Output Ripple Voltage for Buck Switching Regulator (Rev. A)

Switched-mode power supplies (SMPSs) are used to regulate voltage to a certain level. SMPSs have an inherent switching action, which causes the currents and voltages in the circuit to switch and fluctuate. The output voltage also has ripple on top of the regulated steady-state DC value. Designers of power systems consider the output voltage ripple to be both a key parameter for design considerations and a key figure of merit. The online WEBENCH® Power Designer recognizes the key importance of peak-to-peak voltage output ripple voltage—the ripple voltage is calculated and reported in the visualizer [1]. This application report presents a closed-form analytical formulation for the output voltage ripple waveform and the peak-to-peak ripple voltage. This formulation is accurate over all regions of operation and harmonizes the peak-to-peak ripple voltage calculation over all regions of operation. The new analytical formulation presented in this application report gives an accurate evaluation of the output ripple as compared to the simplified linear or root-mean square (RMS) approximations often used. In this application report, the analytical model for output voltage waveform and peak-to-peak ripple voltage for buck is derived. This model is validated against SPICE TINA-TI simulations. This report presents the behavior of ripple peak-to-voltage for various input conditions and choices of output capacitor and compare it against SPICE TINA-TI results. This report analyzes and presents the validity of the linear and the RMS approximation. Examples from the TI portfolio are presented that compare the experimental ripple waveforms in different regimes.