LM2773

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SNVS554A – JANUARY 2008 – REVISED MAY 2013

Output Capacitor and Output Voltage Ripple

The output capacitor in the LM2773 circuit (COUT) directly impacts the magnitude of output voltage ripple. Other

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 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 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

The input capacitor (CIN) is a reservoir of charge that aids a quick transfer of charge from the supply to the flying

capacitors 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 (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 LM2773 with different capacitor setups is

discussed below in Recommended Capacitor Configurations.

Flying Capacitors

The flying capacitors (C1, C2) transfer charge from the input to the output. Flying capacitance can impact both

output current 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 overwhelm the input and output capacitors, resulting in increased input and output

ripple.

In typical high-current applications, 1µF low-ESR ceramic capacitors 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 provided show expected ripple voltage with VIN = 3.6V and a load 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 ensure performance. With any capacitance configuration chosen, always 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.

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