Analog Dialogue, Volume 45 – January 2011
By Glenn Morita
Widely seen as a panacea for solving noise-related issues, capacitors deserve more respect. Designers often think that adding a few capacitors will solve most noise problems, but give little thought to parameters other than capacitance and voltage rating. Like all electronic components, however, capacitors are not perfect. Instead, they possess parasitic effective series resistance (ESR) and inductance (ESL); their capacitance varies with temperature and voltage; they are sensitive to mechanical effects.
Designers must consider these factors when selecting bypass capacitors—as well as for use in filters, integrators, timing circuits, and other applications where the actual capacitance value is important. An inappropriate choice can lead to circuit instability, excessive noise and power dissipation, shortened product life, and unpredictable circuit behavior.
Capacitor Technologies
Capacitors are available in a wide variety of form factors, voltage ratings, and other properties to meet the requirements of diverse applications. Commonly used dielectric materials include oil, paper, glass, air, mica, polymer films, and metal oxides. Each dielectric has specific properties that affect its suitability for a particular application.
In voltage regulators, three major classes of capacitors are commonly used as voltage input- and output bypass capacitors: multilayer ceramic, solid-tantalum electrolytic, and aluminum electrolytic. The Appendix provides a comparison.
Multilayer Ceramic
Multilayer ceramic capacitors (MLCC) combine small size, low ESR, low ESL, and wide operating temperature range, making them the first choice for bypass capacitors. They are not without faults, however. Depending on the dielectric material, the capacitance can vary dramatically with temperature, dc bias, and ac signal level. In addition, the piezoelectric nature of the dielectric material can transform vibration or mechanical shock into an ac noise voltage. In most cases, this noise tends to be on the order of microvolts, but in extreme cases, mechanical forces can generate noise in the millivolt range.
Voltage-controlled oscillators (VCOs), phase-locked loops (PLLs), RF power amplifiers (PAs), and other analog circuits are sensitive to noise on their power-supply rails. This noise manifests itself as phase noise in VCOs and PLLs, amplitude modulation in RF PAs, and display artifacts in ultrasound, CT scans, and other applications that process low-level analog signals. Despite these imperfections, virtually every electronic device uses ceramic capacitors due to their small footprint and low cost. For regulators used in noise-sensitive applications, however, designers must carefully evaluate their side effects.
Solid Tantalum Electrolytic
Solid tantalum capacitors are less sensitive to the effects of temperature, bias, and vibration than ceramic capacitors. A recent variation uses a conductive polymer electrolyte instead of the usual manganese dioxide electrolyte, providing improved surge-current capability and eliminating the need for a current-limiting resistor. Lower ESR is an additional benefit of this technology. Solid tantalum capacitors have a stable capacitance with temperature and bias voltage, so the selection criteria need only account for the tolerance, voltage derating at the operating temperature, and maximum ESR.
Conductive polymer tantalum capacitors with low ESR cost more and are somewhat larger than ceramic capacitors, but may be the only choice for applications that cannot tolerate noise due to piezoelectric effects. The leakage current of tantalum capacitors is much larger than for equal-value ceramic capacitors, however, rendering them unsuitable for some low-current applications.
A drawback of the solid polymer electrolyte technology is that this type of tantalum capacitor is more sensitive to the high temperatures encountered in the lead (Pb)-free soldering process, with manufacturers typically specifying that the capacitors not be exposed to more than three soldering cycles. Ignoring this requirement in the assembly process can cause long-term reliability issues.
Aluminum Electrolytic
Conventional aluminum electrolytic capacitors tend to be large and have high ESR and ESL, relatively high leakage current, and limited service lifetimes—measured in thousands of hours. OS-CON capacitors employ an organic semiconductor electrolyte and an aluminum foil cathode to achieve low ESR. Although related to solid polymer tantalum capacitors, they actually preceded tantalum capacitors by 10 years or more. With no liquid electrolyte to dry out, the service lifetime of OS-CON type capacitors is better than that of conventional aluminum electrolytic capacitors. Most are limited to 105°C, but OS-CON type capacitors capable of 125°C operation are now available.
Although the performance of the OS-CON type capacitor is better than that of conventional aluminum electrolytic capacitors, they tend to be larger and have higher ESR than ceramic or solid polymer tantalum capacitors. Like solid polymer tantalum capacitors, they do not suffer from the piezoelectric effect, so they are suitable for use in low-noise applications.
To be continued
