Abstract:
Current sense amplifiers include a pair of differential output signal lines and a current sensor electrically coupled to the pair of differential output signal lines. A first equalization device is also provided. The first equalization device is electrically coupled to the pair of differential output signal lines and is responsive to a sense amplifier enable signal (SAEN). In addition, according to a preferred aspect of the present invention, a second equalization device is also provided to reduce the likelihood that the differential outputs of the current sense amplifier will oscillate during sense and amplify operations. This second equalization device is also electrically coupled to the pair of differential output signal lines, however, the second equalization device is not responsive to the sense amplifier enable signal. Instead, the second equalization device is preferably responsive to a power supply signal (e.g., Vcc) and/or reference signal (e.g., Vss) and performs a constant or variable equalization function when the sense amplifier is active.

Description:
RELATED APPLICATION 
     This application is related to Korean Application No. 98-59420, filed Dec. 28, 1998, the disclosure of which is hereby incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to integrated circuit devices, and more particularly to current sense amplifiers (CSA). 
     BACKGROUND OF THE INVENTION 
     Data stored in a memory cell may be read through a pair of differential input and output signal lines (e.g., an input and output signal line and a complementary input and output signal line) during a data reading operation. The voltage difference between the input and output line and the complementary input and output line may be very small during the reading operation. To sense the small voltage difference, a sense amplifier is frequency used. 
     There are two common types of sense amplifiers, voltage sense amplifiers (VSA) and current sense amplifiers (CSA). VSAs are used to sense voltage differences and CSAs are used to sense current differences. In general, a current sense amplifier is used when the load of a line connected to an input port of a sense amplifier is large. In a semiconductor memory device, when the capacity of the memory is large, the pair of input and output signal lines are typically long and the load is typically very large. Accordingly, current sense amplifiers are frequently used in connection with large semiconductor memory devices. Also, due to the fact that a current sense amplifier may have a faster sense speed than a voltage sense amplifier, current sense amplifiers are often used in semiconductor memory devices having small data storage capacitors. 
     Referring to FIG. 1, a circuit diagram of a conventional sense amplifier is illustrated. A current source  11  provides the same amount of current to a differential input signal line INPUT and a complimentary differential input signal line INPUTB during the activation state of an sense amplifier enable signal SAEN (e.g., logic high). At this time, when data is loaded on the differential input signal line INPUT and the complimentary differential input signal line INPUTB, a current difference is generated between INPUT and INPUTB according to the voltage difference between them. A differential current sensor  12  detects the current difference between the pair of differential input signal lines INPUT and INPUTB, converts the current difference into a voltage difference, and outputs the converted voltage difference to a differential output signal line OUTPUT and a complementary differential output signal line OUTPUTB. A current sink  14  lets some of the current from the pair of differential output signal lines OUTPUT and OUTPUTB flow to a ground port VSS during the activation state of the sense amplifier enable signal SAEN. An equalization device  13  electrically connects the pair of differential output signal lines OUTPUT and OUTPUTB and equalizes them when the sense amplifier enable signal SAEN is inactive and the current source is inactive. 
     The ratio of the voltage difference between the pair of differential output signal lines OUTPUT and OUTPUTB to the voltage difference between the pair of differential input signal lines INPUT and INPUTB (e.g., the degree of amplification) is called the gain. The gain is controlled by regulating the sizes of PMOS transistors P 13  and P 14  of the differential current sensor  12  and the sizes of NMOS diodes N 11  and N 12  of the current sink  14 . As the gain becomes larger, the sensing speed of the current sense amplifier becomes faster. However, when the gain gets too large, the signals on the pair of differential output signal lines OUTPUT and OUTPUTB may begin to oscillate, as illustrated by FIG.  8 . Such oscillations may cause the levels of OUTPUT and OUTPUTB to become switched, and therefore an incorrect data value may be produced during a reading operation. 
     If the gain of the current sense amplifier is large, the values of OUTPUT and OUTPUTB can oscillate when the power supply voltage VCC is high. The level of the maximum power supply voltage in which oscillation is not generated in the current sense amplifier is called the High-VCC margin. Therefore, in the current sense amplifier, the gain should be appropriately controlled by regulating the sizes of the PMOS transistors P 13  and P 14  and the sizes of the NMOS diodes N 11  and N 12  so that the High-VCC margin is high enough to prevent oscillation. 
     In the conventional current sense amplifier, the High-VCC margin is reduced when the gain is increased in order to make the sensing speed fast. Yet, when the gain is decreased in order to increase the High-VCC margin and reduce the likelihood of oscillations, the sensing speed decreases and the rate at which data can be read from a memory also decreases. Thus, notwithstanding the desired use of current sense amplifiers in large memory devices, there continues to be a need for improved current sense amplifiers that are less susceptible to parasitic oscillators during reading operations. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide improved current sense amplifiers. 
     It is another object of the present invention to provide current sense amplifiers having reduced susceptibility to sensing errors caused by output oscillations. 
     It is still another object of the present invention to provide current sense amplifiers having high-Vcc margin and high sensing speed. 
     These and other objects, advantages and features of the present invention may be provided by a current sense amplifier that comprises a pair of differential output signal lines and a current sensor electrically coupled to the pair of differential output signal lines. A first equalization device is also provided. The first equalization device is electrically coupled to the pair of differential output signal lines and is responsive to a sense amplifier enable signal (SAEN). In addition, according to a preferred aspect of the present invention, a second equalization device is also provided to reduce the likelihood that the differential outputs of the current sense amplifier will oscillate during sense and amplify operations. This second equalization device is also electrically coupled to the pair of differential output signal lines, however, the second equalization device is not responsive to the sense amplifier enable signal. Instead, the second equalization device is preferably responsive to a power supply signal (e.g., Vcc) and/or reference signal (e.g., Vss) and performs a constant or variable equalization function when the sense amplifier is active. 
     To provide a variable equalization function, the second equalization device preferably comprises a voltage divider having a reference node, and a pass transistor having source and drain regions electrically coupled to the pair of differential output signal lines and a gate electrode electrically coupled to the reference node. Here, the resistance provided by the pass transistor determines the degree to which the second equalization device acts to reduce oscillations at the output of the sense amplifier. This resistance may also be controlled by varying the potential of the reference node in the voltage divider. For example, if the voltage divider is connected between a power supply potential and a ground reference potential, an increase in the magnitude of the power supply potential may be used to increase the potential of the reference node and thereby decrease the resistance of the pass transistor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an electrical schematic of a conventional current sense amplifier. 
     FIG. 2 is an electrical schematic of a current sense amplifier according to the present invention. 
     FIG. 3 is an electrical schematic of a preferred equalization device according to a first embodiment of the present invention. 
     FIG. 4 is an electrical schematic of a preferred equalization device according to a second embodiment of the present invention. 
     FIG. 5 is an electrical schematic of a preferred equalization device according to a third embodiment of the present invention. 
     FIG. 6 is an electrical schematic of a preferred equalization device according to a fourth embodiment of the present invention. 
     FIG. 7 is an electrical schematic of a preferred equalization device according to a fifth embodiment of the present invention. 
     FIG. 8 is a timing diagram that illustrates waveforms corresponding to signals at the output of the current sense amplifier of FIG.  1 . 
     FIG. 9 is a timing diagram that illustrates waveforms corresponding to signals at the output of the current sense amplifier of FIG.  2 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in 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 numbers refer to like elements throughout and signal lines and signals thereon may be referred to by the name reference numerals. 
     Referring to FIG. 2, a circuit diagram of a current sense amplifier according to the present invention is illustrated. This current sense amplifier includes a differential current sensor  22 , a first equalization device  23 , a current source  21 , and a second equalization device  25  that inhibits oscillation. The current sense amplifier may also include a current sink  24 . 
     The differential current sensor  22  includes a pair of PMOS current sense transistors P 23  and P 24  operating as latches, an NMOS control transistor N 23 , and a pair of NMOS current sink transistors N 21  and N 22  operating as diodes. The NMOS control transistor N 23  is responsive to the sense amplifier enable signal SAEN. NMOS control transistor N 23  is turned on when the sense amplifier enable signal SAEN is active at a logic  1  level (e.g., during the operation of the current sense amplifier). 
     The first equalization device  23  includes a PMOS equalizing transistor P 25 . Transistor P 25  is electrically connected across the pair of differential output lines OUTPUT and OUTPUTB and to the sense amplifier enable signal line SAEN as illustrated in FIG.  2 . The PMOS equalizing transistor P 25  is responsive to the sense amplifier enable signal SAEN and is turned on when the sense amplifier enable signal SAEN is inactive (e.g., a logic low) during initialization. When it is turned on, the PMOS transistor P 25  equalizes the potentials of the pair of differential output signal lines OUTPUT and OUTPUTB. In other words, the PMOS equalizing transistor P 25  electrically connects the pair of differential output signal lines OUTPUT and OUTPUTB together so they are at the same voltage level when the current sense amplifier turns on in response to an active SAEN. 
     The current source  21  includes a pair of PMOS current source transistors P 21  and P 22 . Transistors P 21  and P 22  are electrically connected to a power supply voltage VCC, the differential input signal lines INPUT and INPUTB, and the sense amplifier enable signal line SAEN as illustrated in FIG.  2 . The pair of PMOS current source transistors P 21  and P 22  are generally called load transistors. The current source  21  is responsive to the sense amplifier enable signal SAEN. The sense amplifier enable signal SAEN is inverted before it is applied to the gates of transistors P 21  and P 22 . P 21  and P 22  are turned on during the activation state of the sense amplifier enable signal SAEN (e.g., a logic high) and provide the same amount of current to the differential input signal line INPUT and the complementary differential input signal line INPUTB. At this time, when data is loaded on the differential input signal lines INPUT and INPUTB, a current difference is established between INPUT and INPUTB according to the voltage difference between them. 
     Although the current source  21  is described in this embodiment as utilizing P-channel transistors in combination with an inverted sense amplifier enable signal SAEN, the P-channel embodiment is used as a descriptive device only and is not meant to limit the embodiment of the current source to P-channel transistors. The P-channel current source  21  can be replaced with an N-channel version of the current source, as known in the art of analog design. 
     According to a preferred aspect of the present invention, the second equalization device  25  electrically connects the pair of differential output signal lines OUTPUT and OUTPUTB together in order to inhibit oscillation of the voltages across the pair of differential output signal lines OUTPUT and OUTPUTB. The second equalization device  25  is not influenced by the value of the sense amplifier enable signal SAEN. Connecting the pair of differential output signal lines reduces the voltage difference between the values of the pair of differential output signal lines OUTPUT and OUTPUTB. In particular, when the second equalization device  25  turns on, current flows from the differential output signal line (OUTPUT or OUTPUTB) having the higher voltage to the differential signal line having the lower voltage through the second equalization device  25 . Accordingly, the voltage difference between the output signal is reduced. Therefore, it is possible to inhibit the gain of the current sense amplifier from becoming excessively large. As a result, the likelihood of oscillation of the output signals is reduced and the High-VCC margin of the current sense amplifier according to the present invention is increased. 
     The first equalization device  23  and the second equalization device  25  may be resistive structures. The resistance of the second equalization device  25  is typically substantially larger than the resistance of the first equalization device  23 . Moreover, the resistance provided by the second equalization device  25  is set at a level sufficient to achieve a desired High-VCC margin and sense speed. 
     Referring now to FIG. 3, the first embodiment of the equalization device  25  for inhibiting oscillation is illustrated. This equalization device  25  includes an NMOS equalizing transistor N 31  acting as a pass transistor, a first NMOS clamp transistor N 32 , and a second NMOS clamp transistor N 33 . The transistors N 31 , N 32 , and N 33  are electrically coupled to the pair of differential output signal lines OUTPUT and OUTPUTB, the power supply voltage VCC, the ground voltage VSS, and to each other as illustrated in FIG.  3 . The first NMOS clamp transistor N 32  provides current and the second NMOS clamp transistor N 33  sinks current so that charges do not accumulate on the gate of the NMOS equalizing transistor N 31 . The first and second clamp transistors N 32  and N 33  also form a voltage divider having an intermediate reference node that is connected to the gate of transistor N 31 . 
     According to a preferred aspect of the first embodiment, the NMOS equalizing transistor N 31  does not turn on when the power supply voltage is at a low level (e.g., the High-VCC margin is held at a low voltage) because oscillation of the values of the differential output signal lines is unlikely when Vcc is low. However, the NMOS equalization transistor N 31  does turn on when the operating power supply voltage Vcc is high and the High-VCC margin is also driven to a high level. Thus, the embodiment of FIG. 3 can automatically provide a desired degree of oscillation clamping as the value of VCC varies from low levels (where the likelihood of oscillation is small) to high levels (where the likelihood is greater). In other words, the NMOS equalizing transistor N 31  preferably only turns on when it is needed (e.g., when High-VCC is set to a high level). 
     A voltage of 2.0 volts can be applied to the gate of the NMOS equalizing transistor N 31  when the operating power supply voltage is 3.0 volts by regulating the sizes (i.e., widths) of the first and second NMOS clamp transistors N 32  and N 33  and thereby regulating the top and bottom resistances of the voltage divider. The voltage difference between the gate and the source of the NMOS equalizing transistor N 31  is reduced (and the drain-to-source resistance of N 31  is increased) for a normal operating power supply voltage (e.g., 3 volts). In addition, because the voltage difference between the pair of differential output signal lines is not reduced when VCC is low, the gain of the current sense amplifier shown in FIG. 2 is not reduced and the high sensing speed of the current sense amplifier is maintained. As the operating power supply voltage (VCC) increases (e.g., to at least 4 volts), the voltage difference between the gate and the source of the NMOS equalizing transistor N 31  increases. Accordingly, the NMOS equalizing transistor N 31  is turned on to a greater extent. As a result, since the voltage difference is reduced when the power supply voltage is high (e.g., the High-VCC margin), the oscillation of the output signals sent to the pair of differential output signal lines OUTPUT and OUTPUTB is inhibited. 
     Referring now to FIG. 4, a second embodiment of the equalization device  25  for inhibiting oscillation is illustrated. This equalization device  25  includes a single NMOS equalizing transistor N 41 . The transistor N 41  is electrically coupled to the pair of differential output signal lines OUTPUT and OUTPUTB and the power supply voltage VCC as illustrated in FIG.  4 . Here, the width of transistor N 41  is preferably chosen to provide a desired amount of on-state clamping resistance. 
     Referring now to FIG. 5, a third embodiment of the equalization device  25  for inhibiting oscillation is illustrated. This equalization device  25  includes a PMOS equalizing transistor P 51 . The transistor P 51  is electrically coupled to the pair of differential output signal lines OUTPUT and OUTPUTB and the ground voltage VSS as illustrated in FIG.  5 . The width of transistor P 51  can also be chosen to achieve a desired amount of clamping. 
     The voltage difference between the pair of differential output signal lines OUTPUT and OUTPUTB is reduced by implementing either one of the embodiments of the second equalization device  25  illustrated in FIG.  4  and FIG.  5 . Since this voltage difference is reduced, the oscillation of the values of the pair of differential output signal lines OUTPUT and OUTPUTB is inhibited. Inhibiting this oscillation allows the High-VCC margin to be increased. 
     Referring now to FIG. 6, a fourth embodiment of the equalization device for inhibiting oscillation  25  is illustrated. The equalization device for inhibiting oscillation according to the fourth embodiment includes a first NMOS clamp transistor N 61  and second NMOS clamp transistor N 62 . Transistors N 61  and N 62  are electrically coupled to the differential output signal lines OUTPUT an OUTPUTB as illustrated in FIG.  6 . These transistors N 61  and N 62  operate as antiparallel-connected diodes. When the voltage of the complementary differential output signal line OUTPUTB is higher than the voltage of the differential output signal line OUTPUT, current flows from OUTPUTB to OUTPUT through the first NMOS clamp transistor N 61 . On the other hand, when the voltage of the differential output signal line OUTPUT is higher than the voltage of the complementary differential output signal line OUTPUTB, current flows from the OUTPUT to OUTPUTB through the second NMOS clamp transistor N 62 . As a result, the voltage difference between the pair of differential output signal lines OUTPUT and OUTPUTB is reduced and the oscillation of the output signals sent to OUTPUT and OUTPUTB is inhibited. 
     Referring to FIG. 7, a fifth embodiment of the equalization device for inhibiting oscillation  25  is illustrated. The equalization device for inhibiting oscillation according to the fifth embodiment includes a first PMOS clamp transistor P 71  and a second PMOS clamp transistor P 72 . The transistors P 71  and P 72  are electrically coupled to the pair of differential output signal lines OUTPUT and OUTPUTB as illustrated in FIG.  7 . When the voltage of the complementary differential output signal line OUTPUTB is higher then the voltage of the differential output signal line OUTPUT, current flows from OUTPUTB to OUTPUT through the second PMOS clamp transistor P 72 . On the other hand, when the voltage of the differential output signal line OUTPUT is higher than the voltage of the complementary differential output signal line OUTPUTB, current flows from OUTPUT to OUTPUTB through the first PMOS clamp transistor P 71 . As a result, since the voltage difference between the pair of differential output signal lines OUTPUT and OUTPUTB is reduced, the oscillation of the output signals is inhibited. 
     According to the equalization devices for inhibiting oscillation of the second through fifth embodiments, the sensing speed of the current sense amplifier may be decreased since the voltage difference between the pair of differential output signal lines OUTPUT and OUTPUTB is reduced in the normal power supply voltage region (e.g., 3 volts). However, it is possible to prevent the sensing speed from decreasing too much by regulating the sizes of the PMOS current sense transistors P 23  and P 24  of the differential current sensor  22  shown in FIG.  2  and the sizes of the NMOS current sink transistors N 21  and N 22  of the current sink  24  shown in FIG.  2 . 
     FIG. 8 illustrates waveforms of the output signals of the conventional current sense amplifier shown in FIG.  1 . FIG. 9 shows waveforms of the output signals of a current sense amplifier according to the present invention shown in FIG.  2 . FIGS. 8 and 9 show simulation results under the same conditions. In FIGS. 8 and 9, OUTPUT( 1 ) and OUTPUTB( 1 ) denote waveforms of the differential output signals sent to the pair of differential output signal lines OUTPUT and OUTPUTB when the power supply voltage VCC is 3.0 volts. OUTPUT( 2 ) and OUTPUTB( 2 ) denote waveforms of the output signals sent to the pair of differential data output lines OUTPUT and OUTPUTB when the power supply voltage VCC is 4.0 volts. OUTPUT( 3 ) and OUTPUTB( 3 ) denote the waveforms of the output signals sent to the pair of differential output signal lines OUTPUT and OUTPUTB when the power supply voltage VCC is 5.0 volts. 
     Referring now to FIG. 8, the output signals sent to the pair of differential output signal lines OUTPUT and OUTPUTB oscillate more as the power supply voltage VCC becomes higher in the conventional current sense amplifier shown in FIG.  1 . Referring now to FIG. 9, in the current sense amplifier according to the present invention shown in FIG. 2, the output signals sent to the pair of differential output signal lines OUTPUT and OUTPUTB oscillate to a much lesser degree even when the power supply voltage VCC increases. 
     In the current sense amplifier according to the present invention, a high sensing speed is maintained in a normal operating power supply voltage region. The High-VCC margin is large since the oscillation of the values of the differential output signal lines is inhibited in the high power supply voltage region by including an equalization device that is electrically coupled between a pair of differential output signal lines. 
     In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.