Application note from Würth Elektronik about the effect of filters that are added to the output of switching regulators. Link here
The output voltage of switching regulators has a voltage ripple that can disturb with electrical power supplied circuits and lead to electromagnetic disturbances. Thus output filters are often used for noise suppression, which may under certain circumstances have an influence on the control loop. To prevent output power losses it may be necessary to compensate the control loop.
No matter what switching regulator topology is used, as a result of the parasitic series resistor ESR and the parasitic inductance ESL of the output capacitor, the output current causes an undesired residual ripple. Depending on the capacitor type selected, a relatively large residual ripple is created, which has varying wave forms. A common electrolytic capacitor, for example, can have a ripple voltage of up to a few hundred millivolt, depending on the output power of the switching regulator. If a ceramic capacitor is chosen, the ripple voltage may only be a few tenth of a Volt.