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LM3354 Datasheet(PDF) 9 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. LM3354
Description  Regulated 90mA Buck-Boost Switched Capacitor DC/DC Converter
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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Filter Capacitor Selection (Continued)
parable to ceramics) and good temperature stability (compa-
rable to tantalums). The Aluminum Polymer Electrolytics
offered by Cornell-Dubilier and Panasonic, and the POS-
CAPs 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
ESR
Lowest
High
Low
Relative Height
Low for Small Values (<10 µF); Taller for
Higher Values
Lowest
Low
Relative Footprint
Large
Small
Largest
Temperature Stability
X7R/X5R-Acceptable
Good
Good
Frequency Stability
Good
Acceptable
Good
V
OUT Ripple Magnitude @ <50 mA
Low
High
Low
V
OUT Ripple Magnitude @ >100 mA
Low
Slightly Higher
Low
dv/dt of V
OUT Ripple @ All Loads
Lowest
High
Low
b) CAPACITOR SELECTION
i) Output Capacitor (C
OUT)
The output capacitor C
OUT directly affects the magnitude of
the output ripple voltage so C
OUT should be carefully se-
lected. The graphs titled V
OUT Ripple vs. COUT in the Typical
Performance Characteristics section show how the ripple
voltage magnitude is affected by the C
OUT value and the
capacitor technology. These graphs are taken at the gain at
which worst case ripple is observed. In general, the higher
the value of C
OUT, the lower the output ripple magnitude. At
lighter loads, the low ESR ceramics offer a much lower V
OUT
ripple than the higher ESR tantalums of the same value. At
higher loads, the ceramics offer a slightly lower V
OUT ripple
magnitude than the tantalums of the same value. However,
the dv/dt of the V
OUT 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 electro-
lytics 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
IN directly affects the magnitude of the
input ripple voltage, and to a lesser degree the V
OUT ripple.
A higher value C
IN will give a lower VIN ripple. To optimize
low input and output ripple as well as size a 10 µF polymer
electrolytic or ceramic, or 15 µF tantalum capacitor is rec-
ommended. This will ensure low input ripple at 90 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 ceram-
ics and polymer electrolytics achieve a lower V
IN 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 10 µF polymer electrolytics are physi-
cally much larger than the 15 µF tantalums and 10 µF
ceramics.
iii) C
FIL
A 1 µF, X7R ceramic capacitor should be connected to pin
C
FIL. This capacitor provides the filtering needed for the
internal supply rail of the LM3354.
Of the different capacitor technologies, a sample of vendors
that have been verified as suitable for use with the LM3354
are shown in
Table 2.
TABLE 2. Capacitor Vendor Information
Manufacturer
Tel
Fax
Website
Ceramic
Taiyo-yuden
(408) 573-4150
(408) 573-4159
www.t-yuden.com
AVX
(803) 448-9411
(803) 448-1943
www.avxcorp.com
Sprague/Vishay
(207) 324-4140
(207) 324-7223
www.vishay.com
Tantalum
Nichicon
(847) 843-7500
(847) 843-2798
www.nichicon.com
Polymer Electrolytic
Cornell-Dubilier (ESRD)
(508) 996-8561
(508) 996-3830
www.cornell-dubilier.com
Sanyo (POSCAP)
(619) 661-6322
(619) 661-1055
www.sanyovideo.com
www.national.com
9




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