The first objective in selecting input capacitors is to reduce the ripple voltage amplitude seen at the input of the module. This reduces the rms ripple current to a level which can be handled by bulk capacitors. Ceramic capacitors placed right at the input of the regulator reduce ripple voltage amplitude.
Depending on what you are trying to accomplish, the amount and type of capacitance can vary. The first objective in selecting input capacitors is to reduce the ripple voltage amplitude seen at the input of the module. This reduces the rms ripple current to a level which can be handled by bulk capacitors.
Taking the temperature and voltage effects is extremely important when selecting a ceramic capacitor. The Multilayer Ceramic Capacitor Selection section explains the process of determining the minimum capacitance of a capacitor based on its tolerance and dc bias characteristics.
Apart from the suitability of different capacitors for specific applications, other important factors that may need to consider include the following: Tolerance – It must be checked if the working of the circuit depends on precision capacitance. A capacitor with the lowest tolerance should be used if it requires narrow capacitance.
The most important parameters are the magnitude of the load transient (ΔI) and the distributed bus impedance to the load. The selection of the output capacitors is determined by the allowable peak voltage deviation (ΔV). This limit should reflect the actual requirements, and should not be specified lower than needed.
There are two key factors for selecting bulk input capaci-tors: 1) overshoot and undershoot requirement of transient response; and 2) allowable ripple current requirement. The ESR of the bulk capacitor (ESRB) and the capaci-tance (CB) need to meet the transient response requirement.