Introduction to Choosing MLC Capacitors For Bypass/Decoupling Applications

Methods to ensure signal integrity using decoupling capacitors have been the topic of many papers in the past as well as in the present. One can find equally many methods of decoupling as well. This paper will illustrate one of these established methods and introduce it in a theoretical sense using the most simplistic of terms. The paper will also describe the methods of the past (in slow speed systems) and the practices of the present (in high speed systems). TECHNICAL INFORMATION Introduction There have been numerous papers and articles published on the subject of bypass/decoupling methods to achieve signal integrity. The purpose and arrangement of this paper is to pull together the ideas from these various technical articles, the intent being a quick introductory guide to decoupling methods. It is meant to be a one-stop guide to define, establish the need, and illustrate a method in the science of decoupling. It should act as an introduction to a novice signal integrity or EMC designers who are looking to choose a Multilayer Ceramic Capacitor (MLC) for a bypass/decoupling (hereafter “decoupling”) application. Definitions Decoupling is a means of overcoming physical and time constraints found usually in a Power Distribution System (PDS) of a digital circuit [2]. Simply put, decoupling reduces switching noise in the PDS. Many times it is mistakenly referred to as filtering. These are two different applications and although a filtering circuit may perform decoupling, it is not optimally designed for that application. The same is true of a decoupling circuit. It may perform filtering, but it was not designed to perform this specific function in a prime manner. Decoupling may be seen as a two terminal application and filtering as a three or four terminal application (Figure 1). Decoupling delivers energy to a specific point and filtering (typically an EMI solution) modifies a signal along a path. The Need for Decoupling As stated earlier, decoupling reduces noise in the PDS. Electrical noise can be caused in a number of different ways. In RF circuitry, oscillators and amplifier circuits generate this noise. In the digital environment, the switching integrated circuits, power supplies and regulators mainly generate this noise. This noise can be thought of as voltage ripple in the PDS. At a given current, I, this ripple voltage or the voltage drop (across an ideal capacitor) can be described by Equation (3): Equation (3) states that a current draw, I, leads to a voltage drop, V. As in most CMOS circuitry, the IC chip only draws current when the transistors are switching, and only a leakage current during the “0” or “1” state. This means that when the IC switches, it draws current; therefore it results in a voltage drop leading to ripple noise in the power distribution system (PDS). Furthermore, with increased processor speeds, the ripple noise is much greater since more logic states draw current, simultaneously. Introduction to Choosing MLC Capacitors For Bypass/Decoupling Applications Yun Chase AVX Corporation 801 17th Avenue South Myrtle Beach, SC 29577 Figure 1: Types vs. Application Filtering Decoupling Design Must Fit Application Three Terminal Two Terminal I = C dV dt