The input capacitor is used to suppress noise at the input of the power supply and reduce the ripple voltage seen at the input. Load current, duty cycle and switching frequency are some of the factors used to determine the magnitude of the input ripple voltage. Ceramic capacitors placed directly at the input of the regulator reduce ripple voltage amplitude. Only ceramics have the extremely low ESR that is needed to reduce the ripple voltage amplitude. These capacitors must be placed close to the regulator input pins to be effective.