SNVSA56B May 2015 – February 2017 LM2776
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 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.