lm2770sdx-12157 National Semiconductor Corporation, lm2770sdx-12157 Datasheet - Page 12

• Manufacturer Part Number: lm2770sdx-12157
• Description: High Efficiency Switched Capacitor Step-down Dc/dc Regulator With Sleep Mode
• Manufacturer: National Semiconductor Corporation

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Application Information
INPUT CAPACITOR AND INPUT VOTLAGE 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 capaci-
tor 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. This can be
observed in the input voltage ripple waveforms in the Typi-
cal Performance Characteristics section. Input voltage,
output current, and flying capacitance also will affect input
ripple levels to some degree.
In typical high-current applications, a 10µF 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
capacitors and/or output capacitor to maintain good overall
circuit performance. Performance of the LM2770 with differ-
ent capacitor setups is discussed below in Recommended
Capacitor Configurations.
FLYING CAPACITORS
The flying capacitors (C
input to the output. Flying capacitance can impact both
output current capability and ripple magnitudes. If flying
Note 13: Refer to the text in the Recommended Capacitor Configurations section for detailed information on the data in this table
CAPACITOR
CONFIGURATION
C
C1 = C2 = 1µF
C
C1 = C2 = 1µF
C
C1 = C2 = 0.47µF
C
C1 = C2 = 0.22µF
IN
IN
IN
IN
= C
= C
= C
= C
OUT
OUT
OUT
OUT
= 2x10µF,
= 10µF,
= 4.7µF,
= 2.2µF,
TABLE 1. LM2770 Performance with Different Capacitor Configurations (Note 13)
IN
) is a reservoir of charge that aids a
1
and C
(V
TYPICAL
RIPPLE
OUTUT
IN
130mV
200mV
2
25mV
50mV
) transfer charge from the
= 3.6V)
(Continued)
(V
TYPICAL
RIPPLE
IN
150mV
260mV
INPUT
35mV
70mV
= 3.6V)
Recommended Minimum V
12
I
OUT
3.0V
3.0V
3.0V
3.0V
capacitance is too small, the LM2770 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 capacitors might overwhelm the input and output ca-
pacitors, resulting in increased input and output ripple.
The flying capacitors should be identical. As a general guide-
line, the capacitance value of each flying capacitor should be
1/10th that of the output capacitor, up to a maximum of 1µF.
This is a recommendation, not a requirement. Polarized
capacitors (tantalum, aluminum electrolytic, etc.) must not be
used for the flying capacitors, however, as they could be-
come reverse-biased during LM2770 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
(ripple voltages, output current capability, etc.).
As previously discussed, input and output ripple voltages
and frequencies will vary considerably with output current
and input voltage. The numbers provided show expected
ripple voltage when V
tween 100mA and 250mA. The table offers first look at
approximate ripple levels and provides a comparison for the
different capacitor configurations presented, but is not in-
tended to be a guarantee of performance.
The columns that provide minimum input voltage recommen-
dations illustrate the effect that smaller flying capacitors have
on charge pump output current capability. Using smaller
flying capacitors increases the output resistance of the
charge pump. As a result, the minimunm input voltage of an
application using small flying capacitance may need to be
set slightly higher to prevent the output from falling out of
regulation when loaded.
= 50mA
I
OUT
3.0V
3.0V
3.1V
3.1V
IN
= 150mA
IN
= 3.6V and load currents are be-
for Different Output Currents
I
OUT
3.1V
3.1V
3.2V
3.2V
= 250mA