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.
Based on the input voltage, the input current RMS current, and the input voltage peak-to-peak ripple you can choose the capacitor looking at the capacitor datasheets. It is recommended to use a combination of Aluminum Electrolytic (AlEl) and ceramic capacitors.
This application note describes the selection considerations of output capacitors, based on load transient and output impedance of processors power rails. Presently, there are no specific tools available for non-Intel processor output capacitors selection in multiphase designs.
For an output filter you choose a capacitor to handle the load transients and to minimize the output voltage ripple. The equation in Figure 3 shows the equation to determine the input current RMS (Root-Mean-Squared) current the capacitor can handle.
Analytical and experimental results show that output capacitors selection is optimized for load transient and output impedance, to fulfill non-Intel processor requirements. D-CAP+ is a trademark of Texas Instruments. High-performance microprocessors require low voltage and high current voltage regulator modules(VRM).
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.