Abstract:
An input buffer having a comparator that receives an input signal, a reference signal and a positive feedback. The comparator compares the input signal relative to the reference signal and generates an output signal transitioning between a first logic state and a second logic state responsive to the magnitude of the input signal transitioning through the magnitude of the reference signal. The comparator intensifies the output signal in response to the positive feedback from the output of the comparator while the output signal transitions from the first logic state to the second logic state.

Description:
TECHNICAL FIELD 
   This invention relates generally to integrated circuits, and more specifically to an apparatus and method for a comparator circuit that uses AC positive feedback to reduce false switching due to slope reversals of a received signal. 
   BACKGROUND OF THE INVENTION 
   Input buffers are commonly used in a wide variety of integrated circuits. Buffers generally perform a number of advantageous functions when used in digital circuits. For example, buffers generally provide a high input impedance to avoid excessively loading circuits to which they are connected, and they have a low output impedance to simultaneously drive electrical circuits without excessive loading. Buffers can condition the signals applied to internal circuits so that the internal signals have well-defined logic levels and transition characteristics. Buffers are used, for example, to couple command, address and write data signals from command, address and data buses, respectively, of memory devices, including dynamic random access memory (“DRAM”) devices. 
   There are also several types of input buffers. For example, there are single ended input buffers in which a single input signal is applied to the buffer to cause the buffer to transition when the input signal transitions through predetermined voltage levels. Single-ended input buffers may also be used to compare the input signal to a reference voltage so that when the input signal transitions through the reference voltage the output of the buffer also transitions. Differential input buffer circuits are useful in digital circuits for determining whether an unknown input voltage is either above or below a fixed reference voltage. A conventional differential input buffer  100  is shown in  FIG. 1  that includes a pair of differential amplifiers  101 ,  103 , and an output coupled to an inverter  140 . The amplifiers  101 ,  103  are connected in parallel between a PMOS transistor  102  that is coupled to a supply voltage V CC  and an NMOS transistor  104  that is coupled to ground. When enabled by an active low signal EN_, the supply voltage V CC  supplies a current through the PMOS transistor  102  to a node  105 . As a result, a constant current is provided to the amplifier  101  and a PMOS transistor  122  coupled to the amplifier  103 . Similarly, the supply voltage V CC  is directly coupled to the gate of the transistor  104  such that a constant current is coupled from a node  115  through the transistor  104 , thereby drawing current through an NMOS transistor  110  coupled to the amplifier  101  and to the amplifier  103 . Therefore, the transistor  122  functions as a current source providing constant current to amplifier  103 , and transistor  110  functions as a current sink to discharge a constant current from amplifier  101 . 
   The two differential amplifiers  101 ,  103  have essentially the same components, but are complementary configured with respect to each other. The differential amplifier  101  includes a pair of PMOS transistors  116 ,  118  whose gates are coupled to each other and to node  105 , and further coupled to the drain of the PMOS transistor  118 . The transistors  116 ,  118  are coupled to each other in a current mirror configuration so they both have the same gate-to-source voltage. As a result, the transistors  116 ,  118  have the same source-to-drain resistance. The drains of the transistors  116 ,  118  are coupled to the drains of two NMOS transistors  112 ,  114  respectively, and receive an input signal V IN  and a reference signal V REF  at their respective gates. When the magnitude of the V IN  signal is at ground potential, the transistor  112  is turned OFF. As a result, an output node  108  at which an OUT_signal is generated is driven high through the PMOS transistor  116 . An inverter  140  having an input coupled to the node  108  thus generates a low DIFF_OUT signal. When the magnitude of the V IN  signal is at V CC , the transistor  112  is turned ON with a significantly higher gate-to-source voltage than the gate-to-source voltage of the PMOS transistor  116 . As a result, the resistance of the transistor  112  is significantly lower than the resistance of the transistor  116 . The voltage at the output node  108  is therefore low enough so that the inverter  140  outputs a high DIFF_OUT signal. As the magnitude of the V IN  signal passes through the magnitude of the V REF  signal, which is typically V CC /2, the NMOS transistors  112 ,  114  have the same gate-to-source voltage and hence the same resistance. Furthermore, the NMOS transistors  112 ,  114  will have the same gate-to-source voltage as the PMOS transistors  116 ,  118 . If the NMOS transistors  112 ,  114  have the same electrical characteristics as the PMOS transistors  116 ,  118 , the PMOS transistors  116 ,  118  will then have the same resistance as the NMOS transistors  112 ,  114 . In such case, the OUT_voltage will be equal to V CC /2. Therefore, decreasing the magnitude of the V IN  signal increases the resistance across the transistor  112 , reducing the current through the transistors  112 ,  116  to cause the magnitude of the OUT_signal to increase. Conversely, increasing the magnitude of the V IN  signal decreases the resistance across the transistor  112 , increasing the current through the transistors  112 ,  116  to cause the magnitude of the OUT_signal to decrease. 
   The amplifier  103  includes components that are the same as the amplifier  101 , and thus for the sake of brevity, the components to the amplifier  103  will not be described in detail. The amplifier  103  has a topology that is complementary to the topology of the amplifier  101  so that the gates of NMOS transistors  128 ,  130  are coupled together in a current mirror configuration so that both transistors  128 ,  130  have the same resistance. As the magnitude of the V IN  signal increases, the resistance of the PMOS transistor  126  increases to cause the magnitude of the OUT_signal to decrease. Conversely, as the magnitude of the V IN  signal decreases, the resistance of the PMOS transistor  126  decreases to cause the magnitude of the OUT_signal to increase. 
   When the magnitude of the V IN  signal decreases below V T , where V T  is the threshold voltage of the NMOS transistor  112 , the transistor  112  is turned OFF and thus no longer responds to changes in the magnitude of V IN . Similarly, when the magnitude of the V IN  signal increases above V CC -V T , where V T  is the threshold voltage of the PMOS transistor  126 , the transistor  126  is turned OFF and thus no longer responds to changes in the magnitude of V IN . Thus, the buffer  100  can operate at all values of V IN  from 0 to V CC , but only one amplifier  101  or  103  is operable with the magnitude of V IN  below V T  or above V CC -V T . 
   When the difference between V IN  and V REF  is small, such as when V IN  transitions through V REF , the integrity of the input signal can be easily compromised by a number of interferences, such as improper bus termination, reflections, signal noise, and V REF  noise. These factors can result in false switching of the buffer  100  as shown in  FIG. 2 . For example, due to the presence of noise, the V REF  signal may fluctuate about its predetermined value, such as V CC /2. As the V IN  signal approaches the V REF  signal, the noise interference on the V REF  signal may overlap the V IN  signal such that a slope reversal  205  occurs, where the buffer  100  detects V REF  to be greater than V IN , when in fact VIN is intended to be greater than V REF , but may not be due to noise and or signal reflections. Consequently, the buffer  100  may falsely switch its output, thereby generating an incorrect response to the input signal and causing delays or resulting in errors to the overall operation of the system or component that relies on the buffer  100 . 
   Therefore, there is a need for a low current input buffer that reduces false switching in the presence of noise due to input signal slope reversals, and restores signal integrity. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a conventional differential input buffer. 
       FIG. 2  is a graphical representation of a false switching occurring to an input signal of the input buffer of  FIG. 1 . 
       FIG. 3  is a block diagram of a differential input buffer having a capacitor coupled feedback to create AC positive feedback according to an embodiment of the invention. 
       FIG. 4  is a schematic diagram illustrating one embodiment of the differential input buffer circuit of  FIG. 3 . 
       FIG. 5  is a functional block diagram illustrating a memory device that includes at least one differential input buffer circuit according to an embodiment of the invention. 
       FIG. 6  is a functional block diagram illustrating a computer system including the memory device of  FIG. 5 . 
   

   DETAILED DESCRIPTION 
   Embodiments of the present invention are directed to an input buffer with AC positive feedback. Certain details are set forth below to provide a sufficient understanding of the invention. However, it will be clear to one skilled in the art that the invention may be practiced without these particular details. In other instances, well-known circuits, control signals, and timing protocols have not been shown in detail in order to avoid unnecessarily obscuring the invention. 
     FIG. 3  shows a block diagram of a buffer  300  according to an embodiment of the invention. The buffer  300  includes a differential comparator  302  that receives an input signal V IN  and a reference signal V REF . Similarly to the conventional input buffer  100 , the comparator  302  of the buffer  300  compares the two input signals and generates an inverted output signal OUT_depending on whether the V IN  signal is above or below the reference V REF . An inverter  304  drives and inverts the OUT_signal to generate a buffer output signal DIFF. However, as previously described, when the V IN  signal approaches the trip point determined as V REF , false switching due to input signal slope reversals can occur. In such cases, the buffer  300  reduces false switching by coupling the DIFF signal at a node  308  to the comparator  302 , thereby providing positive feedback as the output signal transitions from one logic level to another. The feedback loop includes a capacitor  306  that creates AC positive feedback for a small period of time as it charges and discharges in response to the DIFF signal swings. The positive feedback provided by the output signal can overcome noise interferences of the V IN  or V REF  signals when the signal difference is small to maintain signal integrity at the switching point. If V IN  is a periodic signal, such as a clock signal, the capacitance of the capacitor  306  is preferably chosen so that it charges from and discharges into the amplifier  302  over a duration that is less than one-half period of a periodic signal. 
   A differential input buffer  400  according to one embodiment of the invention is shown in  FIG. 4 . Similar to the amplifiers  101 ,  103  of the buffer  100  ( FIG. 1 ), the buffer  400  includes two differential amplifiers  401 ,  403  whose components are essentially the same, but are complementary configured with respect to each other. The buffer  400  includes many of the same components as the buffer  100  operating in the same manner and, in the interest of brevity, these same components will not be described in detail. 
   The buffer  400  differs from the conventional buffer  100  shown in  FIG. 1  in two respects. Most significantly, as explained in greater detail below, the buffer  400  receives AC positive feedback that makes it more immune to false switching resulting from noise. Second the buffer  400  includes three inverters  440 ,  442 ,  444  coupled to the node  108  to invert the OUT_signal and to provide a low impedance to the output at the node  108 . The use of three inverters  440 ,  442 ,  444  provides greater amplification of the OUT_signal so that the DIFF signal transitions high or low to V CC  or to zero, respectively, well prior to the OUT_signal completely transitioning low-to-high or high-to-low. 
   The AC positive feedback mentioned above is provided by applying the DIFF signal at the output of the inverter  444  to the amplifiers  401 ,  403  through respective third inputs of the amplifiers  401 ,  403  at respective nodes  407 ,  409 . The DIFF signal is applied to the nodes  407 ,  409  through respective capacitors  406 ,  408  to provide AC positive feedback to increase the drive of the OUT_signal at the node  108  as it transitions high or low. The AC positive feedback does not change the V REF  trip point, and provides positive feedback for only a small period of time that the DIFF signal is transitioning from one logic level to another. The positive feedback provided as the V IN  signal approaches the V REF  reference results in a more stable, uniform transition characteristic since it counteracts any input signal slope reversals due to noise. For example, assume the OUT_signal is transitioning low and the DIFF signal is transitioning high in response to the V IN  signal transitioning high. The capacitors  406 ,  408  couple the low-to-high transition of the DIFF signal to the nodes  407 ,  409  of the amplifiers  401 ,  403 , causing the voltage at the nodes  407 ,  409  to be driven high. The increased voltage at the node  407  of the amplifier  401  increases the resistance of the PMOS transistor  116 , thereby further decreasing the magnitude of the OUT_signal. The increased voltage at the node  409  of the amplifier  403  decreases the resistance of the NMOS transistor  130 , thereby also decreasing the magnitude of the OUT_signal. Thus, a rising V IN  signal results in a falling OUT_signal and a rising DIFF signal. The rising DIFF signal further decreases the magnitude of the OUT_signal, thereby providing positive feedback during the time that the DIFF signal is rising. The amplifiers  401 ,  403  respond to a falling V IN  signal in the same manner to provide a falling DIFF signal to the nodes  407 ,  409  that decrease the resistance of the PMOS transistor  116  in the amplifier  401  and increase the resistance of the NMOS transistor  130  in the amplifier  403 , thereby further increasing the OUT_signal. 
   The amount of positive feedback that is provided depends primarily on the size of the capacitor and gain of the amplifier at the nodes  407 ,  409 , and are determined as part of the design parameters for the particular buffer  400 . In the ideal case, the AC positive feedback is provided for less than half the clock cycle of the V IN  signal. For example, assuming the OUT_signal is pulled down and the DIFF signal is driven high in response to the OUT_signal. The capacitor  406  couples the low-to-high transition of the DIFF signal to the node  407 . The capacitor  406  is then discharged as current is drawn from the capacitor  406  by the node  407 . The time constant of the capacitor  406  and resistance at the node  407  should be set so that the capacitor  406  is substantially discharged by the time the DIFF signal transitions low. 
   The buffer  400  or another buffer according to an embodiment of the invention is shown in a synchronous dynamic random access memory (“SDRAM”) device  500 . The SDRAM device  500  includes an address register  512  that receives either a row address or a column address on an address bus  514 , preferably by coupling address signals corresponding to the addresses though one embodiment of input buffers  516  according to the present invention. The address bus  514  is generally coupled to a memory controller (not shown). Typically, a row address is initially received by the address register  512  and applied to a row address multiplexer  518 . The row address multiplexer  518  couples the row address to a number of components associated with either of two memory banks  520 ,  522  depending upon the state of a bank address bit forming part of the row address. Associated with each of the memory banks  520 ,  522  is a respective row address latch  526 , which stores the row address, and a row decoder  528 , which applies various signals to its respective array  520  or  522  as a function of the stored row address. The row address multiplexer  518  also couples row addresses to the row address latches  526  for the purpose of refreshing the memory cells in the arrays  520 ,  522 . The row addresses are generated for refresh purposes by a refresh counter  530 , which is controlled by a refresh controller  532 . 
   After the row address has been applied to the address register  512  and stored in one of the row address latches  526 , a column address is applied to the address register  512  and coupled through the input buffers  516 . The address register  512  couples the column address to a column address latch  540 . Depending on the operating mode of the SDRAM  500 , the column address is either coupled through a burst counter  542  to a column address buffer  544 , or to the burst counter  542  which applies a sequence of column addresses to the column address buffer  544  starting at the column address output by the address register  512 . In either case, the column address buffer  544  applies a column address to a column decoder  548  which applies various signals to respective sense amplifiers and associated column circuitry  550 ,  552  for the respective arrays  520 ,  522 . 
   Data to be read from one of the arrays  520 ,  522  is coupled to the column circuitry  550 ,  552  for one of the arrays  520 ,  522 , respectively. The data is then coupled through a read data path  554  to a data output register  556 . Data from the data output register  556  is coupled to a data bus  558  through data output buffers  559 . Data to be written to one of the arrays  520 ,  522  is coupled from the data bus  558  to a data input register  560  through data input buffers  561  according to an embodiment of the invention. The data input register  560  then couples the write data to the column circuitry  550 ,  552  where they are transferred to one of the arrays  520 ,  522 , respectively. A mask register  564  may be used to selectively alter the flow of data into and out of the column circuitry  550 ,  552 , such as by selectively masking data to be read from the arrays  520 ,  522 . 
   The above-described operation of the SDRAM  500  is controlled by a command decoder  568  responsive to command signals received on a control bus  570  though command input buffers  572  according to an embodiment of the invention. These high level command signals, which are typically generated by a memory controller (not shown), are a clock enable signal CKE*, a clock signal CLK, a chip select signal CS*, a write enable signal WE*, a row address strobe signal RAS*, and a column address strobe signal CAS*, which the “*” designating the signal as active low. Various combinations of these signals are registered as respective commands, such as a read command or a write command. The command decoder  568  generates a sequence of control signals responsive to the command signals to carry out the function (e.g., a read or a write) designated by each of the command signals. These command signals, and the manner in which they accomplish their respective functions, are conventional. Therefore, in the interest of brevity, a further explanation of these control signals will be omitted. 
   Although, the memory device illustrated in  FIG. 5  is a synchronous dynamic random access memory (“SDRAM”)  500  that includes the buffer  400  according to an embodiment of the invention, the buffer  400  can be used in other types of memory devices, as well as other types of digital devices. 
     FIG. 6  shows a computer system  600  containing the SDRAM  500  of  FIG. 5 . The computer system  600  includes a processor  602  for performing various computing functions, such as executing specific software to perform specific calculations or tasks. The processor  602  includes a processor bus  604  that normally includes an address bus, a control bus, and a data bus. In addition, the computer system  600  includes one or more input devices  614 , such as a keyboard or a mouse, coupled to the processor  602  to allow an operator to interface with the computer system  600 . Typically, the computer system  600  also includes one or more output devices  616  coupled to the processor  602 , such output devices typically being a printer or a video terminal. One or more data storage devices  618  are also typically coupled to the processor  602  to allow the processor  602  to store data in or retrieve data from internal or external storage media (not shown). Examples of typical storage devices  618  include hard and floppy disks, tape cassettes, and compact disk read-only memories (CD-ROMs). The processor  602  is also typically coupled to cache memory  626 , which is usually static random access memory (“SRAM”), and to the SDRAM  100  through a memory controller  630 . The memory controller  630  is coupled to the SDRAM  500  through the normally control bus  570  and the address bus  514 . The data bus  558  is coupled from the SDRAM  500  to the processor bus  604  either directly (as shown), through the memory controller  630 , or by some other means. 
   From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. For example, many of the components described above may be implemented using either digital or analog circuitry, or a combination of both. Accordingly, the invention is not limited except as by the appended claims.