15 μF polymer
electrolytic, 22 μF ceramic, or 33 μF tantalum capacitor is
recommended. This will ensure low input ripple at 200 mA
load current. If lower currents will be used or higher input
ripple can be tolerated then a smaller capacitor may be used
to reduce the overall size of the circuit. The lower ESR ce-
ramics and polymer electrolytics achieve a lower V
ripple
than the higher ESR tantalums of the same value. Tantalums
make a good choice for small size, very low profile applica-
tions. The ceramics and polymer electrolytics are a good
choice for low ripple, low noise applications where size is
less of a concern. The 15 μF polymer electrolytics are physi-
cally much larger than the 33 μF tantalums and 22 μF ceram-
ics.
www.national.com
8
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