1 Overview

The second-order low-pass LC filter is the most commonly used filter type with DC/DC converters. The filter is used to suppress high-frequency noise or minimize the output voltage ripple. After the addition of the filters on the input and the output sides, the overall output impedance of the DC/DC converter shows high resonance peaks if the filters are undamped. When the load draws current, the voltage at the output can vary due to these resonance peaks in the impedance curve. One other concern is that if the resonant frequency of the filter is in the range of the crossover frequency of the control loop, it affects the control loop stability which further affects the transient behavior of the device. DC/DC converter acts as a load for the input filter which provides negative variable resistance. When this negative dynamic resistance is connected to the input filter, it acts as a negative oscillator circuit. It causes system instability in the case of an undamped system.

This application note discusses in detail how one can design the input and second order LC filter for DC/DC converter and which parameters are important to keep in mind while designing them. TPS6281x -Q1 is used as an example in this application note.