Abstract:
Two or more pulse width modulation stages, each having progressively higher resolution, are utilized to allow the lower resolution stage or stages to operate at lower clock speeds. Later stages are operated at higher clock speeds and thus a smaller portion of the total pulse width modulation circuit utilizes the higher clock speed. Additionally, later stages operate over smaller time intervals in order to reduce usage of the later stages.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of pulse width modulation and, more particularly, to multistage pulse width modulators. 
     BACKGROUND OF THE INVENTION 
     Pulse width modulation circuits (“pulse width modulators”) are utilized in a variety of different applications including digital-to-analog conversion, power supply control, and motor control. For example, pulse width modulators are utilized in digital pulse width modulator power amplifiers for audio amplification. Such devices eliminate the need for an extra digital-to-analog converter and enhance efficiency of the audio amplifier. 
     In general, the resolution R of a pulse width modulator output signal is determined by the ratio of the pulse width modulator counter clock rate F_CLK to the pulse width modulator output pulse frequency F_PWM. For example, for a two-level modulation scheme, R=(F_CLK/F_PWM)+1. Thus, in order to enhance the resolution of a pulse width modulator output signal for a given output pulse frequency, the pulse width modulator counter clock rate F_CLK needs to be increased. 
     Increasing the clock rate for the entire pulse width modulator circuit has certain drawbacks. As the clock rate for a circuit increases, the current consumption of the circuit also increases. Increased current consumption is particularly undesirable in applications in which power consumption is a critical operating characteristic. Increased current consumption leads to undesirable circuit heating. Higher circuit clock rates also increase the chance that noise signals will couple to other parts of the circuit, particularly in devices in which the pulse width modulator is part of an integrated circuit. Additionally, increasing the clock rate of a pulse width modulator increases the complexity of the circuit. 
     Accordingly, the desire and need to have a pulse width modulator design that allows for high resolution operation exist while avoiding some or all of the problems associated with increasing the clock rate of a pulse width modulation circuit. 
     SUMMARY OF THE INVENTION 
     Two or more pulse width modulation stages, each having progressively higher resolution, are utilized to allow the lower resolution stage or stages to operate at lower clock speeds. Later stages are operated at higher clock speeds, and thus a smaller portion of the total pulse-width modulation circuit utilizes the higher clock speed. Additionally, later stages operate over smaller time intervals in order to reduce usage of the later stages. 
     Accordingly, one aspect of the present invention provides an apparatus including a first pulse width modulator stage and a second pulse width modulator stage. The first pulse width modulator stage includes a first comparator operable to receive a first reference signal and a sample signal and to compare the first reference signal to the sample signal. The second pulse width modulator stage is coupled to the first pulse width modulator stage and includes a second comparator operable to receive a second reference signal and the sample signal and to compare the second reference signal to the sample signal. 
     Another aspect of the present invention provides a method of producing a pulse width modulated signal. A first reference signal is generated. The first reference signal is compared to a sample signal. A first output signal is produced depending on the comparison of the first reference signal to a sample signal. A second reference signal is generated. The second reference signal is compared to the sample signal. A second output signal is produced depending on the comparison of the second reference signal to the sample signal. 
     The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. As will also be apparent to one of skill in the art, the operations disclosed herein may be implemented in a number of ways, and such changes and modifications may be made without departing from this invention and its broader aspects. Other aspects, inventive features, and advantages of the present invention, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention and advantages thereof may be acquired by referring to the following description and the accompanying drawings, in which like reference numbers indicate like features. 
     FIG. 1 illustrates exemplary waveforms and signals associated with the pulse width modulation process. 
     FIG. 2 is a simplified block diagram of a portion of a digital-to-analog conversion circuit including an exemplary multistage pulse width modulator according to the present invention. 
     FIG. 3 is a simplified block diagram of an exemplary multistage pulse width modulator according to the present invention. 
     FIGS. 4A and 4B illustrate the levels, waveforms, and signals respectively associated with the first and second stages of an exemplary multistage pulse width modulator according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     The following sets forth a detailed description of at least the best contemplated mode for carrying out the one or more devices and/or processes described herein. The description is intended to be illustrative and should not be taken to be limiting. 
     FIG. 1 illustrates exemplary waveforms and signals associated with the pulse width modulation process. General operation of a pulse width modulator involves comparing a sample signal (e.g., sample  110 ) to a reference signal (e.g., reference  100 ) and producing an output signal  120  depending on the result of the comparison. In a typical example, reference signal  100  is a triangle waveform as shown in FIG.  1 . Pulse width modulators that use triangle waveforms centered on the sample interval as the reference signal are often referred to as double-sided modulators. The timing of reference signal  100  is selected to correspond with the desired sample intervals. For example, one period of reference signal  100  corresponds to the time period for each sample interval, and reference signal  100  is selected so that peaks of the signal correspond to sample interval boundaries. For each sample interval, sample  110  is determined by some appropriate means. For example, each sample value is based on a digital value supplied to the pulse width modulator. 
     In the example of FIG. 1, output signal  120  is low when reference signal  100  is greater than or equal to sample signal  110 . When sample signal  110  is greater than reference signal  100 , output signal  120  is high. Thus, larger sample values result in wider pulses, and smaller sample values result in narrower pulses. Sample values less than the minimum reference signal result in an output pulse that is entirely low, and sample values higher than the maximum reference signal result in an output pulse that is entirely high. 
     A number of variations of this basic scheme may be utilized with the present invention. For example, the comparison utilized to determine output signal  120  is changed so that output signal  120  is high when reference signal  100  is greater than or equal to sample signal  110  and output signal  120  is low when sample signal  110  is greater than reference signal  100 . Alternately, simple circuit elements such as inverters are utilized to transform output signal  120  into the desired format. Different reference signals, such as sawtooth waveforms, may be utilized. Additionally, the timing relationship among the reference signal, the sample signal, and the sample interval are adjusted as necessary to produce the desired result. FIG. 1 is generally representative of the waveforms associated with either analog or digital implementations of pulse width modulators. 
     In an ideal implementation, each output pulse is fully symmetric around the center of the sampling interval, and infinite resolution indicates that signal comparisons are exact so that the transitions from low-to-high values or high-to-low values are precisely defined. A typical digital solution inherently has a finite resolution of the output signal as determined by the number of discrete time steps or intervals within the sample interval and also the signal level intervals associated with each time interval. The effect of finite resolution is illustrated in FIGS. 4A and 4B as will be described below. 
     FIG. 2 is a simplified block diagram of a portion of digital-to-analog conversion circuit  200  including multistage pulse width modulator  230 . In this example, a digital signal  205  is received by filter  210 , which performs any desired or necessary signal filtering tasks. Quantizer  220  samples and processes the received digital signal to produce sample signals for use by multistage pulse width modulator  230 . Quantizer  220  returns information to correction block  240  for adjusting the state of filter  210 . Quantizer  220  also returns information to a gain stage (in this case a subtractive stage) for further signal processing. Multistage pulse width modulator (“PWM”)  230  produces pulses according to the samples received from quantizer  220 . For example, the pulse width modulated output signal  235  is then utilized to control one or more transistors that drive a speaker, to control a power supply, to adjust a motor, or the like. Digital-to-analog conversion circuit  200  is only one example of the types of circuits that utilize a multistage pulse width modulator, such as multistage pulse width modulator  230 . 
     FIG. 3 is a simplified block diagram of one exemplary embodiment of multistage pulse width modulator  230 . Multistage pulse width modulator  230  generally includes two stages  300  and  360  and may include additional stages. Sample register  320  receives data from quantizer  220 . Sample register  320  typically includes enough register space to contain at least one sample signal for comparison. In other examples, sample register  320  may be some other memory structure, such as a buffer or FIFO structure. The reference signal for stage  300  of multistage pulse width modulator  230  is determined by coarse ramp logic system  310 . Coarse ramp logic system  310  typically produces a series of ramp signals approximating an ideal reference signal, such as triangle waveform  100  of FIG. 1 within the resolution limits of the first stage. By setting the resolution of the first stage lower (e.g., with larger time intervals), stage  300  does not need to utilize a higher clock speed signal that a higher resolution stage would typically need. 
     Coarse ramp logic system  310  may be implemented in a number of ways. As illustrated, coarse ramp logic system  310  receives a clock signal and a dividing factor K. When a clock rate corresponding to the received clock signal is divided by the factor K, a new (slower) clock rate upon which the reference signal is based for stage  300  is determined. Coarse ramp logic system  310  includes a counter circuit  311  from which the reference signal is constructed. For example, the counter circuit  311  produces an output signal ranging from zero to a maximum number of signal intervals or levels minus one. The number of signal intervals is directly related to the number of time intervals utilized by the stage. For example, if one complete sample interval is divided into 64 time internals, the corresponding reference signal typically has 32 signal intervals or steps “down” (e.g., moving from the highest reference signal value to the lowest reference signal value in the center of the sample interval) and 32 steps “up”. Thus, the reference signal is approximated by a number of discrete signals having a set time interval and signal interval. As the counter circuit  311  proceeds from zero to the maximum number of signal intervals minus one, the counter signal is inverted for the second half of the sample interval. If necessary, the final output of the counter circuit  311  is inverted to produce the desired waveform shape (e.g., starting from the highest value, moving to the lowest value in the middle of the sample interval, and returning to the highest value at the end of the sample interval). A variety of other schemes and corresponding circuits for producing the reference signal may also be utilized. 
     Both the sample signal and the reference signal are supplied to comparator  330 , which makes the first of two comparisons utilized by stage  300 . Comparator  330  compares the reference signal to the sample signal and determines whether the reference signal is less than or equal to the sample signal. If the result of the comparison is positive (e.g., the reference signal is less than or equal to the sample signal), then comparator  330  produces a corresponding output signal, which is typically a logic true value such as a high signal level. If the result of the comparison is negative, (e.g., the reference signal is not less than or equal to the sample signal), then comparator  330  produces a corresponding output signal, which is typically a logic false value such as a low signal level. The resulting output signal from comparator  330  is sent to output logic  390 , which ensures that modulator  230  produces the proper output signal  235 . 
     Comparator  340  also receives the sample signal and the reference signal. Comparator  340  compares the reference signal to a sum of the sample signal and the signal interval as previously determined by the dividing factor K. If the reference signal is greater than the sum of the sample signal and the signal interval, the result of the comparison is positive. If the reference signal is not greater than the sum of the sample signal and the signal interval, then the result of the comparison is negative. The output signal from comparator  340  is sent to output logic  390 , which ensures that modulator  230  produces the proper output signal  235 . 
     FIG. 4A illustrates the output signal  235  resulting from the comparison of comparator  340 . In this example, nine signal intervals decrease while nine signal intervals increase. Ten corresponding reference signals  420  are in the decreasing direction, and ten reference signals  420  are in the increasing direction. Collectively, the reference signals  420  approximate an ideal reference signal  400 . The sample signal is shown by level  430 . As shown by the first five reference signals  420  (moving from left to right), the result of the comparison performed by comparator  330  is negative (e.g., the reference signal is not less than or equal to the sample signal). For the same five reference signals  420 , the result of the comparison performed by comparator  340  is positive (e.g., the reference signal is greater than the sum of the sample signal and one signal interval). Output  235  remains low, and the result from NOR gate  350  is logically false, (e.g., low value so that stage  360  is not activated). The next reference signal  420  is still greater than sample signal  430 , and the result of the comparison performed by comparator  330  is negative. However, the same reference signal is no longer greater than the sum of the sample signal and one signal interval as determined by comparator  340 . Because the output signal from both comparators is logically false, NOR gate  350  produces a true output signal thereby activating stage  360 . 
     In one embodiment, stage  360  includes fine ramp logic  370  and comparator  380 . In general, fine ramp logic  370  is similar to coarse ramp logic  310  with at least two important differences. Fine ramp logic  370  produces reference signals starting with the value of the reference signal produced by coarse ramp logic  310  when stage  360  is activated. Also, fine ramp logic  370  uses smaller time intervals and signal intervals than those used by coarse ramp logic  310 . For example, fine ramp logic  370  produces reference signals based directly on the clock signal that it receives. Thus, fine ramp logic  370  produces a higher resolution reference signal. However, stage  370  is typically activated only when the higher resolution is needed, (e.g., when the sample signal is within one signal interval of a reference signal generated by coarse ramp logic  310 ). 
     When stage  360  is activated, comparator  380  compares the reference signal from fine ramp logic  370  with the sample signal from sample register  320 . In this example, if the reference signal is less than or equal to the sample signal, the comparison result is positive and a logical true signal (e.g., a high signal) is produced as output signal  235 . If the reference signal is not less than or equal to the sample signal, the comparison result is negative, and a logic false signal (e.g., a low signal) is produced as output signal  235 . 
     As noted above, output logic  390  is utilized to ensure that the proper output signal  235  is produced from the various signals provided by comparators  330 ,  340 , and  380 . FIG. 3 illustrates one possible implementation (using an inverter, an AND gate, and an OR gate) of output logic  390 . An alternative to utilizing output logic  390  is to encode logic on the output to the next stage. For example, when the output of comparator  330  is active, the sample input to comparator  380  is forced high. Similarly, when the output of comparator  340  is active, the sample input to comparator  380  is forced low. However, a variety of different logic structures may be implemented to perform the same task. Additionally, variations in the implementation of other portions of modulator  230  may dictate variation in the implementation of output logic  390 . In general, different encodings of logic for control between stages can be used. 
     FIG. 4B illustrates the output signal  235  resulting from the comparison performed by comparator  380 . Fine ramp logic  370  produces reference signals  440 . In this example, five signal intervals and thus four additional reference signals  440  exist between reference signals  420 , which are produced by coarse ramp logic  310 . Additional reference signals  440  (not illustrated) coincide with reference signals  420  as produced by coarse ramp logic  310 . As stage  360  cycles through each reference signal  440 , the reference signal is compared with the sample signal and the appropriate output value is generated. In one embodiment, operation of stage  360  continues until the signal level corresponding to the next reference signal  420  is reached. At that time, stage  360  is deactivated, and the level of output signal  235  is determined by stage  300 . When comparators  330  and  340  both have negative results, stage  360  is activated. 
     In this example, stage  360  includes only one comparator. The type of comparisons made may vary depending upon the desired form of output signal  235  and the form of the reference signals utilized. The counter utilized in the second stage may also be utilized for the clock divider signal in the first stage. 
     The examples described above illustrate a two stage, and thus two resolution, multistage pulse width modulator. However, additional stages may be added to further enhance the resolution of multistage pulse width modulator  230 . For example, a three stage pulse width modulator might be implemented where the first and second stages are similar to stage  300  but use large and intermediate step sizes, respectively. The third stage would be similar to stage  360  and use a fine step size. Furthermore, one or more additional stages beyond the first two stages are implemented such that the additional stages are optionally activated when additional resolution is desired. 
     A variety of different circuits may be utilized to implement comparators  330 ,  340 , and  380 , including iterative networks of logic gates and adders. Although the examples described have emphasized digital logic implementations, the devices and techniques of the present invention may be implemented utilizing analog circuitry and signals. 
     Signal flow in the multi-stage pulse width modulator is “forward”—going from input towards output—and typically without any loops. Thus, any number of pipe-lining registers is inserted into the circuit as is deemed necessary to operate at the desired speed given a specific process and operating environment. 
     While the invention has been described in light of the embodiments discussed above, one skilled in the art will recognize that certain substitutions may be easily made in the circuits without departing from the teachings of this disclosure. For example, a variety of logic gate structures may be substituted for those shown and still preserve the operation of the circuit. In particular, a NAND gate may be replaced by a NOR gate by appropriate polarity changes of the various signals in accordance with DeMorgan&#39;s law. Also, many circuits using NMOS transistors may be implemented using PMOS transistors instead as is well known in the art, provided the logic polarity and power supply potentials are reversed. The transistor conductivity-type (i.e., N-channel or P-channel) within a CMOS circuit may be frequently reversed while still preserving similar analogous operation. 
     Although the present invention has been described with respect to specific preferred embodiments thereof, various changes and modifications may be suggested to one skilled in the art and it is intended that the present invention encompass such changes and modifications fall within the scope of the appended claims.