Computer-aided design and analysis of circuits and semiconductor – Nanotechnology related integrated circuit design
Reexamination Certificate
2002-10-31
2004-09-07
Siek, Vuthe (Department: 2825)
Computer-aided design and analysis of circuits and semiconductor
Nanotechnology related integrated circuit design
C716S030000, C716S030000
Reexamination Certificate
active
06789241
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electronic circuits, and more particularly, the design of power distribution systems.
2. Description of the Related Art
As computer system technology advances, there is a continuous demand for systems that demand higher power at a relatively low voltage. Designing power distribution systems that can deliver a large amount of current at low voltages is a significant challenge. Tight voltage tolerances (e.g. ±5%) are often times required to ensure the proper performance of silicon chips within a computer system. The lower operating voltages may result in much lower target impedance requirements. At the same time, the clock frequencies at which microprocessors and other types of silicon chips operate are continually increasing, thereby resulting in a much wider frequency range for which target impedance requirements must be met.
Power distributions systems typically include at least one pair of planar conductors (e.g. a power plane and a ground plane), separated by a dielectric layer. A plurality of capacitors may be electrically connected in a parallel configuration between the planar conductors in order to provide a low impedance path for power distribution. Many power distribution systems employ a plurality of ceramic capacitors mounted upon a printed circuit board (PCB). Such capacitors may be chosen based on their ability to meet target impedance requirements for a given frequency. Capacitors having different electrical characteristics may be chosen to meet the target impedance requirements over a wide frequency range.
Electrical characteristics of capacitors which must be considered when designing power distribution systems include capacitance, equivalent series resistance (ESR) and equivalent series inductance (ESL). These characteristics have a significant effect on the electrical response of a given capacitor over a frequency range. At lower frequencies, the impedance provided by a capacitor is dominated by its capacitance. Since capacitors include conductive elements, such as conductive plates and mounting pads or pins, there is an inductance (ESL) associated with them. This inductance dominates the impedance profile of a capacitor at higher frequencies. The point at which the inductive and capacitive reactances cancel each other out is known as the resonant frequency, and the impedance provided by the capacitor at this frequency (which is purely resistive) is known as the ESR.
FIG. 1
illustrates the frequency response for a typical capacitor. At lower frequencies, the impedance decreases with frequency at a rate of approximated −20 dB/decade. At these frequencies, the impedance provided by the capacitor is dominated by capacitance, and may be calculated by the formula
Z
=
1
j
ω
C
,
where Z is the impedance, C is the capacitance, and &ohgr; is the angular frequency. As frequency increases, the capacitor will eventually achieve a state of resonance, as the capacitive reactance will be offset by the inductive reactance. This resonant frequency, sometimes referred to as the series resonant frequency, may be calculated by the formula
F
=
1
2
π
LC
,
where F is the resonant frequency, L is the equivalent series inductance, and C is the capacitance. The impedance provided by a capacitor at its resonant frequency is the capacitor's ESR. At frequencies above the resonant frequency, the impedance provided by a capacitor may be dominated by its ESL. The impedance of the capacitor for frequencies greater than the resonant frequency may be calculated by the formula
Z=j&ohgr;L,
where Z is the impedance, L is the ESL, and &ohgr; is the angular frequency. In designing power distribution systems, a plurality of capacitors having different impedance profiles may be combined in order to achieve a target impedance over a wide frequency range.
Designing power distribution systems and determining the necessary decoupling capacitors may include a significant amount of modeling and simulation. The power distribution system, as well as the various circuitry to which power is to be provided, may be modeled and simulated so as to predict the performance of the power distribution system under various operating conditions.
Performance prediction using modeling and simulation may include determining the effects of certain resonances that may affect power distribution system operation. These resonances may be associated with the parallel plate geometry of the power planes, and may have significant effect on the performance of the power distribution system if not accounted for during the design phase. In particular, there are two types of resonances that typically must be addressed in order to design a power distribution system that is effectively decoupled over a wide frequency bandwidth. One of these types of resonances is known as LC (inductive-capacitive) resonance. LC resonance may result from the inter-plane capacitance (i.e. the capacitance existing between the power and ground planes, including any capacitors electrically coupled between these planes) resonating with the inductance of the mounted decoupling capacitors. LC resonance may result in one or more impedance peaks at frequencies about the series resonant frequency.
The other type of resonance that must be managed is known as cavity resonance. Cavity resonances may be a function of the dimensions (x-y) and geometry of the circuit board or carrier of the power distribution system and the various frequencies of the system (i.e. clock frequencies and associated harmonics). In particular, the relationship between the dimensions of the circuit board and the wavelengths of the various frequencies present may cause impedance peaks and valleys to occur at various physical positions on the board itself. These positions may be located at distances that are multiples of ¼ wavelength, ½ wavelength, {fraction (3/2)} wavelengths, and so on, from the edge of the circuit board. Standing waves at these positions may result in either impedance peaks or impedance valleys. The high impedance peaks, if left unchecked, may result in excess noise in the power distribution system, and may also be problem frequencies for electromagnetic interference (EMI).
Managing both the LC and cavity resonances may be affected by the manner in which the power distribution system and its various components are modeled, as well as the order in which the resonances are dealt with during the design phase. However, it is important that impedance peaks resulting from both of these types of resonance be damped in order to meet target impedance requirements across a wide range of frequencies.
FIGS. 2 and 3
may illustrate the effects of one method of modeling capacitors.
FIG. 2
is a schematic of one embodiment of a traditional electrical model for a capacitor. The embodiment shown is a model of a capacitor based on a series RLC circuit. The model includes a resistor representing the capacitor's ESR value, a capacitor representing its capacitance value, and an inductor representing its ESL value. The model may be implemented as a SPICE model or other type of mathematical for simulation on a computer system.
The capacitor model of
FIG. 2
may be useful for simulation at lower frequencies, but may be inadequate for higher frequencies. As previously stated, power distribution systems typically include a pair of planar conductors separated by a dielectric, which may act as a capacitor at lower frequencies. At higher frequencies, a pair of planar conductors may develop the impedance resonances discussed above. Impedance peaks resulting from these resonances are sometimes referred to as anti-resonances, or parallel resonances. The traditional series RLC circuit model of a capacitor may be unable to correctly predict the frequency or frequencies at which anti-resonances occur.
FIG. 3
is a graph illustrating the simulated and measured performance of a capacitor mounted between two power planes over a range
Anderson Raymond E.
Chun Sungjun
Smith Larry D.
Kivlin B. Noäl
Meyertons Hood Kivlin Kowert & Goetzel P.C.
Siek Vuthe
Sun Microsystems Inc.
Tat Binh
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