LM3352MTC-2.5 National Semiconductor, LM3352MTC-2.5 Datasheet - Page 8

• Manufacturer Part Number: LM3352MTC-2.5
• Description: Regulated 200 mA Buck-Boost Switched Capacitor DC/DC Converter
• Manufacturer: National Semiconductor

www.national.com
ESR
Relative Height
Relative Footprint
Temperature Stability
Frequency Stability
V
V
dv/dt of V
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
(
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 (
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
b) CAPACITOR SELECTION
i) Output Capacitor (C
The output capacitor C
the output ripple voltage so C
lected. The graphs titled V
Performance Characteristics section show how the ripple
voltage magnitude is affected by the C
pacitor technology. These graphs are taken at the gain at
which worst case ripple is observed. In general, the higher
the value of C
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
magnitude than the tantalums of the same value. However,
the dv/dt of the V
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.
OUT
OUT
<
1.2 mm) packages. This makes the tantalums attractive
Ripple Magnitude
Ripple Magnitude
OUT
Ripple
OUT
, the lower the output ripple magnitude. At
OUT
@
ripple with the ceramics and polymer
OUT
All Loads
OUT
@
@
OUT
<
>
)
directly affects the magnitude of
50 mA
100 mA
Ripple vs. C
OUT
should be carefully se-
TABLE 1. Comparison of Capacitor Technologies
>
OUT
1 MHz).
OUT
value and the ca-
Lowest
Low for Small Values (
Higher Values
Large
X7R/X5R-Acceptable
Good
Low
Low
Lowest
in the Typical
OUT
ripple
OUT
Ceramic
8
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
(
Taiyo-yuden which are suitable for use with regulators. Ce-
ramics are taller and larger than the tantalums of the same
capacitance value.
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.
ii) Input Capacitor (C
The input capacitor C
input ripple voltage, and to a lesser degree the V
A higher value C
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
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.
<
>
10 µF); Taller for
1 µF) are achievable from companies such as
IN
will give a lower V
IN
IN
High
Lowest
Small
Good
Acceptable
High
Slightly Higher
High
directly affects the magnitude of the
)
Tantalum
IN
ripple. To optimize
Low
Low
Largest
Good
Good
Low
Low
Low
Electrolytic
Polymer
OUT
IN
ripple.
ripple