LM2770

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SNVS318E – NOVEMBER 2004 – REVISED MAY 2013

Due to the PFM nature of the LM2770, output voltage ripple is highest at light loads. To eliminate this ripple,

consider running the LM2770 in sleep mode when load currents are 20mA or less. Sleep mode disables the

charge pump and enables the internal low-noise bypass linear regulator (LDO).

INPUT CAPACITOR AND INPUT VOLTAGE RIPPLE

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 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. This can be observed in the input voltage ripple

waveforms in the Typical 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 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 capacitors and/or output capacitor

to maintain good overall circuit performance. Performance of the LM2770 with different capacitor setups is

discussed below in RECOMMENDED CAPACITOR CONFIGURATIONS.

FLYING CAPACITORS

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 capacitors, resulting in increased input and output

ripple.

The flying capacitors should be identical. As a general guideline, 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 become 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

currents are between 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 intended to ensure performance.

The columns that provide minimum input voltage recommendations 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.

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