Several dielectric technologies compete for this board space. High Cap MLCC’s are sometimes selected when the total energy needs are low enough. When actually needed capacitance approaches and exceeds around 30 µF to 40 µF of delivered capacitance, polymers become a more sensible choice. No piezoelectric noise issues are sometimes reasons that a polymer would be selected over an MLCC. When dealing with a broad band application that requires low impedance, just one polymer capacitor will deliver a much low impedance across a wide frequency range. MLCCs have a very narrow low impedance curve sometimes referred to as a V-notch. Given this narrow range of capability, a series of MLCCs must be used to keep the impedance low across the range. The MLCC’s capacitance instability across voltage, temperature and time means that an advertised 100 µF MLCC value will only deliver a small fraction of that value in a true application condition. In the example shown in the lower right hand corner, when placing a 100 µF MLCC against a 100 µF Polymer, the outcome of a 5 V application working at 65oC with a switch frequency of 300 kHz will only deliver about 20% of its advertised value. Add in any aging effects and the resulting capacitance is even less. Polymers however remain stable across these application conditions. While some capacitance is lost due to switch frequency, around 95% of the capacitance value still remains. When replacing a 100 µF MLCC, remember that it may only need 1/3 of the capacitance value in a polymer capacitor solution.