data/text/DLP1/899542

WEBENCH® Tools/TPSM846C24: Input Capacitor Selection

Part Number: TPSM846C24 Tool/software: WEBENCH® Design Tools Hi team, My customer wants to replace D/S recommended 330uF input bulk capacitor to MLCC due to availability issue. He wants to know if it is OK to replace bulk cap by 47uF x7 (329uF) MLCC, or fewer. 47uF MLCC will be 16V rated and it is going to decrease -30% - -53% capacitance due to DC bias. My customer don't want to use neither aluminum cap nor tantalum cap. Please help us move forward with 47uF MLCC. Regards, Itoh

Hi Itoh-san, It should be fine to use ceramic capacitors but some considerations should be kept in mind. What are the input voltage ripple and transient requirements? Does the proposed total ceramic input cap with derating due to DC bias still allow them to meet their input ripple requirements? You can refer to this app note for guidance on determining cap value. The other consideration is what the input source inductance is. If the input supply is coming in through long inductive leads or if an input filter inductor is used before the TPSM846C24, then this L-C can have high a high Q that can lead to input ringing, especially if all ceramic capacitors are used. The higher ESR of an aluminum/tantalum helps dampen out this resonance and is why we often include a bulk cap on our EVMs and other boards where the input is coming from a bench supply. If all ceramic input cap is used with an inductive source, then other methods may be needed to dampen the effective input filter as discussed in this app note on input filter design.