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LM2771 Datenblatt(PDF) 8 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Teilenummer LM2771
Bauteilbeschribung  Low-Ripple 250mA Switched Capacitor Step-Down DC/DC Converter
Download  10 Pages
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Hersteller  NSC [National Semiconductor (TI)]
Direct Link  http://www.national.com
Logo NSC - National Semiconductor (TI)

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Application Information (Continued)
Net capacitance of a ceramic capacitor decreases with in-
creased DC bias. This degradation can result in lower ca-
pacitance than expected on the input and/or output, resulting
in higher ripple voltages and currents. Using capacitors at
DC bias voltages significantly below the capacitor voltage
rating will usually minimize DC bias effects. Consult capaci-
tor manufacturers for information on capacitor DC bias char-
acteristics.
Capacitance characteristics can vary quite dramatically with
different application conditions, capacitor types, and capaci-
tor manufacturers. It is strongly recommended that the
LM2771 circuit be thoroughly evaluated early in the
design-in process with the mass-production capacitors of
choice. This will help ensure that any such variability in
capacitance does not negatively impact circuit performance.
The table below lists some leading ceramic capacitor manu-
facturers.
Manufacturer
Contact Information
AVX
www.avx.com
Murata
www.murata.com
Taiyo-Yuden
www.t-yuden.com
TDK
www.component.tdk.com
Vishay-Vitramon
www.vishay.com
OUTPUT CAPACITOR AND OUTPUT VOLTAGE RIPPLE
The output capacitor in the LM2771 circuit (C
OUT) directly
impacts the magnitude of output voltage ripple. Other promi-
nent factors also affecting output voltage ripple include input
voltage, output current and flying capacitance. Due to the
complexity of the regulation topology, providing equations or
models to approximate the magnitude of the ripple can not
be easily accomplished. But one important generalization
can be made: increasing (decreasing) the output capaci-
tance will result in a proportional decrease (increase) in
output voltage ripple.
In typical high-current applications, a 4.7µF low-ESR ce-
ramic output capacitor is recommended. Different output
capacitance values can be used to reduce ripple, shrink the
solution size, and/or cut the cost of the solution. But chang-
ing the output capacitor may also require changing the flying
capacitor and/or input capacitor to maintain good overall
circuit performance. Performance of the LM2771 with differ-
ent capacitor setups in discussed in the section Recom-
mended Capacitor Configurations.
High ESR in the output capacitor increases output voltage
ripple. If a ceramic capacitor is used at the output, this is
usually not a concern because the ESR of a ceramic capaci-
tor is typically very low and has only a minimal impact on
ripple magnitudes. If a different capacitor type with higher
ESR is used (tantalum, for example), the ESR could result in
high ripple. To eliminate this effect, the net output ESR can
be significantly reduced by placing a low-ESR ceramic ca-
pacitor in parallel with the primary output capacitor. The low
ESR of the ceramic capacitor will be in parallel with the
higher ESR, resulting in a low net ESR based on the prin-
ciples of parallel resistance reduction.
INPUT CAPACITOR AND INPUT VOLTAGE RIPPLE
The input capacitor (C
IN) is a reservoir of charge that aids a
quick transfer of charge from the supply to the flying capaci-
tor during the charge phase of operation. The input capacitor
helps to keep the input voltage from drooping at the start of
the charge phase when the flying capacitor is connected to
the input. It also filters noise on the input pin, keeping this
noise out of sensitive internal analog circuitry that is biased
off the input line.
Much like the relationship between the output capacitance
and output voltage ripple, input capacitance has a dominant
and first-order effect on input ripple magnitude. Increasing
(decreasing) the input capacitance will result in a propor-
tional decrease (increase) in input voltage ripple. Input volt-
age, output current, and flying capacitance also will affect
input ripple levels to some degree.
In typical high-current applications, a 1µF low-ESR ceramic
capacitor is recommended on the input. Different input ca-
pacitance values can be used to reduce ripple, shrink the
solution size, and/or cut the cost of the solution. But chang-
ing the input capacitor may also require changing the flying
capacitor and/or output capacitor to maintain good overall
circuit performance. Performance of the LM2771 with differ-
ent capacitor setups is discussed below in Recommended
Capacitor Configurations.
FLYING CAPACITOR
The flying capacitor (C
1) transfers charge from the input to
the output. Flying capacitance can impact both output cur-
rent capability and ripple magnitudes. If flying capacitance is
too small, the LM2771 may not be able to regulate the output
voltage when load currents are high. On the other hand, if
the flying capacitance is too large, the flying capacitor might
overwhelm the input and output capacitors, resulting in in-
creased input and output ripple.
Polarized capacitors (tantalum, aluminum electrolytic, etc.)
must not be used for the flying capacitor, as they could
become reverse-biased during LM2771 operation.
RECOMMENDED CAPACITOR CONFIGURATIONS
The data in Table 1 can be used to assist in the selection of
a capacitor configuration that best balances solution size
and cost with the electrical requirements of the application.
As previously discussed, input and output ripple voltages will
vary with output current and input voltage. The numbers
provided show expected ripple voltage when V
IN = 3.6V and
load currents are between 10mA and 200mA. The table
offers first look at approximate ripple levels and provides a
comparison for the different capacitor configurations pre-
sented, but is not intended to be a guarantee of perfor-
mance.
www.national.com
8


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