With the introduction of its AO-CAP (A700 series), component manufacturer Kemet Electronics of Greenville, S.C., becomes another source for surface-mount aluminum polymer capacitors. Kemet joins Panasonic in producing these low-ESR, high-frequency capable components, which are finding applications in power-management circuits associated with advanced microprocessors. As processor speeds continue to rise, demand for surface-mount aluminum polymer capacitors will likely increase as well.
The AO-CAP components will be offered in the same case sizes as Kemet's tantalum chip capacitors. Initially, they will be available in values up to 220 µF with voltage ratings as high as 6.3 V. However, the company plans to release a 2-V part specified at 390 µF by year's end, to be followed by a 470-µF unit next year. In addition, there are plans for 16- or 20-V models next year.
According to Kemet's John Prymak, aluminum polymer capacitors fill a void left by existing ceramic and tantalum components. Both ceramic and tantalum parts currently present difficulties in high-speed surface-mount applications. Tantalums offer fairly high values of capacitance, but they suffer from high ESR as well as a roll-off in capacitance at high frequencies.
On the other hand, ceramics have low ESR and good frequency response but are difficult to obtain in high values. Multilayer ceramic capacitors can be fabricated in larger values but may suffer cracking in surface-mount assembly. Prymak notes that cracking occurs with packages larger than a 1210 case (3.2 by 2.5 mm), limiting surface-mount ceramic capacitance options to about 22 µF. If designers opt to parallel multiple ceramic capacitors to obtain the desired value, a large number of components will likely be re-quired.
In contrast, aluminum polymer capacitors offer the high capacitance values listed above while still exhibiting lower ESR and better high-frequency re-sponse than tantalum equivalents. A comparison of two 47-µF capacitors, a tantalum in a 7.3- by 4.3- by 3.1-mm package (T491D) and an AO-CAP of 7.3 by 4.3 by 2.0 mm (A700), shows the advantages of the aluminum polymer style (see the figure).
The AO-CAP employs a stacked construction consisting of aluminum as the anode material, aluminum oxide as the dielectric, and a conductive organic polymer for the counter electrode material. A molded case and wraparound terminations protect the device against flex-induced cracking. The design is lead-free, and the device operates from −55°C to 105°C with no voltage derating.
AO-CAP is jointly marketed by Kemet and TTI, the Fort Worth, Texas-based distributor. For more information, contact Cathy Walensky of TTI by phone at (817) 740-9000. Or, point your browser to www.ttiinc.com or www.kemet.com.