Abstract:
A method of controlling and maintaining a constant slew rate at an output of a buffer is provided. The method includes the following steps: (a) receiving, (i) a first input signal and (ii) at least one of a control voltage using the buffer; (b) generating a threshold voltage using a first reference voltage generator; (c) comparing (i) the threshold voltage with an output of the buffer using at least one of a comparator; (d) determining a phase difference using a phase detector; (e) producing a DC voltage using a loop filter; (f) generating a reference voltage; (g) receiving the DC voltage and the reference voltage using an amplifier; (h) amplifying the difference between (a) said DC voltage, and (b) the reference voltage to obtain a control voltage using the amplifier; and (i) feeding the control voltage to the buffer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to Indian patent application no. 201641009007 filed on Mar. 15, 2016, the complete disclosure of which, in its entirely, is herein incorporated by reference. 
       BACKGROUND 
     Technical Field 
       [0002]    The embodiments herein generally relate to slew rate, and more particularly, to a method for controlling and maintaining a constant slew rate. 
       Description of the Related Art 
       [0003]    Slew rate is defined as the rate of change in output voltage with time. The slew rate may be measured using an oscilloscope. In high speed input output (IO) buffers, matching the impedance with the transmission line connecting receivers is required to preserve the signal integrity at the inputs of the receivers. Matching of IO buffer resistances may be achieved at a good accuracy, but due to varying capacitive loads at the IO buffer outputs and variable operating frequencies, matching the reactance becomes difficult. Here, in these situations the magnitude of the reflections is affected by the rise time and fall time of the inputs and outputs. Sharper inputs result in higher magnitudes of ISI (intersymbol Interference). In the cases of data bus, NEXT (Near-end crosstalk) and FEXT (Far-and crosstalk) occur which are undesirable. Hence, the slew rates at the inputs have to be controlled and reduced to minimize the above-mentioned effects. The slew rate should remain constant across variations in load, process, temperature, and supply voltage. 
         [0004]      FIG. 1  illustrates a block diagram of a typical output buffer in accordance with an exemplary scenario. The block diagram includes a pre-driver slew rate compensation circuit  102 , a driver impedance compensation circuit  104 , a compensation circuit  106 , a temperature detection circuit  108  and a voltage detection circuit  110 . The pre-driver slew rate compensation circuit  102  receives a slew rate control signal from the compensation circuit  106 . The driver impedance compensation circuit  104  receives an impedance control signal from the compensation circuit  106 . The temperature detection circuit  108  detects the temperature of the typical output buffer and provides a signal based on the detection to the compensation circuit  106 . Similarly, the voltage detection circuit  110  detects a supply voltage provided to the typical output buffer and provides a signal based on this detection to the compensation circuit  106 . 
         [0005]    The compensation circuit  106  controls only the slew rate at the output of the pre-driver slew rate compensation circuit  102 , which drives the driver impedance compensation circuit  104 . The slew rate at the pre-driver slew rate compensation circuit  102  is inversely proportional to the product of the resistance of the pre-driver slew rate compensation circuit  102  and an input capacitance of the driver impedance compensation circuit  104 . The compensation circuit  106  generates signals which adjusts the output resistance of the pre-driver slew rate compensation circuit  102  and hence maintains the RC product constant. However, to have slew rate depend only on the RC product requires very sharp rising/falling edges at the input to the pre-driver slew rate compensation circuit  102 . Hence, the circuit drives the pre-driver slew rate compensation circuit  102  must be very strong which inevitably consumes more power and area. 
         [0006]    Due to practical constraints, there will be finite rise/fall time at the input of the pre-driver slew rate compensation circuit  102  which results in error in the slew rate at the output of the pre-driver slew rate compensation circuit  102  across process, voltage and temperature (PVT). Also the main disadvantage of this technique is that it assumes constant slew rate at the input of a circuit that results inconstant output slew rate. If there are variations in the load, process, temperate and voltage, it results in huge variation of the slew rate. The compensation circuit  106  tries to compensate for (a) temperature using the temperature detection circuit  108  and (b) voltage using the voltage detection circuit  110 , but since there is no feedback mechanism, there will be error in the slew rate. Also, there is no account for variations in load and process. 
         [0007]    Accordingly, there remains a need for controlling and maintaining the slew rate at the output of the buffer accurately to a desired value and is independent of the variations in the output load, process, temperature, and supply voltages. 
       SUMMARY 
       [0008]    In view of a foregoing, an embodiment herein provides a method for controlling and maintaining a constant slew rate at an output of a buffer. The method includes the following steps of: (a) receiving, using the buffer, (i) a first input signal and (ii) at least one of a control voltage; (b) generating, using a first reference voltage generator, a threshold voltage; (c) comparing, using at least one of a comparator, the threshold voltage with an output of the buffer to obtain an output digital signal; (d) determining, using a phase detector, a phase difference; (e) producing, using a loop filter, a DC voltage from an output of the phase detector; (f) generating, using a second reference voltage generator, a reference voltage; (g) receiving, by an amplifier, the DC voltage from the loop filter and the reference voltage from the second reference voltage generator; (h) amplifying, using the amplifier, the difference between (a) the DC voltage from the loop filter and (b) the reference voltage to obtain a control voltage; and (i) feeding the control voltage to the buffer, wherein the slew rate at the output of the buffer is determined using the control voltage. 
         [0009]    In one aspect, a slew rate locked loop circuit for controlling and maintaining a constant slew rate at an output of a buffer is provided. The buffer receives (i) a first input signal and (ii) at least one of a control voltage. The slew rate locked loop circuit includes a slew rate detection unit, a loop filter, a second reference voltage generator and an amplifier. The slew rate detection unit includes a first reference voltage generator, a first comparator, a second comparator and a phase detector. The first reference voltage generator generates (i) an upper threshold voltage (V h ) and (ii) a lower threshold voltage (V l ). The first comparator compares the upper threshold voltage (V h ) with the output of the buffer to obtain a first output digital signal. The second comparator compares the lower threshold voltage (V l ) with the output of the buffer to obtain a second output digital signal. The phase detector determines a phase difference between the first output digital signal and the second output digital signal. The phase difference is directly proportional to the slew rate at the output of the buffer. The loop filter produces a DC voltage from an output of the phase detector. The second reference voltage generator generates a reference voltage. The amplifier receives the DC voltage from the loop filter and the reference voltage generated by the second reference voltage generator. The amplifier amplifies the difference between (i) the DC voltage from the loop filter, and (ii) the reference voltage to obtain a control voltage. The control voltage is fed back to the buffer. The slew rate at the output of the buffer is determined using the control voltage. 
         [0010]    In an embodiment, the output of the phase detector is high when the output of the buffer is between the upper threshold voltage (V h ) and the lower threshold voltage (V h ). The output of the phase detector is low when the output of the buffer is not between the upper threshold voltage (V h ) and the lower threshold voltage (V l ). 
         [0011]    In another embodiment, the output of the phase detector is low when the output of the buffer is between the upper threshold voltage (V h ) and the lower threshold voltage (V l ). The output of the phase detector is high when the output of the buffer is not between the upper threshold voltage (V h ) and the lower threshold voltage (V l ). 
         [0012]    In yet another embodiment, the constant slew rate is obtained when an error between an output of the loop filter and the reference voltage of the second reference voltage generator becomes zero. 
         [0013]    In yet another embodiment, a time taken (t) from the output of the buffer to swing from the upper threshold voltage (V h ) to the lower threshold voltage (V l ) is controlled by an equation which is slew rate=(V h −V l )/t. 
         [0014]    In yet another embodiment, the upper threshold voltage (V h ) and the lower threshold voltage (V l ) of the first reference voltage generator and the reference voltage of the second reference voltage generator track a power supply to equalize power supply variations. 
         [0015]    In another aspect, a slew rate locked loop circuit for controlling and maintaining a constant slew rate at an output of a buffer is provided. The buffer receives (i) a first input signal and (ii) at least one of a control voltage. The circuit includes a slew rate detection unit, a loop filter, a second reference voltage generator and an amplifier. The slew rate detection unit includes a first reference voltage generator, a comparator and a phase detector. The first reference voltage generator generates a threshold voltage. The comparator compares the threshold voltage with the output of the buffer to obtain an output digital signal. The phase detector determines a phase difference between the output digital signal and a second input signal. The phase difference is directly proportional to the slew rate at the output of the buffer. The loop filter produces a DC voltage from an output of the phase detector. The second reference voltage generator generates a reference voltage. The amplifier (a) receives the DC voltage from the loop filter and the reference voltage generated by the second reference voltage generator, and (b) amplifies the difference between (i) the DC voltage from the loop filter and (ii) the reference voltage to obtain a control voltage. The control voltage is fed back to the buffer. The slew rate at the output of the buffer is determined using the control voltage. 
         [0016]    In an embodiment, the slew rate of the buffer is directly proportional to the reference voltage of the second reference voltage generator which is constant. 
         [0017]    In another embodiment, the constant slew rate is obtained when an error between the output of the loop filter and the reference voltage of the second reference voltage generator becomes zero. 
         [0018]    The above-mentioned technique can be used to maintain and control the output slew rate of buffers constant across process, voltage and temperature (PVT). 
         [0019]    These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The embodiments herein will be better understood from the following detailed description with reference to the drawings, in which: 
           [0021]      FIG. 1  illustrates a block diagram of a typical output buffer in accordance with an exemplary scenario; 
           [0022]      FIGS. 2A-2B  illustrate a block level implementation of a slew rate locked loop according to an embodiment herein; 
           [0023]      FIG. 3  illustrates a block level implementation of a slew rate locked loop including a comparator according to an embodiment herein; 
           [0024]      FIG. 4  illustrates one or more buffers calibrated using a replica buffer according to an embodiment herein; 
           [0025]      FIG. 5A  illustrates a graphical view of operational waveforms for the slew rate locked loop when comparators produce a high output voltage of  FIGS. 2A-2B  according to an embodiment herein; 
           [0026]      FIG. 5B  illustrates a graphical view of operational waveforms for the slew rate locked loop when the comparators produce a low output voltage of  FIGS. 2A-2B  according to an embodiment herein; 
           [0027]      FIG. 6  illustrates two simple architectures of a buffer according to an embodiment herein; and 
           [0028]      FIG. 7  is flow diagram illustrating a method for controlling and maintaining a slew rate at an output of the buffer of  FIGS. 2A-2B  according to an embodiment herein. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0029]    The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein. 
         [0030]    Various embodiments provide methods and circuits to control and maintain a constant slew rate at an output of a buffer. As mentioned, there remains a need for a method and a circuit which controls and maintains the slew rate at the output of the buffer accurately to a desired value and is independent of the variations in the output load, process, temperature, and supply voltages. Referring now to the drawings, and more particularly to  FIGS. 2A through 7 , where similar reference characters denote corresponding features consistently throughout the figures, there are shown preferred embodiments. 
         [0031]      FIGS. 2A-2B  illustrate a block level implementation of a slew rate locked loop according to an embodiment herein. The block level implementation of a slew rate locked loop includes a buffer  202 , a slew rate detection unit  218 , a loop filter  212 , a second reference voltage generator  214 , and an amplifier  216 . The slew rate detection unit  218  includes a first reference voltage generator  204 , a first comparator  206 , a second comparator  208  and a phase detector  210 . A slew rate of the buffer  202  is what should be controlled and maintained. In one embodiment, (i) a first input signal and (ii) at least one of a control voltage are fed to the buffer  202  as inputs. The first reference voltage generator  204  generates (i) an upper threshold voltage (V h ) and (ii) a lower threshold voltage (V l ). In one embodiment, a time taken (t) from an output of the buffer  202  to swing from the upper threshold voltage (V h ) to the lower threshold voltage (V l ) is also controlled. The slew rate is then controlled which obeys the equation, the slew rate=(V h −V l )/t. 
         [0032]    The first comparator  206  compares the upper threshold voltage (V h ) with the output of the buffer  202  to obtain a first output digital signal. The second comparator  208  compares the lower threshold voltage (V l ) with the output of the buffer  202  to obtain a second output digital signal. The phase detector  210  determines a phase difference between the first output digital signal and the second output digital signal. In an embodiment, the phase difference is directly proportional to the slew rate at the output of the buffer  202 . 
         [0033]    The loop filter  212  produces a DC voltage of an output of the phase detector  210 . In an embodiment, the output of the phase detector  210  is high when the output of the buffer  202  is between the upper threshold voltage (V h ) and the lower threshold voltage (V l ). In another embodiment, the output of the phase detector  210  is low when the output of the buffer  202  is not between the upper threshold voltage (V h ) and the lower threshold voltage (V l ). In yet another embodiment, the output of the phase detector  210  is low when the output of the buffer  202  is between the upper threshold voltage (V h ) and the lower threshold voltage (V l ). In yet another embodiment, the output of the phase detector  210  is high when the output of the buffer  202  is not between the upper threshold voltage (V h ) and the lower threshold voltage (V l ). The second reference voltage generator  214  generates a reference voltage. The amplifier  216  receives (i) the DC voltage from the loop filter  212  and (ii) the reference voltage generated by the second reference voltage generator  214 . The amplifier  216  amplifies the difference between (i) the DC voltage from the loop filter  212 , and (ii) the reference voltage to obtain a control voltage. The control voltage is fed back to the buffer  202 . In an embodiment, the slew rate at the output of the buffer  202  is determined using the control voltage. 
         [0034]    In another embodiment, the constant slew rate is obtained when an error between an output of the loop filter  212  and the reference voltage of the second reference voltage generator  214  becomes zero. The slew rate at the output of the buffer  202  is directly proportional to the reference voltage of the second reference voltage generator  214  which is constant. Hence, the slew rates remain constant independent of load, process, and temperature. The upper threshold voltage (V h ) and the lower threshold voltage (V l ) of the first reference voltage generator  204  and the reference voltage of the second reference voltage generator  214  track the power supply to equalize power supply variations. 
         [0035]    With reference to  FIGS. 2A-2B ,  FIG. 3  illustrates a block level implementation of a slew rate locked loop including a comparator  302  according to an embodiment herein. The slew rate locked loop includes the comparator  302 , and is implemented when some following conditions are met. In one embodiment, the first condition is when the upper threshold voltage (V h ) is equivalent to a source voltage (V DD ) and the lower threshold voltage (V l ) is equivalent to half of the source voltage (V DD ). In another embodiment, the second condition is when the upper threshold voltage (V h ) is equivalent to half of the source voltage (V DD ) and the lower threshold voltage (V l ) is equivalent to zero. The comparator  302  compares the output of the buffer  202  with a threshold voltage generated by the first reference voltage generator  204  to obtain an output digital signal. In an embodiment, the phase detector  210  determines a phase difference between the output digital signal and a second input signal. The loop filter  212  produces a DC voltage from an output of the phase detector  210 . The second reference voltage generator  214  generates a reference voltage. The amplifier  216  receives the DC voltage from the loop filter  212  and the reference voltage generated by the second reference voltage generator  214 . The amplifier  216  further amplifies (i) the DC voltage from the loop filter  212  and (ii) the reference voltage to obtain a control voltage. The control voltage is fed back to the buffer  202 . In another embodiment, the slew rate at the output of the buffer  202  is determined using the control voltage. 
         [0036]      FIG. 4  illustrates one or more buffers  402 A-N calibrated using a replica buffer according to an embodiment herein. Calibration of slew rates of the one or more buffers  402 A-N lead to huge consumption of power and area. The calibration of the slew rate of the one or more buffers  402 A-N distributes control voltages to rest that saves power and area. 
         [0037]      FIG. 5A  illustrates a graphical view of operational waveforms for the slew rate locked loop circuit when the comparators ( 206  and  208 ) produce a high output voltage of  FIGS. 2A-2B  according to an embodiment herein. The graphical view of the operational waveforms includes a first upper threshold voltage wave  502 , a first lower threshold voltage wave  504 , a first buffer output wave  506 , a first comparator and a second comparator wave with the high output voltage  508 , and a first phase detector wave  510 . The first comparator  206  produces a high output voltage when the output of the buffer  202  is above the upper threshold (V h ). In an embodiment, the first comparator  206  produces low output voltage (V l ) when the output the buffer  202  is below the upper threshold (V h ). The second comparator  208  produces the high output voltage when the output of the buffer  202  is above the lower threshold (V l ). In an embodiment, the second comparator  208  produces low output voltage when the output of the buffer  202  is below the lower threshold (V l ). 
         [0038]      FIG. 5B  illustrates a graphical view of operational waveforms for the slew rate locked loop circuit when the comparators ( 206  and  208 ) produce a low output voltage of  FIGS. 2A-2B  according to an embodiment herein. The graphical view of the operational waveforms includes a second upper threshold voltage wave  512 , a second lower threshold voltage wave  514 , a second buffer output wave  516 , the first comparator and the second comparator wave with the low output voltage  518  and a second phase detector wave  520 . The first comparator  206  produces a low output voltage when the output of the buffer  202  is above the upper threshold (V h ). In an embodiment, the first comparator  206  produces high output voltage when the output of the buffer  202  is below the upper threshold (V h ). In an embodiment, the second comparator  208  produces the low output voltage when the output of the buffer  202  is above the lower threshold (V l ). In an embodiment, the second comparator  208  produces high output voltage when the output of the buffer  202  is below the lower threshold (V l ). 
         [0039]      FIG. 6  illustrates two simple architectures of a buffer according to an embodiment herein. The architectures include one or more negative channel metal-oxide semiconductor (NMOS) transistors and one or more positive channel metal-oxide semiconductor (PMOS) transistors. The one or more NMOS transistor receives a control voltage A and the one or more PMOS transistor receives a control voltage B. The slew rate locked loop is applicable for any voltage controlled buffer. 
         [0040]      FIG. 7  is a flow diagram illustrating a method for controlling and maintaining a slew rate at an output of the buffer  202  of  FIGS. 2A-2B  according to an embodiment herein. At step  702 , (i) a first input signal and (ii) at least one of a control voltage are received as inputs using the buffer  202 . At step  704 , a threshold voltage is generated using the first reference voltage generator  204 . At step  706 , the threshold voltage is compared with an output of the buffer  202  to obtain an output digital signal using at least one of a comparator. At step  708 , a phase difference is determined using a phase detector  210 . At step  710 , a DC voltage from an output of the phase detector  210  is produced using a loop filter  212 . At step  712 , a reference voltage is generated using a second reference voltage generator  214 . At step  714 , the DC voltage from the loop filter  212  and the voltage from the second reference voltage generator  214  are received by an amplifier  216 . At step  716 , (i) the DC voltage from the loop filter  212  and the reference voltage are amplified to obtain a control voltage using the amplifier  216 . At step  718 , the control voltage is fed to the buffer  202 . In an embodiment, the slew rate at the output of the buffer  202  is determined using the control voltage. 
         [0041]    The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein may be practiced with modification within the spirit and scope of the appended claims.