lm2773tlx National Semiconductor Corporation, lm2773tlx Datasheet - Page 9

• Manufacturer Part Number: lm2773tlx
• Description: Low-ripple 1.8v/1.6v Spread-spectrum Switched Capacitor Step-down Regulator
• Manufacturer: National Semiconductor Corporation

(+80%, -20%) and vary significantly over temperature (Y5V:
+22%, -82% over -30°C to +85°C range; Z5U: +22%, -56%
over +10°C to +85°C range). Under some conditions, a 1µF-
rated Y5V or Z5U capacitor could have a capacitance as low
as 0.1µF. Such detrimental deviation is likely to cause Y5V
and Z5U capacitors to fail to meet the minimum capacitance
requirements of the LM2773.
Net capacitance of a ceramic capacitor decreases with in-
creased DC bias. This degradation can result in lower capac-
itance 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 capacitor man-
ufacturers for information on capacitor DC bias characteris-
tics.
Capacitance characteristics can vary quite dramatically with
different application conditions, capacitor types, and capaci-
tor manufacturers. It is strongly recommended that the
LM2773 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.
OUTPUT CAPACITOR AND OUTPUT VOLTAGE RIPPLE
The output capacitor in the LM2773 circuit (C
pacts the magnitude of output voltage ripple. Other prominent
factors also affecting output voltage ripple include input volt-
age, output current and flying capacitance. Due to the com-
plexity 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 capacitance will re-
sult in a proportional decrease (increase) in output voltage
ripple.
In typical high-current applications, a 4.7µF low-ESR ceramic
output capacitor is recommended. Different output capaci-
tance values can be used to reduce ripple, shrink the solution
size, and/or cut the cost of the solution. But changing the out-
put capacitor may also require changing the flying capacitor
and/or input capacitor to maintain good overall circuit perfor-
mance. Performance of the LM2773 with different capacitor
setups in discussed in the section Recommended Capacitor
Configurations.
High ESR in the output capacitor increases output voltage
ripple. If a ceramic capacitor is used at the output, this is usu-
ally not a concern because the ESR of a ceramic capacitor 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 sig-
nificantly reduced by placing a low-ESR ceramic capacitor in
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OUT
) directly im-
9
parallel with the primary output capacitor. The low ESR of the
ceramic capacitor will be in parallel with the higher ESR, re-
sulting in a low net ESR based on the principles of parallel
resistance reduction.
INPUT CAPACITOR AND INPUT VOLTAGE RIPPLE
The input capacitor (C
quick transfer of charge from the supply to the flying capaci-
tors 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 capacitors are 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,
first-order effect on input ripple magnitude. Increasing (de-
creasing) the input capacitance will result in a proportional
decrease (increase) in input voltage ripple. Input voltage, out-
put 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 changing
the input capacitor may also require changing the flying ca-
pacitor and/or output capacitor to maintain good overall circuit
performance. Performance of the LM2773 with different ca-
pacitor setups is discussed below in Recommended Capac-
itor Configurations.
FLYING CAPACITORS
The flying capacitors (C
to the output. Flying capacitance can impact both output cur-
rent capability and ripple magnitudes. If flying capacitance is
too small, the LM2773 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 over-
whelm the input and output capacitors, resulting in increased
input and output ripple.
In typical high-current applications, 1µF low-ESR ceramic ca-
pacitors are recommended for the flying capacitors. Polarized
capacitors (tantalum, aluminum electrolytic, etc.) must not be
used for the flying capacitor, as they could become reverse-
biased during LM2773 operation.
RECOMMENDED CAPACITOR CONFIGURATIONS
The data in Table 1 can be used to assist in the selection of
capacitance configurations 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 pro-
vided show expected ripple voltage with V
current of 300mA. The table offers a first look at approximate
ripple levels and provides a comparison of different capacitor
configurations, but is not intended to be a guarantee of per-
formance. With any capacitance configuration chosen, al-
ways verify that the performance of the ripple waveforms are
suitable for the intended application. The same capacitance
value must be used for all the flying capacitors.
IN
1
) is a reservoir of charge that aids a
, C
2
) transfer charge from the input
IN
= 3.6V and a load
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