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LM2776

SNVSA56A – MAY 2015 – REVISED FEBRUARY 2016

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NOTE

In high-current applications, TI recommends a 10-µF, 10-V low-ESR ceramic output

capacitor. If a small output capacitor is used, the output ripple can become large during

the transition between PFM mode and constant switching. To prevent toggling, a 2-µF

capacitance is recommended. For example, a 10- µF, 10-V output capacitor in a 0402

case size typically only has 2-µF capacitance when biased to 5 V.

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 is in parallel with the higher ESR, resulting in a low net ESR based on the principles of parallel

resistance reduction.

8.2.2.5 Input Capacitor

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 and first-order effect on input ripple magnitude. Increasing (decreasing) the input capacitance results in

a proportional decrease (increase) in input voltage ripple. Input voltage, output current, and flying capacitance

also affects input ripple levels to some degree.

In typical applications, a 1-µF low-ESR ceramic capacitor is recommended on the input. When operating near the

maximum load of 200 mA, a minimum recommended input capacitance after taking into the DC-bias derating is 2

µF or larger. Different input capacitance values can be used to reduce ripple, shrink the solution size, and/or cut

the cost of the solution.

8.2.2.6 Flying Capacitor

The flying capacitor (C1) transfers 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 LM2776 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, TI recommends 0.47-µF or 1-µF 10 V low-ESR ceramic capacitors 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 LM2776 operation.