LM2771

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SNVS435 – MARCH 2006

OUTPUT CAPACITOR AND OUTPUT VOLTAGE RIPPLE

prominent 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 capacitance will result 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

capacitance values can be used to reduce ripple, shrink the solution size, and/or cut the cost of the solution. But

changing 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 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 usually 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 significantly reduced by

placing a low-ESR ceramic capacitor 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 principles of parallel

resistance reduction.

INPUT CAPACITOR AND INPUT VOLTAGE RIPPLE

capacitor 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 proportional decrease (increase) in input voltage ripple. Input voltage, 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 capacitance 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 capacitor and/or output capacitor

to maintain good overall circuit performance. Performance of the LM2771 with different capacitor setups is

discussed below in RECOMMENDED CAPACITOR CONFIGURATIONS.

FLYING CAPACITOR

current 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 increased 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

200mA. The table offers first look at approximate ripple levels and provides a comparison for the different

capacitor configurations presented, but is not intended to be a guarantee of performance.

Copyright © 2006, Texas Instruments Incorporated

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