1 Introduction

As an excellent alternative to traditional bulky and expensive passive filters, active electromagnetic interference filters (AEFs) can help designers deal with ever-increasing EMI challenges, improve power density, and reduce the cost of power solutions. References [1] and [2] demonstrate an approximate 50% size reduction and >75% volume reduction with an AEF implemented in the Texas Instruments LM25149-Q1 buck controller.

Most AEFs use operational amplifier (op amp)-based active circuits to sense noise and inject an appropriate cancellation signal to reduce EMI, such as the AEF integrated into the LM25149-Q1. To achieve the best performance with this kind of AEF, the op-amp circuits need to be stable and the op amp should not saturate. Otherwise, the AEF would have worse performance and may even inject additional noise into the system [3]. This article discusses the proper compensation and damping techniques to achieve stability and the best performance of an AEF.