Selection of filter capacitors in embedded applications

As capacitor manufacturing continues to move toward smaller packages, an ideal
solution for high capacitance, low ESR, and low voltage applications is 3-D multi-anode coated (conformal coated) chip capacitors.
Introduction
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 capacitance required to meet the device's energy conversion requirements 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 move toward smaller packages, an
ideal solution for high capacitance, low ESR, and low voltage applications is 3-D multi-anode coated (conformal coated) chip capacitors.
High Capacitance and Low ESR Technology
There are a variety of technologies that have been used to optimize the capacitance per unit volume. For example, coated chip tantalum capacitor technology eliminates the
lead frame structure of conventional molded solid tantalum capacitors, and this technology, similar to semiconductor special packaging, 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 pressed surface mount tantalum (SMD) capacitors. However, designers also need to minimize ESR,
and this requirement has stimulated a variety of candidate solutions.
Polymer Aluminum CapacitorsPolymer
aluminum capacitors have very low ESR, in the range of 10 μm or less, filling
the application space between high capacitance multilayer ceramic capacitors (MLCC) and tantalum polymer capacitors. However, although they meet the required
ESR requirements 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.
Solid Tantalum CapacitorsSolid
tantalum capacitors are available in standard and low ESR types. Both types are made with the usual lead frame structure. Solid
Tantalum Low ESR types have ESR values ​​in the range of 100 μm at 100 KHz. Since ESR values ​​are dependent on the surface of the anode
, larger case sizes generally have lower ESR values. Extensive powder development work in solid tantalum capacitors
has resulted in new lower ESR levels. Improvements in surge voltage have also made solid tantalum technology more robust.
Polymer Tantalum CapacitorsPolymer
tantalum capacitors use new highly conductive polymers. Highly conductive polymers are used for the cathode instead of manganese dioxide.
The improved 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, significantly lower than conventional solid tantalum capacitors of similar size. In fact
, the lead frame structure is the main constraint on the available capacitance for a given case size. Multi-Anode Tantalum Capacitors
Today, the dual requirements of high volume and low ESR are being addressed by a 3-D packaging approach, a multi
-anode tantalum capacitor that eliminates the conventional lead frame. This structure achieves high capacitance in a miniaturized SMD package that
is pin-compatible with conventional die-cast tantalum devices. Importantly, the technology achieves very low and stable ESR.
The main electrical and mechanical parameters of the multi-anode capacitors include:
High capacitance: typically >1000 F;
Very low and stable ESR over the operating temperature range;
Low inductance;
Wide voltage rating range: 4V, 6.3V and 10V;
Low DCL < 60 A;
Small size, low thickness 3D chip package;
No lead frame;
Standard pins, compatible with conventional die-cast tantalum capacitor dimensions
Bulk Decoupling Capacitor Applications
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 formats are also emerging, especially for
SBCs based on 16-bit and 32-bit processors. In addition, PC/104 SBCs must also achieve stack-through connections of 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 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.
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Power requirements are usually determined by the microprocessor or microcontroller manufacturer based on the voltage regulation module (VRM). Most systems
are built based on a synchronous buck converter that can provide multiple voltage values. Typically, they will provide 1.5~1.8V, 3.3V
and 5.0V
The processor core voltage, or VCORE, is often a major challenge in selecting low ESR bulk capacitors.
Evaluation of Appropriate Capacitor Technology
Analyze the processor manufacturer's recommendations for core voltage, such as specifying an appropriate filter capacitor for VCORE .
New processors that require a 1.5 V core voltage have the following example requirements: Output voltage = 1.5 V to 1.8 V;
Output ripple voltage = 2% of output voltage;
Output Current > 14 A;
Output Filter Capacitance = 3900 F/4V, ESR< 3 mTo
investigate the effect of this new packaging technology, the previously described capacitor technologies were evaluated to determine
the best technology in terms of board layout, component height, and electrical performance as an integral output filter capacitor for a PC/104SBC. However,
existing aluminum electrolytic capacitors were excluded because they exceeded the maximum height of 4.0mm (0.16”).
Capacitor technologies were reviewed to determine an implementation that would have the smallest total footprint on the printed circuit board (PCB) and the lowest ESR while meeting the height
constraints.
Although polymer tantalum capacitors have very good ESR, the overall capacitance requirements require more individual mounted capacitors. To
achieve the necessary bulk capacitance, 18 255D series 330 F capacitors would be required, occupying a total of 558 mm2 (0.88
inch2) of board space. This is significantly higher than an arrangement of 4 Vishay 597D multi-anode tantalum capacitors.