Application of filter capacitors in embedded systems

Selecting bulk decoupling capacitors for energy storage/transfer processing in microprocessor systems is a complex matter. Due to the emphasis on physical size of the product, processor manufacturers generally only specify the amount of capacitance required to meet the energy conversion requirements of the device without considering the available space reserved for a suitable capacitor arrangement. Processors used in embedded

single-board computers also require higher capacitor charging and discharging performance, thereby requiring a low time constant. As capacitor manufacturing continues to advance into smaller packaging applications, an ideal solution for high capacitance, low ESR and low voltage applications is 3-D multi-anodized (conformalcoated) chip capacitors.

High Capacitance and Low ESR Technology

There are a variety of technologies that have been developed to optimize the capacitance per unit volume. For example, coated tantalum chip capacitor technology eliminates the lead frame structure of conventional molded solid tantalum capacitors, and this technology, similar to special packaging for semiconductors , greatly reduces the average size. Vishay has developed coated tantalum chip technology for capacitors that meet NASA's requirements. These products far exceed the volumetric efficiency of conventional molded surface mounted tantalum capacitors (SMD). However, designers also need to minimize ESR, and this requirement has stimulated a variety of candidate solutions.

Polymer aluminum capacitors

Polymer aluminum capacitors have very low ESR, in the range of 10m or less, filling the application space between high capacitance multilayer ceramic capacitors (MLCC) and tantalum polymer capacitors. However, although they meet the ESR requirements required in filtering applications, their volumetric efficiency is generally much lower than that of tantalum technology. In applications where assembly space is at a premium, this technology must give way to other technologies such as tantalum technology.

Solid Tantalum Capacitors

Solid tantalum capacitors are available in standard and low ESR types. Both types are made with the usual lead frame construction. Solid tantalum low ESR types have ESR values ​​in the 100m range at 100KHz. Since the ESR value is determined by the outer surface of the anode, larger form factors generally have lower ESR values. Extensive powder development work on solid tantalum capacitors has resulted in new, lower ESR levels. Improvements in surge voltage have also been made to make solid tantalum technology more powerful.

Polymer tantalum capacitors

Polymer tantalum capacitors use a new type of highly conductive polymer. A highly conductive polymer is used for the cathode instead of manganese dioxide. The improvement in conductivity of the polymer cathode results in lower impedance and lower ESR. The low impedance also results in excellent high frequency filtering response. Polymer tantalum capacitor technology has the lowest ESR, which is significantly lower than conventional solid tantalum capacitors of similar size. In fact, the lead frame structure is the main limit on the available capacitance for a given form factor.

Multi-anode tantalum capacitors

Today, the dual requirements of high volume and low ESR are being solved by a 3-D packaging method, which is a multi-anode tantalum capacitor that eliminates the conventional lead frame. This structure achieves high capacitance in a miniaturized SMD package and is pin-compatible with conventional molded tantalum devices. Importantly, this technology achieves very low and stable ESR. The main electrical properties and mechanical parameters

of multi-anode capacitors include: high capacitance: generally >1000F; very low and stable ESR within the operating temperature range; low inductance; wide rated voltage range: 4V, 6.3V and 10V; low DCL <60A; small size, low thickness 3D chip package; no lead frame; standard pins, compatible with the size of conventional molded tantalum capacitors Body decoupling capacitor application

A large number of today's embedded controllers are built using a single-board computer (SBC). The dominant industry standard is PC/104, which specifies a 3.8" x 3.6" form factor. New, smaller proprietary form factors are also emerging, particularly SBCs based on 16-bit and 32-bit processors. In addition, PC/104 SBCs must also achieve stack-through connections for multiple PC/104 boards to take advantage of the maximum mounting component height of 4.0mm (0.16”).

A considerable number of designers also prefer to use a microcontroller or microprocessor plus selected peripheral components to make their own custom embedded controller solutions. These solutions may be implemented directly on the PCB, and are also subject to the same compression space limitations as ordinary SBCs.

Therefore, the materials and packaging structure must be such that a capacitor can fit into the very small space between the CPU and the chipset without exceeding the strict height restrictions.

Power requirements are usually determined by the microprocessor or microcontroller manufacturer based on the voltage regulation module (VRM). Most systems are based on a synchronous buck converter that can provide multiple voltage values. Typically, they will provide voltages of 1.5-1.8V, 3.3V and 5.0V to the processor core, processor and chipset I/O, and various basic electrical units on the general board. The processor core voltage or VCORE is usually a major difficulty in selecting low ESR bulk capacitors.

Evaluation Analyze the processor manufacturer's recommendations regarding core voltage, such as specifying an appropriate filter capacitor for VCORE. A new processor requiring a 1.5V core voltage has the following example requirements:
Output voltage = 1.5V to 1.8V;
Output ripple voltage = 2% of output voltage;
Output current > 14A;
Output filter capacitor = 3900F/4V, ESR < 3mTo

investigate the effect of this new packaging technology, the previously described capacitor technologies were evaluated to determine the best technology for board layout, component height, and electrical performance as an integral output filter capacitor for a PC/104 SBC. However, existing aluminum electrolytic capacitors were excluded because they exceeded the maximum height of 4.0mm (0.16"). The

various capacitor technologies were reviewed to determine the implementation with the smallest total footprint on the printed circuit board (PCB) and the lowest ESR while meeting the height restrictions. A comprehensive table of all Vishay technology options is compiled below.

Although the polymer tantalum capacitors have good ESR, the overall capacitance value requirements require more individual mounted capacitors. To achieve the required volumetric capacitance, 18 255D series 330Fs are required, occupying a total of 558mm2 (0.88in2) of board space. This is significantly higher than an arrangement of 4 Vishay 597D multi-anode tantalum capacitors.

Multi-anode technology was chosen for this application as it occupies the smallest space and has the best ESR. 4 multi-anode 597Ds are mounted in parallel on the board, occupying an area of ​​124mm2 (0.19in2). This results in better volumetric efficiency than other technologies. The ESR of the parallel capacitor arrangement is <3m, meeting the target application requirements.