Abstract:
A method and system of voltage compensated integrated circuits. Operating characteristics of integrated circuitry are enhanced by application of voltage compensation.

Description:
This is a continuation of application(s) application Ser. No. 10/439,665 filed on May 16, 2003 now U.S. Pat. No. 6,831,494 which is hereby incorporated by reference to this specification which designated the U.S. 
    
    
     FIELD OF THE INVENTION 
     Embodiments in accordance with the present invention relate to the design and operation of integrated circuits. Specifically, an inverter circuit and a ring oscillator circuit are disclosed in embodiments in accordance with the present invention. 
     BACKGROUND 
     Semiconductor devices, e.g., digital logic integrated circuits, are generally operated with a positive supply voltage. However, the characteristics of such circuitry, e.g., operating frequency, gate delay and the like, vary widely with changes in supply voltage. 
     SUMMARY OF THE INVENTION 
     A method and system of voltage compensated integrated circuits are disclosed. Specifically, a voltage compensated inverter circuit is disclosed in accordance with one embodiment of the present invention. Also, a voltage compensated ring oscillator circuit is disclosed in accordance with another embodiment of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a voltage compensated inverter circuit, in accordance with embodiments of the present invention. 
         FIG. 2  illustrates a voltage compensated inverter circuit, in accordance with embodiments of the present invention. 
         FIG. 3  illustrates a voltage compensated ring oscillator circuit, in accordance with embodiments of the present invention. 
         FIG. 4  illustrates a flow diagram of a method of operating an integrated circuit, in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description of the present invention, voltage compensated integrated circuits, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be recognized by one skilled in the art that the present invention may be practiced without these specific details or with equivalents thereof. In other instances, well-known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention. 
     Voltage Compensated Integrated Circuits 
     Embodiments of the present invention are described in the context of design and operation of integrated semiconductors. More particularly, embodiments of the present invention relate to voltage compensated integrated circuits. It is appreciated, however, that elements of the present invention may be utilized in other areas of semiconductor operation. 
     Although the following description of embodiments in accordance with the present invention describes semiconductors formed in p-type materials, it is to be appreciated that symmetries with n-type materials are well known. For example, in general, exchanging p-type materials and processes with n-type materials and processes, and changing voltages from positive to negative can create symmetric structures and behaviors. 
     Consequently, embodiments in accordance with the present invention are well suited to semiconductors formed in n-type materials, and such embodiments are considered within the scope of the present invention. 
     Variations in the operating of integrated circuits corresponding to changes in supply voltage are well known. For example, the same integrated circuit generally operates faster at a higher supply voltage than it does at a lower voltage. 
     It is oftentimes desirable to minimize such variations in operation of integrated circuits with respect to variations in supply voltage. For example, a design may desire for gate delay characteristics to remain relatively constant over a range of supply voltages. As another example, it may be advantageous to operate an integrated circuit during periods of unusually wide supply voltage fluctuations, for example, during a power supply ramp up or ramp down. 
     A typical power supply for an integrated circuit does not instantaneously switch from an “off” state, supplying no power, to an “on” state, supplying voltage and current levels within an operating tolerance range for the integrated circuit. There is usually a finite period of time during which the voltage and current levels smoothly transition from zero to rated levels. Frequently, bulk and distributed capacitance of a printed wiring board associated with the integrated circuit and power supply also influence such transitions. This period is generally known or described as a “ramp,” “voltage ramp” or “ramp up” period. Similarly, when a power supply switches to an off state, voltage and current levels ramp “down.” 
     It may be desirable to operate an integrated circuit during ramp up and ramp down periods. For example, during ramp down, a computer system, e.g., a computer powered by removable batteries, may desire to perform “housekeeping” tasks, e.g., to save information from volatile memory to non-volatile memory. By way of further example, it may be advantageous to operate certain circuits, e.g., a timing source, during a ramp up period. 
     In order to perform such tasks during ramp up or ramp down, it is desirable to utilize circuits with greater resistance to voltage variations than are available under the conventional art. It is further known to adjust the supply voltage of integrated circuits, e.g., a microprocessor, to optimize a relationship between performance requirements and power consumption of an integrated circuit. Such operations can be enabled, or a range of voltage adjustment can be extended, by utilizing circuits with greater resistance to voltage variations than are available under the conventional art. 
       FIG. 1  illustrates a voltage compensated inverter circuit  100 , in accordance with embodiments of the present invention. Voltage compensated inverter circuit  100  comprises a conventional inverter circuit  110  and voltage compensating feedback circuit  120 . Voltage compensating feedback circuit  120  is coupled to conventional inverter circuit  110 . 
     Voltage compensating feedback circuit  120  comprises p-type transistor element  130  and n-type transistor element  140 . It is to be appreciated that voltage compensating feedback circuit  120  forms an inverter circuit, and that the input of voltage compensating feedback circuit  120  is coupled to the output of conventional inverter circuit  110 . Likewise, the output of voltage compensating feedback circuit  120  is coupled to the input of conventional inverter circuit  110 . 
     Transistor elements  130  and  140  are constructed so as to have a higher threshold voltage than the transistor elements comprising conventional inverter circuit  110 . In accordance with embodiments of the present invention, such a higher threshold voltage may be achieved by constructing transistor elements  130  and  140  as “thick gate” devices. It is appreciated that conventional inverter circuit  110  can be produced with conventional gate thicknesses as are standard to a particular logic family for a particular semiconductor process. Embodiments in accordance with the present invention are well suited to other techniques of adjusting threshold voltages of transistor devices, e.g., varying gate length and/or changing doping concentrations. 
     For example, for an exemplary 0.13 micron process, a standard gate can have a thickness of about 17 Angstroms, corresponding to a typical threshold voltage of about 250 mV, and begins to operate at about 350 mV. A “thick” gate can have a thickness of about 70 Angstroms, a typical threshold voltage of about 500 mV, and begin to operate at about 600 mV. 
     In general, semiconductor devices operate faster at higher supply voltages than at lower supply voltages. For example, conventional inverter circuit  110  will generally have a higher maximum frequency, faster switching time and lower gate delay at a higher supply voltage than at a lower supply voltage. Similarly, voltage compensating feedback circuit (inverter)  120  will generally have a higher maximum frequency, faster switching time and lower gate delay at a higher supply voltage than at a lower supply voltage. 
     When voltage compensating feedback circuit  120  is operating, it will oppose the operation of conventional inverter circuit  110 . More specifically, voltage compensating feedback circuit  120  tends to lower the maximum frequency and switching time, and increase the gate delay of conventional inverter circuit  110 . 
     Because voltage compensating feedback circuit  120  is constructed with devices having different voltage characteristics, e.g., threshold voltage, than the voltage characteristics of devices comprising conventional inverter circuit  110 , the effects of voltage compensating feedback circuit  120  upon conventional inverter circuit  110  will vary with frequency. More specifically, voltage compensating feedback circuit  120  will oppose the operation of conventional inverter circuit  110  to a greater or lesser extent at different supply voltages. 
     For a low range of supply voltages, e.g., supply voltages below about 500 mV, voltage compensating feedback circuit  120  will have substantially no effect upon the operation of conventional inverter circuit  110 . More specifically, the operating characteristics of voltage compensated inverter circuit  100  will be substantially dominated by the operating characteristics of conventional inverter circuit  110 . 
     For a middle range of voltages, e.g., supply voltages between about 600 mV and about 1.4 V, voltage compensating feedback circuit  120  will affect the operation of conventional inverter circuit  110  to a varying degree. Within such a middle range of voltages, voltage compensating feedback circuit  120  becomes “stronger,” or has a higher current output, and operates faster with increasing voltage. More specifically, the operating characteristics of voltage compensated inverter circuit  100  will be increasingly influenced by voltage compensating feedback circuit  120 , corresponding with increasing supply voltage in this range. 
     It is to be appreciated that, because voltage compensating feedback circuit  120  is coupled in opposition to conventional inverter circuit  110 , as voltage compensating feedback circuit  120  becomes “stronger” and faster with increasing supply voltage, this behavior opposes characteristics of conventional inverter circuit  110  to become faster with increasing supply voltage. As a beneficial consequence, voltage compensated inverter circuit  100  exhibits less dependence of operating characteristics on supply voltage than conventional inverter circuit  110  alone. 
     The size, shape and threshold voltages of transistor devices  130  and  140  to achieve a desired feedback characteristic can be determined through a process of simulation. The size, configuration and characteristics of conventional inverter circuit  110  are typically provided to a designer as standard elements of a logic family for a specific semiconductor process. Size and shape of transistor devices in integrated circuits are generally given as a ratio, e.g., “⅝.” The first number specifies the width of the cell, e.g., “5,” and the second number specifies the length of the cell, e.g., “8.” The numbers typically indicate the dimension in microns. For an exemplary voltage compensated inverter circuit  100  constructed in a 0.13 micron process, conventional inverter circuit  110  can comprise a 10/8p-type device (not shown) and a ⅝n-type device (not shown). Device  130  can be a 5/1.6 thick gate p-type device, and device  140  can be a 2.5/1.6 thick gate n-type device. 
     Because feedback circuit  120  comprises transistor devices, e.g., transistor devices  130  and  140 , having significant differences in construction from transistor devices comprising conventional inverter circuit  110 , e.g., substantial differences in area and gate thickness, characteristics of feedback circuit  120  vary differently with supply voltage than do characteristics of conventional inverter circuit  110 . More specifically, a frequency-voltage characteristic of feedback circuit  120  will typically have a different shape than a frequency-voltage characteristic of conventional inverter circuit  110 . 
     For circuits intended to operate at supply voltages up to and within a middle range of voltages, e.g., supply voltages from 0 volts up to about 1.4 V in the present example, such differences between the shape of frequency-voltage characteristics for the two stages may be discounted. 
     However, above a certain supply voltage, for example about 1.4 volts, differences between the shape of frequency-voltage characteristics for the two stages can become significant. For such supply voltages, the frequency-voltage characteristics of voltage compensating feedback circuit  120  can “overpower” conventional inverter circuit  110 , producing a slight decrease in maximum operating frequency with increasing supply voltage for the combined voltage compensated inverter circuit  100 . 
     In order to advantageously overcome such higher voltage characteristics of voltage compensated inverter circuit  100 , an additional stage of feedback can be added.  FIG. 2  illustrates a voltage compensated inverter circuit  200 , in accordance with embodiments of the present invention. In addition to the elements of voltage compensated inverter circuit  100  ( FIG. 1 ), voltage compensated inverter circuit  200  comprises an additional, or second feedback circuit  220 . Second feedback circuit  220  comprises a p-type transistor device  230  and an n-type transistor device  240 . 
     It is to be appreciated that second feedback circuit  220  forms an inverter circuit. It is to be further appreciated that second feedback circuit  220  is coupled to voltage compensating feedback circuit  120  so as to provide feedback to voltage compensating feedback circuit  120 . More specifically, the output of voltage compensating feedback circuit  120  drives the input of second feedback circuit  220  and the output of second feedback circuit  220  drives the input of voltage compensating feedback circuit  120 . Additionally, it is to be appreciated that second feedback circuit  220  actually reinforces conventional inverter circuit  110 . 
     Transistor elements  230  and  240  are constructed so as to have a higher threshold voltage than the transistor elements comprising voltage compensating feedback circuit  120  (which comprises transistor elements with higher threshold voltages than the transistor elements comprising conventional inverter circuit  110 ). In accordance with embodiments of the present invention, such a higher threshold voltage may be achieved by constructing transistor elements  230  and  240  to comprise “thicker” gates than those of voltage compensating feedback circuit  120 . Embodiments in accordance with the present invention are well suited to other techniques of adjusting threshold voltages of transistor devices, e.g., varying gate length and/or changing doping concentrations. 
     Second feedback circuit  220  serves to oppose the capability of voltage compensating feedback circuit  120  to overpower conventional inverter circuit  110  at higher supply voltages. As a beneficial consequence, voltage compensated inverter circuit  200  has more consistent operating characteristics, e.g., maximum frequency, switching and delay times, over a wider range of supply voltages in comparison with voltage compensated inverter circuit  100 . 
     It is to be appreciated that additional feedback stages beyond the two shown in  FIG. 2  can be added to improve the “flatness” of frequency response with respect to supply voltage of a voltage compensated circuit. Further, additional feedback stages can increase a range of supply voltages over which a desired flat frequency response to that supply voltage can be achieved. Embodiments in accordance with the present invention are well suited to a plurality of feedback stages. 
     A ring oscillator circuit generally comprises an odd number of inverter stages coupled in a ring configuration. It is to be appreciated that other inverting circuits, e.g., a NAND gate, are well suited to embodiments in accordance with embodiments of the present invention. A ring oscillator will oscillate, or “ring,” at a frequency determined, in part, by switching delay times of the inverter stages and the number of inverter stages. Ring oscillators are a straight-forward source of oscillating clock signals in integrated circuits, and are an ideal frequency source for many applications. Since a ring oscillator does not require external components, e.g., a crystal, ceramic resonator or external resistors and/or external capacitors, ring oscillators can be implemented at lower cost and in a smaller size than many other clock sources. 
     However, ring oscillators are sometimes not used in integrated circuit designs because their frequency of operation is determined (in part) by the switching delay time of each inverter stage. As has been discussed previously, switching delay time of a conventional inverter stage varies with supply voltage. Consequently, the frequency of a conventional ring oscillator varies with supply voltage. Because a conventional ring oscillator frequently has many stages, the variation of switching delay time due to supply voltage changes for a single stage is magnified by the number of stages, producing great frequency variations in response to supply voltage changes for the entire oscillator. 
       FIG. 3  illustrates a voltage compensated ring oscillator circuit  300 , in accordance with embodiments of the present invention. Voltage compensated ring oscillator circuit  300  comprises three stages. Each stage is comprised of a voltage compensated inverter circuit  100  as described above. Ring oscillators constructed with other voltage compensated circuits, e.g., voltage compensated inverter circuit  200  or circuits comprising more stages of feedback, are well suited to embodiments in accordance with the present invention. The frequency of operating of a ring oscillator, e.g., voltage compensated ring oscillator circuit  300 , depends on the delay inherent to each inverter stage and the number of stages provided in the ring. 
     Because the stages comprising voltage compensated ring oscillator circuit  300  have a stable frequency response over a range of supply voltages, voltage compensated ring oscillator circuit  300  exhibits a similar desirable stable frequency response over a range of supply voltages. Voltage compensated ring oscillator circuit  300  can be advantageously utilized to provide a stable frequency, e.g., for use as a microprocessor clock or to control a charge pump circuit, during a power supply ramp up and/or ramp down period. Further, voltage compensated ring oscillator circuit  300  can beneficially provide a stable frequency while a supply voltage of an integrated circuit, e.g., a microprocessor, is changed in order to optimize a relationship between performance requirements and power consumption of the integrated circuit. 
       FIG. 4  illustrates a flow diagram of a method  400  of operating an integrated circuit, in accordance with embodiments of the present invention. In block  410 , an output signal from a first circuitry is generated. The first circuitry comprises a first transistor device having a first threshold voltage. 
     In block  420 , an output signal from a second circuitry is generated. The second circuitry comprises a second transistor device having a second threshold voltage. The second circuitry takes the output signal from the first circuitry as an input. The first threshold voltage differs from the second threshold voltage. 
     In block  430 , an input to the first circuitry is driven with the output signal from the second circuitry. In this novel manner, the second circuit provides a feedback signal to the first circuitry, enhancing the stability of characteristics, e.g., maximum frequency, switching and delay times, with respect to supply voltage of the combination of the two circuits in comparison to the characteristics of the first circuitry alone. 
     Embodiments in accordance with the present invention, voltage compensated integrated circuits, are thus described. While the present invention has been described in particular embodiments, it should be appreciated that the present invention should not be construed as limited by such embodiments, but rather construed according to the below claims.