Abstract:
A pulse-width modulation (PWM) controller to supply power to electronic components using a phase lock loop (PLL) is presented. A PWM controller comprises an input node operable to receive a reference signal and a phase-locked loop (PLL). The PLL comprises an oscillator operable to receive an error-correction signal and to generate an oscillator signal having a frequency that is related to the error-correction signal, a phase-frequency detector (PFD) coupled to the oscillator and operable to receive the reference signal and to generate the error-correction signal based upon a phase difference between the reference signal and a feedback signal, and a suppression circuit coupled to the PFD and operable to periodically enable the PFD to generate the error-correction signal.

Description:
CROSS-RELATED APPLICATION  
       [0001]    This application is related to U.S. application Ser. No. ______ (Atty. Docket No. 1938-005-03) titled PHASE-LOCK LOOP HAVING PROGRAMMABLE BANDWIDTH which was filed on the same day as this application and is hereby incorporated by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    A graphics board is a printed-circuit board that typically includes at least one graphics processor and other electronic components that process and display graphics or other video data in a computer system. FIG. 1 is a block diagram of a conventional graphics board  100  that includes a graphics processor  105 . Typically, one of the electronic components connected to the graphics processor  105  is a double-data-rate random-access memory (DDS RAM) chip  106 . Both the graphics processor  105  and the DDR RAM  106  typically have high power requirements as compared to other electronic components. For example, the graphics processor  105  typically requires 5-15 amps (A) of power at 1.6 volts (V), and the DDR RAM  106  typically 5-10 A and 10-20 A at 1.25 V and 2.5 V, respectively. Because the processor  105  and DDR RAM  106  have such high power requirements, pulse-width-modulated (PWM) switching power supplies  110   a,    110   b,  and  110   c  are typically provided for the graphics processor  105  and the DDR RAM  106 . Typically, the PWM power supplies  10   a,    110   b,  and  110   c  each include a separate PWM-controller chip, although, these controllers can be integrated into the graphics processor  105  and DDR RAM  106  chips, respectively.  
           [0003]    Ideally, the operating frequencies of the PWM power supplies  110   a,    110   b,  and  110   c  are the same. If, however, these frequencies are different, undesirable “beat” frequencies can result. A beat frequency is equal to the difference between the two frequencies. Unfortunately, the beat frequency can cause undesirable artifacts to appear in a video display.  
           [0004]    One technique for reducing or eliminating the beat frequency is to have a master clock chip  115  that generates a master clock signal for all three PWM power supplies  110   a,    110   b,  and  110   c.  The PWM controllers  120   a,    120   b,  and  120   c  will typically divide down the frequency of the master clock signal to a desired PWM frequency. For example, a typical frequency for the PWM power supplies  110   a,    110   b,  and  1   10   c  can range from 100 kilohertz to 1 megahertz, and the master clock frequency may be an order of magnitude above the PWM frequency. By providing the same master clock frequency to all the PWM controllers  120   a,    120   b,  or  120   c,  ideally all of the PWM signals should have the same frequency thus eliminating any beat frequency.  
           [0005]    But, providing a master clock signal can have several disadvantages. Because the PWM controllers  120   a,    120   b,  and  120   c  have high-impedance clock inputs noise may cause jitter and other artifacts on the master clock signal. Furthermore, the master clock signal paths to the PWM controllers  120   a,    120   b  and  120   c  may have different propagation delays. Such jitter, artifacts, and signal delays may cause the PWM signals generated by the PWM controllers  120   a,    120   b  and  120   c  to have different frequencies. Again, having different frequencies may give rise to a beat frequency that may cause visual artifacts in the video display. Furthermore, the master clock chip which takes up space on the graphics board  100  and, thus, increases component count, overall cost, and manufacturing complexity.  
           [0006]    Another technique for reducing or eliminating the beat frequency is, instead of using a master clock chip  115 , for two of the PWM controllers  120   b,  and  120   c,  (slaves) of the graphics board  105  to lock onto the PWM signal of the other PWM controller  120   a  (master) using a phase-locked loop (PLL). The slave PLLs can each generate one or more slave-PWM output signals that are phase locked to the master-PWM signal, and, that have the same frequency as the master-PWM signal. One problem with using a slave PLL, however, is that because it typically operates at a relatively low bandwidth (e.g. 100 Hz to 100 kHz) the PLL typically requires relatively large passive filter components (typically a capacitor) to set the bandwidth. Such a component is typically too large to be integrated onto a PWM controller chip  120 , and thus, must be located on the graphics board  100  external to the PWM controller chip  120 . Unfortunately, such an external component occupies space on the graphics board  100  and, thus, often increases the component count, overall cost and manufacturing complexity of the graphics board  105 . Furthermore, the external component requires that the PWM controller chip  120  have an additional coupling pin and thus, often increases the size, cost, and manufacturing complexity of the PWM controller chip  120   
           [0007]    One technique for eliminating the external filter component requirement is to provide the PLL with a variable-gain charge pump. Such a charge pump includes multiple, parallel-output-drive stages that can be selectively activated to increase or decrease the output current, and thus the gain, of the charge pump. By increasing or decreasing the charge pump gain, one can respectively increase or decrease the PLL bandwidth. A problem with this technique, however, is that the multiple drive stages occupy a significant area of the PWM controller chip  120  that includes the PLL.  
         SUMMARY OF THE INVENTION  
         [0008]    In one embodiment of the invention, a PWM controller with an integrated PLL comprises an input node operable to receive a reference signal from an internal source such as a master clock or external source such as a master-PWM signal generated from another PWM controller. The PLL comprises an oscillator operable to receive an error-correction signal and to generate an oscillator signal having a frequency that is related to the error-correction signal, a phase-frequency detector (PFD) coupled to the oscillator and operable to receive the reference signal and to generate the error-correction signal based upon a phase difference between the reference signal and a feedback signal, and a suppression circuit coupled to the PFD and operable to periodically enable the PFD to generate the error-correction signal.  
           [0009]    There are several advantageous aspects to this embodiment of the invention. First, PWM controllers (slave) that are locked into phase with a master PWM signal allow a user to eliminate the need for additional clocking circuitry for all PWM power supplies. In the past, external clocks were used to synchronize all PWM controllers. By providing a master PWM signal generated from a designated master-PWM controller, additional space on printed circuit boards can be preserved.  
           [0010]    In another embodiment of the invention, an integrated phase-locked loop (PLL) includes a programmable delay that allows the PLL to have a relatively low bandwidth without the need for an external component. Providing such a suppression circuit in a PLL provides advantages that include reducing the PFD gain. By lowering the PFD gain, one lowers the PLL bandwidth such that one can use a filter capacitance that is small enough to be integrated onto a chip that includes the PLL. In addition, such a suppression circuit allows one to use a charge pump having a single output stage.  
           [0011]    Another advantage is that using slave PWM controllers also allows a user to adjust the phase of each slave PWM signals so as to minimize power supply ripple. Specifically, because the PWM power supplies for the various electronic components are supplied from a main power supply, imperfect filtering and large current requirements cause ripple effects on the power supply. Ripple effects will cause artifacts on a display. By offsetting when specific electronic components draw power from the main power supply through phase-shifting, the ripple effects can be reduced.  
           [0012]    Another advantage is that a slave PWM controller can be configured to determine when a synchronization mode is required. When synchronization mode is required, the slave PWM controller is phase-locked with a master PWM controller. If, however, the slave PWM controller should operate in an independent mode, an internal frequency is generated for use by the PWM controller. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0014]    [0014]FIG. 1 is a block diagram of a conventional graphics board that synchronizes multiple PWM power supplies with a master clock signal;  
         [0015]    [0015]FIG. 2 is a block diagram of a graphics board that utilizes an embodiment of a PWM controller according to an embodiment of the invention;  
         [0016]    [0016]FIG. 3 is a block diagram of a PLL according to an embodiment of the invention;  
         [0017]    [0017]FIG. 4A is a schematic diagram of a typical embodiment of some aspects of the PLL of FIG. 3;  
         [0018]    [0018]FIG. 4B is a schematic diagram of another typical embodiment of some aspects of the PLL of FIG. 3;  
         [0019]    [0019]FIG. 5 is a schematic diagram of a frequency divider circuit of the PLL of FIG. 3 according to an embodiment of the invention;  
         [0020]    [0020]FIG. 6 is a block diagram of a typical PWM controller with an integrated PLL according to an embodiment of the invention;  
         [0021]    [0021]FIG. 7 is a Wireless-Area-Network (WAN) transmitter/receiver that can incorporate the PLL of FIG. 3 according to an embodiment of the invention; and  
         [0022]    [0022]FIG. 8 is a block diagram of a computer system that can incorporate the graphics board of FIG. 2 according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]    The following discussion is presented to enable a person skilled in the art to make and use the invention. The general principles described herein may be applied to embodiments and applications other than those detailed below without departing from the spirit and scope of the present invention. The present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed or suggested herein.  
         [0024]    [0024]FIG. 2 shows a block diagram of a typical graphics board  100  that utilizes an embodiment of the invention. As was the case with respect to FIG. 1, the graphics board  100  includes a graphics processor  105  connected to a DDR RAM  106 . Different from the prior art of FIG. 1, however, each of these components are driven by a PWM power supply  210   a,    210   b,  and  210   c  having respective PWM controllers  220   a,    220   b,  and  220   c  with integrated PLLs. The PWM controllers  220  are described below in conjunction with FIG. 6, and the PLLs are described below in conjunction with FIG. 3. The graphics processor  105  is driven by a single PWM power supply  210   a  and the DDR RAM  106  is driven by a pair of PWM power supplies  210   b  and  210   c.  In this embodiment, the PWM power supply  210   a  is the master and the PWM power supplies  210   b  and  210   c  are the slaves, although any one of the supplies  210   a - 210   c  can be the master with the remaining two supplies being the slaves. The master PWM controller  220   a  generates a master PWM signal in a conventional manner, and the slave PWM controllers  210   b  and  210   c  each include an integrated PLL (not shown in FIG. 2) that locks onto the master PWM signal and generates a respective slave PWM signal having the same frequency as the master PWM signal. By precisely synchronizing the PWM frequencies of the slave PWM power supplies  210   b  and  210   c  with the PWM frequency of the master PWM power supply  210   a,  beat frequencies are virtually eliminated. The difference, however, between the PLLs of the controllers  220   a - 220   c  and conventional PLLs is that they can have a relatively low loop bandwidth approximately 1 to 3 kHz in this embodiment -without requiring an external filter element or variable-gain charge pump. Furthermore, as discussed below in conjunction with FIGS.  3 - 6 , in some embodiments one can program the PLLs with the desired bandwidth, or can program the slave PLLs to generate slave PWM signals that have respective phase shifts with respect to the master PWM signal.  
         [0025]    [0025]FIG. 3 is a block diagram of a PLL  300  according to an embodiment of the invention. The PLLs of the PWM controllers  220   a - 220   c  of FIG. 2 can be the same as or similar to the PLL  300 . But, the PLL  300  can be used in virtually any application that calls for a PLL.  
         [0026]    The PLL  300  includes a phase frequency detector (PFD)  302 , an error-correction signal suppression circuit  321 , a conventional charge pump  310 , a conventional filter  361 , a conventional VCO  312 , and an optional frequency divider circuit  313 . As discussed below, the suppression circuit  321  allows one to adjust the loop bandwidth of the PLL  300  without the need for the filter  361  to incorporate a large capacitor or other filter element and without the need for the charge pump  310  to have multiple, switchable-output stages for gain control.  
         [0027]    Generally, the PLL  300  receives a reference signal  341  and produces an output signal  340  having a frequency that is the same as or that is a multiple of the frequency of the reference signal. Furthermore, the reference and output signals are typically in phase with one another, although in one embodiment the divider circuit  313  can impart a predetermined phase shift to the output signal as discussed below in conjunction with FIG. 5. With the exception of the suppression circuit  321 , each part of the PLL  300  will only be described in brief detail as PLLs are well known in the art.  
         [0028]    The PFD  302  detects a difference between the phases of the reference signal  341  and a feedback signal  342 , and generates a phase-error signal (UP or DOWN) that has a duration that is proportional to the phase difference. Specifically, the phase-error signal activates the charge pump  310  so as to “push” the VCO  312  in a direction that will cause the frequency of the output signal  340  to be in phase with the reference signal  341  and to have a frequency equal to N (the divisor of the circuit  313 ) times the frequency of the reference signal. The “direction” of the push depends upon the direction of the phase difference. For example, if the PFD  302  determines that the feedback signal  342  leads the reference signal  341  (feedback frequency higher than reference frequency), then the PFD  302  will send a DOWN pulse  306  to the charge pump  310 . The DOWN pulse has a duration that is proportional to the phase difference and causes the VCO  361  to reduce the frequency of the output signal  340 . If, however, the PFD  302  determines that the feedback signal  342  lags the reference signal  341  (feedback frequency lower than reference frequency), then the PFD  302  will send an UP pulse  305  to the charge pump  310 . The UP pulse has a duration that is proportional to the phase difference and causes the VCO  312  to increase the frequency of the output signal  340 .  
         [0029]    The charge pump  310  generates a phase-correction pulse having a duration that is equal to that of the received UP or DOWN phase-error pulse, and the filter  361 , which is typically a capacitor (not shown) coupled in parallel to the output of the charge pump  310 , integrates the pulse to provide a control voltage. The VCO  312  generates the output signal  340  having a frequency that is proportional to the level of the control voltage, and the divide circuit  313  generates the feedback signal  342  from the output signal  340 . As discussed below, the suppression circuit  321  allows the filter capacitor to be small enough for integration onto the chip that incorporates the PLL  300 , and eliminates the need for the charge pump  310  to have an adjustable gain.  
         [0030]    The suppression circuit  321 , working in conjunction with other logic circuitry, decreases the loop bandwidth of the PLL  300  by introducing programmable error-correction suppression into the loop. The suppression circuit  321  causes a decrease in loop bandwidth by enabling the PFD  341  to generate the error-correction signal only periodically. In one embodiment, the PFD  302  generates error-correction pulses, and the suppression circuit  321  suppresses a pre-determined number of the error-correction pulses. Longer periods between successive enablements of the PFD  341  provides for a lower loop bandwidth, and vice versa. Consequently, the loop has the highest bandwidth, and thus the PLL  300  corrects phase errors at its fastest, when the pulse suppression circuit  321  does not suppress any pulses, i.e., no error-correction pulses are eliminated. Furthermore, because it is programmable, the suppression circuit  321  allows one to change the loop bandwidth without changing the values of the elements that compose the filter  261 , and allows one to set the loop bandwidth to a relatively low value without requiring large, external (to the chip incorporating the PLL  300 ) filter elements.  
         [0031]    Specifically, in one embodiment, the suppression circuit  321  counts the cycles of the reference and feedback signals  341  and  342  (these signals are virtually identical when the PLL  300  is in lock), and allows the PFD  302  to provide the error-correction signal to the charge pump  310  only every X cycles, where X is the count value with which the suppression circuit  321  is programmed. For example, where X=5, the charge pump  310  receives an error-correction signal UP or DOWN only once every five cycles of the signals  341  and  342 . As compared with no error-correction signals being suppressed, a suppression rate of X=5 lowers the loop bandwidth by decreasing the number of error-correction pulses, and thus increases the time required for the PLL  300  to correct for phase differences between the reference and feedback signals  341  and  342 . Although the suppression rate X is described as being programmable so that one can select the desired loop bandwidth, the suppression circuit  321  may be designed such that the value of X is fixed. Furthermore, where the value of X is programmable, one should analyze the loop transfer function of the PLL  300  to insure that the programmed value of X does not cause the PLL to become unstable.  
         [0032]    [0032]FIG. 4A is a schematic diagram of the PFD  302  and the suppression circuit  321  of FIG. 3 according to an embodiment of the invention. The PFD  302  includes a phase-difference detect circuit  401 , enable multiplexers  403  and  405 , optional feed forward circuit  407 , and an optional lock-detect circuit  409 . The suppression circuit  321  includes a programmable counter  411  and a logic circuit. Each of these circuits is described in greater detail below.  
         [0033]    The phase-difference detect circuit  401  includes a pair of flip-flops  415  and  416  for detecting the respective edges—the rising edges in this embodiment—of the reference signal  341  and the feedback signal  342 , and a reset circuit  418  for resetting the flip-flops after they have detected the corresponding edges of both the reference  341  and feedback  342  signals More specifically, in response to the reference signal  341  transitioning from a logic-0 to a logic-1 (rising edge), the flip-flop  415  generates a logic-1 for an intermediate-up signal (IUP). Likewise, in response to the feedback signal  342  transitioning from a logic-0 to a logic-1, the flip-flop  416  generates a logic-1for an intermediate-down signal (IDOWN). Consequently, if IUP transitions to logic-1before IDOWN transitions to logic-1, the feedback signal lags the reference signal by a phase difference that is proportional to the time difference between the logic-1transitions of IUP and IDOWN. Conversely, if IUP transitions to logic-1after IDOWN, the feedback signal  342  leads the reference signal  341  by a phase difference that is proportional to the time difference between the logic-1transitions of IUP and IDOWN. Moreover, if IUP and IDOWN transition to logic-1at the same time, the feedback signal  342  is in phase with the reference signal  341  for that cycle. As discussed above in conjunction with FIG. 3, the UP and DOWN signals provided by the multiplexers  403  and  405  control the charge pump  310 , which in turn controls the VCO  312 , to force the feedback signal  342  to have the same phase and frequency as the reference signal  341 .  
         [0034]    The reset circuit  418  includes an AND gate  417 that generates a RESET signal  419  for resetting the flip-flops  415  and  416  after the lagging one of the pulses IUP and IDOWN transitions to a logic 1. The flip-flops  415  and  416 , now reset, are then ready for the next logic-0-to-logic-1transitions of the reference signal  341  and the feedback signal  342 . Because during reset there is a finite propagation delay through the AND gate  417 , an optional OR gate  421 , the flip flops  415  and  416 , and the inverters  422   a  and  422   b,  the durations of IUP and IDOWN at active logic-1levels are extended. If IUP and IDOWN were passed directly to the charge pump  310  (FIG. 3), then these extended durations would be passed to the charge pump as well. Because it is sometimes desired to reduce or eliminate these extended durations, the PFD  302  may include the feed-forward circuits  407  and the multiplexers  403  and  405  to generate the signals UP and DOWN having reduced durations. The operation of the feed-forward circuits  407  is further discussed in commonly owned U.S. patent application Ser. No. 60,359,270, entitled PHASE DETECTOR AND METHOD FOR A SHORTENING PHASE-ERROR CORRECTION PULSE, which is incorporated herein by reference.  
         [0035]    The suppression circuit  321  controls the loop bandwidth of the PLL  300  (FIG. 3) by suppressing some of the error-correction pulses, thus reducing the bandwidth of the PLL  300 . Generally, the counter  411  is programmed with a count value and uses the reset signal from the AND gate  417  as a clock signal. The counter  411  counts up or down from the count value for each reset pulse (which has the same frequency as the reference signal  341  and the feedback signal  342  when the PLL is in lock mode) until the counter reaches a predetermined value such as zero. When the counter reaches the predetermined value, it enables the multiplexers  403  and  405  via the logic  413  to generate the signals UP and DOWN. The counter  411  then resets and begins the process again.  
         [0036]    An embodiment of the suppression circuit  321  is now described in detail. The counter  411  is ripple counter formed from of three flip-flops (not shown individually). Data is loaded into the flip-flops when a load signal  437  is high. The counter  411  counts down when a pulse is detected from the output of the AND gate  417  until all flip-flop outputs are low. Once the flip-flops have all transitioned to low, the load signal  437  resets the flip-flops and the process begins again. While loading the flip-flops, the multiplexers  403  and  405  are enabled. Between loading cycles, however, the multiplexers  403  and  405  are disabled.  
         [0037]    Because sometimes it is desirable to deactivate the suppression circuit  321  until the PLL  300  locks the feedback signal  342  onto the reference signal  341 , the lock-detect circuit  409  may be included. For example, to decrease the capture time of the PLL  300 —the capture time is the amount of time that the PLL  300  requires to locate and lock onto the frequency of the reference signal—one may want the PLL  300  to have maximum bandwidth during signal capture. Including an adaptive frequency synthesizer (not shown) in the PLL  300  is one way to reduce the PLL&#39;s capture time. The lock-detect circuit  409  combined with the suppression circuit  321  and a programmable loop filter resistor (not shown) with a variable value (the resistor value is dependent upon the PFD gain for loop stability) can be used to implement the adaptive frequency synthesizer. By deactivating the suppression circuit  321  during signal capture when the adaptive frequency synthesizer is required to change the VCO frequency quickly, the PLL can locate and lock onto the reference signal within a minimal amount of time. And, by activating the suppression circuit  321  during lock mode, the PLL  300  can maintain the superior noise performance of a smaller loop bandwidth.  
         [0038]    During each cycle of the reference signal when the feedback signal is locked thereto, IUP and IDOWN will be the same virtually the entire cycle. Therefore, the lock-detect circuit  409  effectively compares the percentage of time that IUP and IDOWN are the same to a predetermined threshold. If the measured percentage is greater than the threshold, then the lock-detect circuit  409  declares lock and enables the pulse suppression circuit  321  via a NAND gate  430 . Otherwise, the lock-detect circuit  409  disables the suppression circuit  321  until lock is achieved.  
         [0039]    Still referring to FIG. 4A, as discussed above in conjunction with FIG. 3, the suppression circuit  321  allows the filter  361  to have a smaller capacitance that can be integrated onto a chip when the loop bandwidth of the PLL  300  is at a point where conventional PLLs would require an external capacitor. Furthermore, the suppression circuit  321  allows one to use a regular charge pump  310 , i.e., a charge pump with a single output stage that is not constructed to have multiple, switchable-output stages for gain adjustment. This allows the charge pump  310  to produce a relatively high-valued error-correction pulse when operating and thus to have a relatively high signal-to-noise ratio. Further, it often reduces the amount of layout space that would otherwise be required by an adjustable charge pump.  
         [0040]    [0040]FIG. 4B is a schematic diagram of another embodiment of the PFD  302  and the suppression circuit  321  of FIG. 3. Again, the PFD  302  includes a phase-difference detect circuit  401 , enable multiplexers  403  and  405 , optional feed forward circuit  407 , and an optional lock-detect circuit  409 . The suppression circuit  321  includes a programmable counter  411 , a logic circuit, and inverters  490   a  and  490   b,  which maintain the loop perturbations at a frequency high enough for the low-pass filter  261  (FIG. 3) to filter out. Specifically, each error-correction pulse causes perturbations in the loop even if UP and DOWN are simultaneously active to indicate zero phase error. One cause of these perturbations is the turning on and off of the charge pump  310  (FIG. 3). When the feedback signal  342  is locked to the reference signal  341  and no error-correction pulses are suppressed, the perturbations have a fundamental frequency equal to the frequency of the reference signal. Because the filter  261  typically has a cutoff frequency that is significantly lower than the frequency of the reference signal, the filter removes virtually all of the perturbations. But when the suppression circuit  321  suppresses error-correction pulses, then the perturbations have a lower fundamental frequency. But if the fundamental perturbation frequency is near or significantly below the cutoff frequency of the filter  261 , then the filter may pass some of the perturbation energy, which may cause jitter or other undesirable noise in the VCO output signal  340  (FIG. 3).  
         [0041]    Consequently, to maintain the fundamental frequency of the perturbations at a frequency high enough for filter  261  to remove the perturbations, the inverters  490   a  and  490   b  simultaneously generate UP and DOWN from the reset signal—which has the same frequency as the reference signal  341  when the PLL  300  is in lock mode—when the circuit  321  is suppressing the error-correction pulses IUP and IDOWN from the flip-flops  415  and  416 . Specifically, before the counter  411  reaches the predetermined value X, it tristates inverters  492   a  and  492   b  to uncouple IUP an IDOWN from the multiplexers  403  and  405 . At the same time, the inverters  490   a  and  490   b  couple the reset signal (generated when both IUP and IDOWN are logic 1) to the multiplexers  403  and  405 , which simultaneously generate UP and DOWN equal to logic 1 for the duration of the reset signal. Because UP and DOWN are active logic 1 for the same duration, the charge pump  310  imparts a net zero phase correction to the VCO  312 . But because the charge pump is active, it does generate a perturbation. Consequently, the inverters  490   a  and  490   b  allow the suppression circuit  321  to suppress error correction without suppressing perturbations. To avoid signal conflict at the multiplexors  403  and  405 , however, the counter  411  tristates the inverters  490   a  and  490   b  when it reaches the predetermine value X, and thus when it is not suppressing the error-correction pulses UP and DOWN. Specifically, when the counter  411  reaches the predetermined suppression rate value, a DEC_OUT signal  495  is generated. Each inverter  490   a  and  490   b  is coupled to this signal and is held in tristate while the DEC_OUT signal  495  is present. The DEC_OUT signal  495  goes low after the counter  411  resets and the error-correction signal UP or DOWN has been generated.  
         [0042]    [0042]FIG. 5 is schematic diagram of the frequency divider circuit  313  of FIG. 3 according to an embodiment of the invention. The frequency divider  313  receives the output signal  340  as an input to a multiplexor  501  which provides pulses to a series of flip-flops  510 . Each flip-flop in the series of flip-flops provides an input for the next flip-flop in the series. As a result, any one of the flip-flop outputs Q 1 -Q 6  (selectable via a multiplexer  511 ) can be used as a frequency divider  313  output that is an exact 1/N multiple of the output signal  340 .  
         [0043]    Still referring to FIG. 5, another optional feature of the frequency divider circuit  313  is that it allows one to introduce a predetermined phase shift into the output signal  340  with respect to the reference signal  341  (FIG. 3). Delay gates  520  generate signals PH  90 , PH  120 , PH  150 , PH  180 , and PH  210  which all have a predetermined frequency and have phase shifts with respect to the output signal  340  of 90, 120, 150, 180, and 210 degrees, respectively. Therefore, using a multiplexer  513  to select one of these signals as the feedback signal  342  introduces a corresponding phase shift into the output signal  340 . As discussed above in conjunction with FIG. 2 and below in conjunction with FIG. 6, offsetting the phases of the slave PWM signals with respect to the master PWM signal may reduce ripple on the main power supply by staggering the times when the PWM supplies draw power from the main supply. In one alternative of this embodiment, the designer pre-selects the phase shifts, which do not change during operation of the PWM supplies. Alternatively, the PWM supplies can monitor ripple on the main supply and dynamically shift the relative phases of the slave PWM signals so as to maintain a desired level of ripple on the main power supply.  
         [0044]    [0044]FIG. 6 is a block diagram of one of the PWM controllers  220   a,    220   b,  and  220   c  of FIG. 3 according to an embodiment of the invention. There are two modes in which the PWM controller  220  operates. In an independent mode, the PWM controller  220  does not lock the output signal  340  to the reference signal  341  or to any other reference. A master PWM controller, such as the PWM controller  220   a  of FIG. 2, typically operates in the independent mode. In a PLL-mode, the PLL  300  of the PWM controller  220  synchronizes the output frequency  340  to the reference signal  341  received from the master PWM controller  220   a  or from another source via the synchronization input  200 . The slave PWM controllers  220   b  and  220   c  of FIG. 2 typically operate in PLL-mode.  
         [0045]    When in PLL-mode, an FS/synch input  601  receives the reference signal  341  from a master PWM controller. In FIG. 2, the PWM controller  220   a  for the graphics processor  105  is an example of a master PWM controller, but, alternatively, some other PWM controller can be the master depending on the design of a particular system. Most commercially available PWM controllers  220  make the PWM signal available on a pin, and thus can serve as a master.  
         [0046]    If not in PLL-mode, a resistor  650  is connected between the FS/synch input  601  and either ground (not shown) or a power supply  652 . A voltage-to-current converter  651  converts the voltage that the resistor  650  generates at the input  601  into a current that the logic  600  converts into a VCO control voltage on the line  602 . Therefore, one selects a value for the resistor  650  that causes the VCO  312  to generate an output signal  340  having the desired frequency.  
         [0047]    The PWM controller  220  can automatically determine which mode, independent mode or PLL-mode, in which to operate. To make this determination, a reference-signal detector  619 , which may be part of the block logic  600 , senses pulses from a Schmitt trigger  603 , which is connected to the FS/synch  601  terminal. If the PLL mode is disabled (default condition) but the reference-signal detector  619  senses pulses for a first predetermined time, then the reference-signal detector  619  determines that a master reference signal is present at the input terminal  601  and enables the PLL  300  via line  620  and a switch  660 . Conversely, if the PLL-mode is enabled and the reference-signal detector  619  senses pulses of the feedback signal  342  for a second predetermined time without simultaneously detecting pulses from the Schmitt trigger  603 , the reference-signal detector  619  disables the PLL  300  via line  620  and the switch  660 . The first and second predetermined times may be fixed or may be programmable. The detector  619  detects a signal by discharging a capacitor every time it detects an edge of the signal. In between edges the capacitor charges to a logic level that enables a counter (not shown). If the counter reaches a predetermined count value (corresponding to the first or second predetermined time), then the reference-signal detector  619  determines that the no signal is present. But as long as edges are present, the counter never reaches the predetermined count value. The reference-signal detector  619  includes at least two of these detect circuits, so there are at least two predetermined count values, a first one corresponding to the first predetermined time and a second one corresponding to the second predetermined time. These predetermined count values may be fixed or programmable.  
         [0048]    The block logic  600  also detects whether the value of the resistor  650  is either too high or too low, and, if the resistor is out of range, sets the VCO  312  to generate a predetermined maximum (resistor value too low) or minimum (resistor value to high) frequency. The voltage-to-current converter  651  also includes a current limiter so that such an undervalued resistor  650  does not cause an over-current condition.  
         [0049]    While the PWM controller  220  operates in the PLL (slave) mode, the PLL  300  operates, as discussed above in conjunction with FIGS.  3 - 5 , to lock the feedback signal  342  to the reference signal  341 . The frequency divider circuit  313  provides one or more slave PWM signals—here two such signals PWM 1  and PWM 2 —to a conventional PWM ramp generator (not shown), which generate a corresponding number of ramps (not shown) for regulating the PWM supply  210  (FIG. 2). As discussed above in conjunction with FIG. 5, the frequencies of PWM 1  and PWM 2  are integer multiples—six in one in embodiment—of the frequency of the reference signal  341 . In addition, PWM 1  and PWM 2  may have predetermined phase shifts with respect to the reference signal  341 . Furthermore, in one embodiment, the suppression circuit  321  is programmable to have a count value in the range of 32-1024. Moreover, the filter  361  or another portion of the PLL  300  may include programmable resistance values that allow one to adjust the loop gain to maintain loop stability for a particular count value.  
         [0050]    [0050]FIG. 7 is a Wireless-Area-Network (WAN) transmitter/receiver  700  that can incorporate the PLL  300  of FIG. 3 according to an embodiment of the invention. In addition to the PFD  302 , charge pump  310 , VCO  312 , frequency divider  313 , suppression circuit  321  and the filter  361  (omitted from FIG. 7 for clarity), the PLL  300  includes a terminal  718  for receiving the reference signal and a local-oscillator (LO) distributor  720  for distributing the output of the VCO  312  as an LO signal. In addition to the PLL  300 , the transmitter/receiver  700  includes a transmitter  704 , and a receiver  706 . The transmitter  704  includes a mixer  722  that modulates the LO with a differential base-band data signal received from a computer (not shown) via data terminals  724  and  726 . The transmitter  704  then provides this modulated data signal to a transmit-terminal  728  for wireless transmission to a remote receiver (not shown). Similarly, the receiver  706  receives a modulated data signal from a remote wireless transmitter (not shown) via a terminal  730 , and includes a mixer  732  that demodulates the received data signal with the LO signal and provides a differential demodulated data signal to the computer via the terminals  724  and  726 . The PLL  300  is operable to synchronize the LO signal from the VCO  312  to the reference signal received on terminal  718 . In one embodiment, the suppression circuit  321  is programmable to implement a count value of 0-7. The transmitter/receiver also includes other circuits that are conventional, and that are thus omitted from FIG. 7 for brevity.  
         [0051]    [0051]FIG. 8 is a block diagram of a general-purpose computer system  820  that incorporates the graphics board  200  of FIG. 2 according to an embodiment of the invention. The computer system  820  (e.g., personal or server) includes one or more processing units  821 , system memory  822 , and a system bus  823 . The system bus  823  couples the various system components including the system memory  822  to the processing unit  821 . The system bus  823  may be any of several types of busses including a memory bus, a peripheral bus, and a local bus using any of a variety of bus architectures. The system memory  822  typically includes read-only memory (ROM)  824  and random-access memory (RAM)  825 . Firmware  826  containing the basic routines that help to transfer information between elements within the computer system  820  is also contained within the system memory  822 . The computer system  820  may further include a hard disk-drive system  827  that is also connected to the system bus  823 . Additionally, optical drives (not shown), CD-ROM drives (not shown), floppy drives (not shown) may be connected to the system bus  823  through respective drive controllers (not shown) as well.  
         [0052]    A user may enter commands and information into the computer system  820  through input devices such as a keyboard  840  and pointing device  842 . These input devices as well as others not shown are typically connected to the system bus  823  through a serial port interface  846 . Other interfaces (not shown) include Universal Serial Bus (USB) and parallel ports  840 . A monitor  847  or other type of display device may also be connected to the system bus  823  via an interface such as the graphics card  200 .