Ceramic input caps offer low ripple and high peak current capability. Why there is often an electrolytic cap in parallel?
Hi Ilkka.
MLCCs are great for the reasons that you’ve stated, since they have low ESR. However, often designs still need additional bulk capacitance that you can find with an electrolytic cap or a POSCAP.
MLCC have a very low ESR and ESL which is good for high frequency decoupling. Often a larger parallel higher ESR capacitor (aluminum etc.) provides damping in the audio frequency range for the loop stability for the output (Bode plot) loop and for the input loop (negative input impedance of a regulated DC-DC). So you can benefit from paralleling MLCC with higher loss (ESR) capacitors to get good high frequency EMC behavior the MLCC provide and good damping for the regulation loops with higher ESR bulk capacitors like aluminum or polymer.