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

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.

TABLE 1. LM2770 Performance with Different Capacitor Configurations (Note 13)

CAPACITOR

CONFIGURATION

TYPICAL

OUTUT

RIPPLE

(V

TYPICAL

INPUT

RIPPLE

(V

Recommended Minimum V

I

I

I

C

C1=C2=1µF

25mV

35mV

3.0V

3.0V

3.1V

C

C1=C2=1µF

50mV

70mV

3.0V

3.0V

3.1V

C

C1 = C2 = 0.47µF

130mV

150mV

3.0V

3.1V

3.2V

C

C1 = C2 = 0.22µF

200mV

260mV

3.0V

3.1V

3.2V

Note 13: Refer to the text in the Recommended Capacitor Configurations section for detailed information on the data in this table

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