The Growing Importance of Inductance in Tantalum Capacitors

Until recently, no one paid much attention to the equivalent series inductance (ESL) of tantalum capacitors other than to recognize that it was generally higher than that found in MLCCs. Tantalum capacitors were considered good for bulk charge storage at relatively low frequencies, but high frequency performance was thought to be the unquestioned domain of the MLCC. But that is now starting to change as power distribution networks (PDNs) for high-performance microprocessors, especially those used in mobile computing platforms, evolve. In many instances it has been shown that a very few high-performance, low-inductance tantalum capacitors can outperform competing decoupling solutions that employ a larger number of conventional aluminum, tantalum, and ceramic capacitors. With the advent of high-bandwidth voltage regulator modules (VRMs), PDNs with almost ideal impedance characteristics to frequencies above 10 MHz can be designed using very few capacitors, as long as those capacitors have moderately high capacitance, very low ESL, and predictable, low ESR. These characteristics are the key to the recent success of tantalum polymer capacitors in an application that was once dominated by MLCCs. This paper briefly discusses the electrical theory behind the new success of tantalum polymer capacitors in midrange decoupling applications. The physical design of these low ESL tantalum capacitors is described and some of the challenges and strategies of measuring low-inductance capacitors are discussed.