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High-speed
Circuit Decoupling By Rick Price, ITW Paktron, Lynchburg, VA Power distribution design had been given a low priority in the total systems design for microprocessors. Although power distribution design was an important part of the design process, digital design engineers would safely assume having a centralized "ideal" power supply, could ignore many of the inherent losses in the system, and only needed to apply a few rules-of-thumb to determine how many bulk and high frequency decoupling capacitors would be needed to compensate for power demand sag and noise spikes. Today's high performance microprocessors place more demands on the power supply along with imposing much tighter tolerance requirements. This translates to higher power distribution costs and an immediate shift upwards in design priority, with proper power distribution design practices now becoming a critical part of the design process. Microprocessors are being designed with lower voltage requirements in order to reduce overall power dissipation. These lower voltages require power supplies with much higher current capabilities (up to three times that of previous microprocessors, with load-change transients as high as 30 amps/µsecond). The inductance of a system (caused by cables, power planes, etc.) slows a power supply's ability to respond to these rapidly changing current requirements. Both bulk and high frequency bypass capacitors are required because of the relatively slow speed at which a power supply (or a DC-to-DC converter) can react. For example, a microprocessor's current transients are on the order of 1-20 ns while a typical voltage converter has a reaction time of 1-100 µs. Properly selected bulk capacitors will slow the transient requirement seen by the power source to a rate that the power source can supply by furnishing energy to the system until the power source can react to the demand. At the same time, properly selected high frequency capacitors will slow the transient requirement seen by the bulk capacitors to a rate at which the bulk capacitors can supply. It is usually recommended to use local regulation (the use of a power supply or regulator in close proximity to the microprocessor). While local regulation reduces distribution losses, physical logistics and a variety of thermal dissipation problems can limit the use of this approach. The latest generation of microprocessor is power hungry and future devices will be even more demanding. What is needed is a truly low impedance (low ESR and ESL) power decoupling device capable of extremely high operating frequency and packing enough capacitance to hold up the power train.
Unfortunately, this defies the impedance and parasitic loss characteristics of the typical decoupling capacitors in use today.
Recent developments in capacitor technology may allow for significant improvement in power distribution design by replacing the current MLC/tantalum-electrolytic two stage capacitive network decoupling system with a higher performance MLP/"OS-CON" system. The lower capacitance (low ESR and ESL) parallel set of MLC chip capacitors currently being used for the high frequency decoupling could be replaced with fewer units made with metallized polymer, and the high capacitance bank used for the bulk decoupling could be replaced with an "OS-CON" type electrolytic device made with a solid polymer based electrolyte system which would have lower loss and better frequency response than the tantalum-electrolytic capacitors presently used. Of particular interest to a digital design engineer is the high frequency decoupling bank (currently tens of MLC capacitors in parallel) and its proposed replacement with fewer, perhaps even a single capacitive device based upon ultra-thin, polymer film capacitor technology.
The Issue Noise bypass at high frequency requires a local capacitor network with a small lead-to-lead spacing so that the inductance can be held to a minimum.
Fig. 2 Noise and voltage droop caused by a change in logic state. A low resistance bypass is needed for the noise while high capacitance is needed to hold up the voltage. This has led to the convention of using tens of small MLC capacitors in parallel banks to limit the inductance, while reaching a capacitance value that is appropriate for the di/dt pulse of the bypass function. It has been shown that it is more efficient to use short-and-wide dimensioned chips, but this has also proven itself to be cost prohibitive
because it is limited by the fragile structure of ceramic capacitors.
Larger MLC capacitors are subject to cracking for several reasons including
temperature coefficient differences between the chip capacitor and the
circuit mounting material. An alternate capacitor dielectric system
is proposed that can eliminate the problem of chip cracking and allow
the production of "stick" rather than "chip" shaped
decoupling capacitors. The functionality and reliability of the high
frequency decoupling bank can be increased by the elimination of the
multiple solder joints, and the use of a better decoupling capacitor
based upon polymer film in place of the ferro-electric MLC capacitor.
The Design Approach In order to limit inductance, the proposed MLP high frequency decoupling capacitor bank would use a 5.0 mm (18XX series or approximately 0.200") terminal spacing stacked stick design, although several other options are being investigated.
Fig. 3 Power train bus impedance components. The relatively high capacitance values will be achieved by using ultra-thin films and making use of the length of the stick. The 5.0 millimeter stick has low inductance and ultra low ESR (see table below), while at very high frequencies the small lead spacing facilitates current distribution. Using 0.9 micron film, values of 5.0 microfarads per linear inch have been designed. For the higher value current hold up function, stick capacitors of 10 millimeter spacing are being designed which will have values of greater than 40 microfarads per linear inch (up from the 10 microfarads available today). Samples of these sub-micron film dielectric devices will be available by mid-September, 1997.
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