SNVSA56B May 2015 – February 2017 LM2776
The peak-to-peak output voltage ripple is determined by the oscillator frequency, the capacitance and ESR of the output capacitor COUT:
In typical applications, a 1-µF 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.
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.