Patent Publication Number: US-7212052-B2

Title: Jitter suppressing delay locked loop circuits and related methods

Description:
RELATED APPLICATIONS 
   This application claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 2003-74677, filed on Oct. 24, 2003, the entire content of which is incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to a semiconductor integrated circuit, and more particularly, to a delay locked loop circuit. 
   BACKGROUND OF THE INVENTION 
   Delay locked loop circuits are circuits that may be used to generate an internal clock signal that has a phase that leads a reference clock signal by a predetermined time. Internal clock signals are often used in semiconductor integrated circuits which operate in synchronization with an external clock signal and have a relatively high degree of integration such as, for example, Rambus DRAM (RDRAM) and Synchronous DRAM (SDRAM) circuits. 
   More specifically, in many conventional semiconductor circuits, an external clock signal is input to a clock buffer through an input pin to generate an internal clock signal. A data output buffer outputs data to an external circuit in synchronization with the internal clock signal. The internal clock signal is delayed from the external clock signal by a predetermined time in the clock buffer. The output data from the data output buffer is also delayed from the internal clock signal by a predetermined time. As a result, the output data may be output after a long delay time with respect to the external clock signal such that the output data access time (tAC) becomes long. 
   In order to reduce the output data access time (tAC), a delay locked loop may be used to generate the internal clock signal such that the phase of the internal clock signal leads the phase of the external clock signal by a predetermined time. Use of such an internal clock signal may allow the output data to be output without delay with respect to the external clock signal. In other words, the delay locked loop receives the external clock signal and generates an internal clock signal that has a phase that leads the external clock signal by a predetermined time. The internal clock signal may be used as a clock signal of each part of the circuit, such as the data output buffer. 
     FIG. 1  is a block diagram of a conventional delay locked loop circuit  100 . As shown in  FIG. 1 , the conventional delay locked loop includes a phase detector  11 , a charge pump  12 , and a voltage controlled delay line  13 . 
   SUMMARY OF THE INVENTION 
   Pursuant to embodiments of the present invention, delay locked loop circuits are provided that include (1) a delay locked loop that generates a delay locked loop output signal, (2) a delay circuit that receives the delay locked loop output signal and generates one or more delayed versions of the delay locked loop output signal and (3) a phase interpolator that receives the delay locked loop output signal and the one or more delayed versions of the delay locked loop output signal. In certain embodiments of the present invention, the delay circuit may comprise a plurality of serially connected delay cells. Each of these delay cells may delay signals input thereto for at time equal to one clock period of an external clock signal that is input to the delay locked loop. 
   In these delay locked loop circuits, the phase interpolator may be configured to interpolate the phase of the delay locked loop output signal and the phases of the one or more delayed versions of the delay locked loop output signal to generate an internal clock signal. These circuits may also include a first weighting circuit that is configured to weight the phases of the delay locked loop output signal and the one or more delayed versions of the delay locked loop output signal before the delay locked loop output signal and the one or more versions of the delay locked loop output signal are received by the phase interpolator. 
   In certain embodiments of the present invention, the delay locked loop may include a phase detector that compares the phase of an external clock signal and the phase of the delay locked loop output signal. In other embodiments, the delay locked loop may further include a second weighting circuit that is configured to weight the phases of the delay locked loop output signal and the one or more delayed versions of the delay locked loop output signal and a second phase interpolator that is configured to interpolate the phases of the delay locked loop output signal and the one or more delayed versions of the delay locked loop output signal that are weighted by the second weighting circuit. In these embodiments, the delay locked loop may include a phase detector that compares the phase of an external clock signal and an output of the second phase interpolator. 
   According to further embodiments of the present invention, delayed locked loop circuits for generating an internal clock signal having a phase that leads an external clock signal by a predetermined time are provided which include a delayed lock loop and a jitter suppressor circuit. In these circuits, the delay locked loop may include a phase detector that is configured to compare the phases of the external clock signal and a second signal and a voltage controlled delay line that is configured to produce a delayed version of the external clock signal, wherein the magnitude of the delay is based on the output of the phase detector. The jitter suppressor may include a variable delay unit that is configured to receive the delayed version of the external clock signal and generate a plurality of variable delay unit output signals and a first phase interpolator that is configured to interpolate phases of the delayed version of the external clock signal and at least some of the plurality of variable delay unit output signals to generate the internal clock signal. The delay locked loop may also include a first weighting circuit coupled between the variable delay unit and the first phase interpolator that is configured to weigh the phases of at least some of the delayed version of the external clock signal and the plurality of variable delay unit output signals so that at least some of the signals input to the first phase interpolator comprise phase weighted signals. 
   In certain of these embodiments, the second signal may be the delayed version of the external clock signal. In other embodiments, the delay locked loop circuit may further include a second weighting circuit that is coupled between the variable delay unit and the first phase interpolator that is configured to weight the phases of at least some of the delayed version of the external clock signal and the plurality of variable delay unit output signals to produce a plurality of phase weighted output signals and a second phase interpolator that is configured to interpolate the phase weighted output signals of the second weighting circuit to produce a second phase interpolated output signal. In such embodiments, the second signal may be the second phase interpolated output signal. 
   Pursuant to still further embodiments of the present invention, methods of generating an internal clock signal from an external clock signal are provided. Pursuant to these methods, the external clock signal is input to a delay locked loop to produce a preliminary internal clock signal, and a plurality of delayed versions of the preliminary internal clock signal are then produced. The phases of the preliminary internal clock signal and the plurality of delayed versions of the preliminary internal clock signal are then weighted using a first set of weights to produce a plurality of phase weighted signals, and the phases of the phase weighted signals are interpolated to generate the internal clock signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention, and which are incorporated in and constitute a part of this application, illustrate certain embodiments of the invention and together with the description serve to explain the principle of the invention. In the drawings: 
       FIG. 1  is a block diagram of a conventional delay locked loop; 
       FIG. 2  is a block diagram of a delay locked loop circuit having a jitter suppressor according to certain embodiments of the present invention; 
       FIGS. 3A and 3B  illustrate examples of jitter that may occur in a delay locked loop circuit; and 
       FIG. 4  is a block diagram of a delay locked loop circuit having a jitter suppressor according to further embodiments of the present invention. 
   

   DETAILED DESCRIPTION 
   The present invention will now be described more fully with reference to the accompanying drawings, in which typical embodiments of the invention are shown. This invention, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout. 
   It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. 
   As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
   It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element without departing from the teachings of the disclosure. 
   The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
   Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense expressly so defined herein. 
     FIG. 2  is a block diagram of a delay locked loop circuit having a jitter suppressor according to certain embodiments of the present invention. As shown in  FIG. 2 , the circuit includes a delay locked loop  100   a  and a jitter suppressor  200   a.    
   The delay locked loop  100   a  in the embodiment of  FIG. 2  may have a structure that is identical to the structure of the conventional delay locked loop  100  illustrated in  FIG. 1 . That is, the delay locked loop  100   a  may include a phase detector  11 , a charge pump  12 , and a voltage controlled delay line (VCDL)  13 . 
   As shown in  FIG. 2 , the phase detector  11  compares the phase of an input external clock signal CLK with the phase of the fed-back output clock signal of the delay locked loop  100   a , and outputs the result as a detection signal to the charge pump  12 . The charge pump  12  converts the output signal of the phase detector  11  into a voltage signal using capacitors. The voltage controlled delay line  13  receives the external clock signal CLK and delays the external clock signal CLK by a delay time that corresponds to the output voltage signal of the charge pump  12 . The voltage controlled delay line  13  then outputs the delayed clock signal as the internal clock signal. As noted above, the output signal of the voltage controlled delay line  13  is also fed-back to serve as one of the inputs to the phase detector  11 . 
   The external clock signal CLK that comprises the input signal to the delay locked loop circuit may contain jitter. If the jitter is not suppressed, it may degrade the performance of the semiconductor device, particularly as the input/output frequency is increased. 
   As is also shown in  FIG. 2 , the jitter suppressor  200   a  includes a variable delay unit and a phase interpolator  30 . The variable delay unit is connected in series with the output terminal of the delay locked loop  100   a  and delays the output signal of the delay locked loop  100   a  by a predetermined time. As shown in  FIG. 2 , the variable delay unit may comprise a plurality of serially connected delay cells  21 ,  22 ,  23  and  24 , each of which delay the signal input to the delay cell by one clock cycle T. As can be seen in  FIG. 2 , the phase interpolator  30  receives the output signal from the delay locked loop  100   a  and the delayed signals from the delay cells  21 ,  22 ,  23  and  24  of the variable delay unit and interpolates their phases. The phases are interpolated with assignment of weights a 0 , a 1 , a 2 , a 3  and a 4 . The weighting circuit  40  that applies the weights may be part of the phase interpolator, part of the delay unit, and/or implemented as separate circuit elements. It will be understood that, as used herein, the term “weighting circuit” is intended to encompass all such embodiments. The weighting circuit  40  is configured to weight the phases of the delay locked loop output signal and the one or more delayed versions of the delay locked loop output signal before the delay locked loop output signal and the one or more versions of the delay locked loop output signal are received by the phase interpolator  30 . The delay locked loop circuit generates a weighted average value by interpolating the weighted phases of the current output signal and the output signals of k prior clock cycles (in  FIG. 2 , k=4). This can be expressed as an equation below:
 
 Y ( z )=( a 0 +a 1 ×z+a 2 ×z   2   +a 3 ×z   3 + . . . )  ×X ( z )  (Eq. 1)
 
   In Equation 1, X(z) represents the jitter of the input clock signal x(n) and Y(z) represents the jitter of the final output signal y(n). If the input clock signal has alternating positive and negative jitter, that is, if x(n) has the jitter shown in  FIG. 3A , the jitter can be substantially removed from y(n) by making Y(z)=(1+z)×X(z), that is, y(n)=x(n)+x(n−1). 
   In another example, if x(n) has the jitter shown in  FIG. 3B , the jitter can be substantially removed from y(n) by making Y(z)=(1+z 2 )×X(z), that is, y(n)=x(n)+x(n−2). 
   As still another example, the jitter shown in both  FIGS. 3A and 3B  can be substantially eliminated by making Y(z)=(1+z)×(1+z 2 )×X(z)=(1+z+z 2 +z 3 )×X(z), that is, y(n)=x(n)+x(n−1)+x(n−2)+x(n−3). 
   As illustrated by the above examples, if the interpolation weights a 0 , a 1 , . . . , ak are properly selected, the jitter of the final output signal from the delay locked loop circuit with respect to the arbitrary input jitters can be reduced, minimized or eliminated altogether. 
     FIG. 4  is a block diagram of a delay locked loop circuit having a jitter suppressor according to further embodiments of the present invention. The delay locked loop circuit depicted in  FIG. 4  generates several signals that are provided by sequentially delaying the output signal of the conventional delay locked loop  100   b  by clock period T. The jitter suppressor circuit  200   b , then interpolates the phases of all of these signals, operating in a similar maimer as the jitter suppressor circuit  200   a  of  FIG. 2 . A first weighting circuit  40  is configured to weight the phases of the delay locked loop output signal and the one or more delayed versions of the delay locked loop output signal before the delay locked loop output signal and the one or more versions of the delay locked loop output signal are received by the phase interpolator  30 . The delay locked loop circuit of  FIG. 4  further includes a second phase interpolator  31  that interpolates phases with interpolation weights of b 0 , b 1 , b 2 , b 3  and b 4  provided by a second weighting circuit  50 . As shown in  FIG. 4 , the output signal of the second phase interpolator  31  is fed back to the input terminal of the phase detector  11  of the delay locked loop  100   b  and compared with the phase of the input clock signal. This is in contrast to the embodiment depicted in  FIG. 2 , in which the output signal of the delay locked loop  100   a  is fed back to the phase detector  11  and compared with the phase of the input clock signal. 
   The relation between the input jitter and output jitter of the delay locked loop circuit of  FIG. 4  can be expressed as follows: 
   
     
       
         
           
             
               
                 
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                       + 
                       
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                       + 
                       … 
                     
                   
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                     X 
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   As described above, the present invention has an advantage that can suppress the jitter occurring in the delay locking. 
   While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents.