Abstract:
A data output circuit of a semiconductor memory device matches an equalizing level of voltages at data lines in a pair with a logic threshold voltage of data output buffers. The data output circuit having an equalizing transistor connected between first and second nodes connected to the outputs of a sense amplifier, includes a threshold voltage control circuit disposed between the sense amplifier and the data output buffers for allowing a threshold voltage of the data output buffers to match with the equalizing level of the voltages at the first and second nodes. The threshold voltage control circuit has the same structure and characteristics as that of the output buffers, so as to ensure that the logic threshold voltage of the data output buffers matches with the equalizing level of the voltages at the first and second nodes.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to a data output circuit of a semiconductor memory device and, more particularly to a circuit for matching an equalizing level of data lines in a pair with a logic threshold voltage of data output buffers. 
     Generally, data is read out and written from/into a memory cell of a semiconductor memory device by selecting a word line and a bit line, and then transferred to the exterior of a chip. The data read out is amplified by a sense amplifier and then, is output to the exterior via a data output circuit. 
     Referring to FIG. 1, shown is a conventional data output circuit for outputting data stored on the memory cell to the exterior of the chip. The data stored on the memory cell 1 is amplified by a sense amplifier 2. The data output from the sense amplifier 2 is input to circuits 3 and 5 each having four MOS transistors controlled by a pair of selection control signals MSi and/MSi. The circuits 3 and 5 are respectively configured to have two PMOS transistors (3a, 3b; 5a, 5b) connected in series to the power supply voltage Vcc and two NMOS transistors (3c, 3d; 5c, 5d) connected in series to the ground voltage Vss. In case of the circuit 3, if the selection control signal MSi and the data on the line 1 are at the logic &#34;high&#34;, the PMOS transistor 3aand the NMOS transistors 3c, 3d are all turned on and the PMOS transistor 3b is turned off, so that a voltage at a first node NO1 goes to the logic &#34;low&#34;. On the contrary, a voltage at a second node /NO1 goes to the logic &#34;high&#34;. To increase a sensing speed of the data, the voltages at the first and second nodes NO1 and/NO1 are initially equalized by an NMOS transistor 11 upon receiving an address transition detection pulse PEQ (i.e., an equalizing signal) at a gate thereof. 
     If a buffer enable signal PIO of the logic &#34;low&#34; is applied to first and second data output buffers each comprised of NAND gates 13 and 15, the outputs of the NAND gates 13 and 15 are both at the logic &#34;high&#34;. Thus, the voltages at third and fourth nodes NO2, /NO2 go to the logic &#34;low&#34; and accordingly, pull-up and pull-down NMOS transistors 17 and 19 are both turned off. However, if the buffer enable signal PIO goes to the logic &#34;high&#34;, the outputs of the NAND gates 13 and 15 become &#34;high&#34; and &#34;low&#34; respectively and as a result, the NMOS transistors 17 and 18 are respectively turned off and turned on. Thus, the output data becomes at the logic &#34;low&#34;. 
     However, when the equalizing levels of the first and second nodes NO1 and/NO1 do not match with a logic threshold voltage (or a triggering voltage) of the first and second data output buffers 13 and 15, problems are raised which will be explained hereinbelow with reference to FIGS. 2A and 2B. 
     Referring first to FIG. 2A, shown is a case where the equalizing level of the voltages at the first and second nodes NO1 and/NO1 is set to be higher than the logic threshold voltage of the data output buffers 13 and 15. In this case, when the potential difference between the first and second nodes NO1 and/NO1 is not developed (i.e., the voltages at NO1 and/NO1 are equalized), the input voltages of the respective nodes NO1 and/NO1 may be both misrecognized as the logic &#34;high&#34;. At this moment, if the buffer enable signal PIO changes to the logic &#34;high&#34;, the NMOS transistors 17 and 19 are both turned on, resulting in a realoperation of the data output circuit. 
     Referring to FIG. 2B, illustrated is a case where the equalizing level of the voltages at the first and second nodes NO1 and/NO1 is lower than the logic threshold voltage of the data output buffers 13 and 15. In this case, it takes longer for a potential at the first or second node NO1,/NO1 to become at the logic &#34;high&#34; level, upon receiving data from the sense amplifier 2. Accordingly, the operation time of the data output buffers 13 and 15 is delayed after the buffer enable signal PIO enabled to the logic &#34;high&#34;, reducing the overall operation speed of the data output circuit. A letter &#34;L&#34; representing the data output time interval (time delay) of FIG. 2B is normally 2ns. Thus, it is needed to match the equalizing level of the data lines in a pair with the logic threshold voltage of the data output buffers. However, it becomes extremely difficult to match them since the equalizing level of the voltages at the data lines varies according to the various process parameters of the memory device. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention is to provide a data output circuit used for a semiconductor memory capable of preventing malfunctions during a data read operation, and outputting a data output at high speed. 
     It is another object of the present invention to provide a data output circuit used for a semiconductor memory device capable of outputting data at high speed, irrespective of a mismatching of an equalizing level of the voltages at the data lines with a logic threshold voltage of data output buffers. 
     It is also another object of the present invention to provide a data output circuit used for a semiconductor memory device capable of minimizing an operation time delay and preventing a glitch phenomenon. 
     According to an aspect of the present invention, a data output circuit of a semiconductor memory device having an equalizing transistor connected between first and second nodes connected to the outputs of a sense amplifier, includes a threshold voltage control circuit disposed between the sense amplifier and data output buffers for allowing a threshold voltage of the data output buffers to match with the equalizing level of the voltages at the first and second nodes. The threshold voltage control circuit has the same structure and characteristics as that of the output buffers, so as to ensure that the threshold voltage of the data output buffers matches with the equalizing level of the voltages at the first and second nodes. Further, the threshold voltage control circuit includes a NAND gate receiving the voltage at the first or second node in response to a chip enable signal, and an NMOS transistor responding to the equalizing signal, connected between the input and output of the NAND gate. 
    
    
     BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS 
     For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which: 
     FIG. 1 illustrates a data output circuit of a memory device according to a prior art; 
     FIGS. 2A and 2B are diagrams illustrating operation waveforms of FIG. 1; 
     FIG. 3 illustrates a data output circuit according to the present invention; 
     FIG. 4A illustrates a threshold voltage control circuit according to the present invention; voltage control circuit of FIG. 4A; and 
     FIG. 4B is a curve showing an input-output characteristic of the threshold voltage control circuit of FIG. 4A; and 
     FIG. 5 is a diagram illustrating operation waveforms of FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 3 and FIGS. 4A and 4B, complementary input signals generated from a sense amplifier 2 are input to first and second data output buffers 13, 15 each comprised of a NAND gate, and also provided to first and second threshold voltage control circuits 2125 and 2327 for controlling the voltage levels of first and second nodes NO1 and/NO1 to become the same as a logic threshold voltage level of the first and second data output buffers 13 and 15. An equalizing transistor 11 comprised of an NMOS transistor receives an equalizing signal PEQ through a gate thereof. A channel of the equalizing transistor 11 is formed between input terminals of the first and second data output buffers 13 and 15. It should be noted that the equalizing transistor 11 comprised of an NMOS transistor in this embodiment can be substituted with a PMOS transistor by inverting the logic state of the equalizing signal PEQ. The first and second threshold voltage control circuits 2125 and 2327 are respectively connected to first and second nodes NO1 and/NO1. Further, NAND gates 25 and 27 respectively constituting the first and second threshold voltage control circuits 2125 and 2327 have the same structure and characteristics as that of the first and second data output buffers 13 and 15, so that the logic threshold voltages of the NAND gates 25 and 27 are the same as those of the first and second data output buffers 13 and 15. Each of the NAND gates 25 and 27 receives a chip enable signal CS of the power supply voltage level. The NMOS transistors 21 and 23 commonly receive the equalizing signal PEQ and the channels thereof are respectively connected between the first node NO1 and the output of the NAND gate 25 and between the second node/NO1 and the output of the NAND gate 27. 
     Operations according to the embodiment of the present invention will be explained. If the equalizing signal PEQ is at the logic &#34;high&#34;, the equalizing transistor 11 and the NMOS transistors 21 and 23 are all turned on. The voltage of the first node NO1 is applied both to the first data output buffer 13 and to the NAND gate 25 of the first threshold voltage control circuit 2125. However, while the equalizing signal PEQ is enabled, the buffer enable signal PIO is disabled and therefore the first and second data output buffers 13 and 15 are disabled. Similarly, the NAND gate 27 of the second threshold voltage control circuit 2327 and the second data output buffer 15 commonly receive the voltage at the second node/NO 1. By the equalization operation, the voltages of the first and second nodes NO1 and/NO1 become at the same level. Thereafter, if the chip enable signal CS changes to the logic &#34;high&#34;, the NAND gates 25 and 27 are enabled. Then, the outputs of the NAND gates 25 and 27 are supplied to the first and second nodes NO1 and/NO1 via the NMOS transistors 21 and 23, respectively. Accordingly, the voltages at the points where the load line a (see FIG. 4A) of the NMOS transistors 21 and 23 meets the load line b of the NAND gates 25 and 27 determine the voltages of the first and second nodes NO1 and /NO1. In more detail, it is noted that the chip enable signal CS and the voltages at the first and second nodes NO1 and/NO1 are respectively fed-back to the input terminals of the NAND gates 25 and 27; thus, the voltages at the first and second nodes NO1 and/NO1 can be determined according to the voltages of the first and second nodes NO1 and/NO1 themselves. 
     If, for example, the voltage at the first node NO1 is at the level lower than a logic threshold voltage of the NAND gate 25, the output of the NAND gate 25 becomes at the logic &#34;high&#34;. Alternatively, if the voltage at the first node NO1 is higher than the logic threshold voltage of the NAND gate 25, the output of the NAND gate 25 becomes the logic &#34;low&#34;. It is therefore understood that the load line b of the NAND gate 25 changes rather drastically around the logic threshold voltage of the NAND gate 25. However, since the NMOS transistors 21 and 23 receive the equalizing signal PEQ through gates thereof, they remains turned on during the equalizing period, so that the voltages at the output terminals of the NAND gates 25 and 27 change linearly with respect to the input voltages on the line a (or the nodes NO1 and/NO1). However, in case where the logic &#34;low&#34; level lower than the logic threshold voltage of the NAND gates 25 and 27 is input to the NAND gates 25 and 27 and the outputs thereof go to the logic &#34;high&#34;, because the outputs of the NMOS transistors 21 and 23 are connected to the respective outputs of the NAND gates 25 and 27, the logic &#34;high&#34; output of the NAND gates 25 and 27 increases the input levels (the voltages at the first and second nodes NO1 and/NO1) of the NAND gates 25 and 27 via the NMOS transistors 21 and 23. This operation continues until the voltages at the first and second nodes NO1 and/NO1 increase up to the level of the logic threshold voltage of the NAND gates 25 and 27. When the voltages at the first and second nodes NO1 and/NO1 reach the logic threshold voltage of the NAND gates 25 and 27, the outputs of the NAND gates 25 and 27 become at the logic &#34;low&#34; and the voltages at the first and second nodes NO1 and/NO1 are no longer increased, because the voltages at both ends of the channels of the respective NMOS transistors 21 and 23 become the same to each other at a certain point X (see FIG. 4B). That is, the voltages at the first and second nodes NO1 and/NO1 are determined at the logic threshold voltages of the NAND gates 25 and 27. 
     As described so far and illustrated in FIG. 4B, the characteristic curves Ca and Cb each representing the voltages at the load lines a and b meet each other at the logic threshold voltage Vth of the NAND gates 25 and 27. The voltage at the conjunction X becomes the equalizing levels of the first and second nodes NO1 and/NO1. It is further noted that since the NAND gates 25 and 27 have the same structure and characteristics as that of the data output buffers 13 and 15, the threshold voltage of the data output buffers 13 and 15 are the same as that of the NAND gates 25 and 27. As a result, the equalizing levels of the first and second nodes NO1 and/NO1 become the same as the logic threshold voltage of the data output buffers 13 and 15. After the equalizing level had been determined, the equalizing signal PEQ is disabled, the buffer enable signal PIO is enabled, and the chip enable signal CS are disabled, thereby fixing voltages at the first and second nodes NO1 and/NO1 to the logic threshold voltage level of the data output buffers 13 and 15. Thereafter, the data output from the sense amplifier 2 is transferred to the first and second nodes NO1 and/NO1. Then, since the logic threshold voltage of the data output buffers 13 and 15 matches exactly with the equalizing levels of the first and second nodes NO1 and/NO1, only the voltage at either the first node NO1 or the second node/NO1 is recognized as the logic &#34;high&#34; state while the voltage at the other node is recognized as the logic &#34;low&#34;; therefore, only one of the pull-up and pull-down transistors 17 and 19 is selectively enabled. Accordingly, the glitch phenomenon illustrated in FIG. 2A does not occur. Also, the time delay while recognizing the logic &#34;high&#34; by one of the data output buffers 13 and 15 as shown in FIG. 2B is removed. This is illustrated in FIG. 5. 
     In the examples of the embodiments of the present invention provided so far, only the NMOS transistors and NAND gates have been mentioned as the means for controlling the threshold voltage. However, NOR, AND, OR gates and inverters can be used as long as a threshold voltage of the same level as the threshold voltage of the data output buffers can be provided. In case AND gates are used instead of NAND gates 25 and 27 having the same logic threshold voltage, a chip enable signal SC has a power voltage level. However, in case NOR gates or OR gates are used, the logic &#34;low&#34; signal of the ground level can be used for the chip enable signal SC. Also, PMOS transistors or CMOS transistors can be used instead of NMOS transistors 21 and 23 connected to the output terminal of the NAND gates 25 and 27. The transistors must be turned on during the equalization period. 
     The threshold voltage controlling circuits 2125 and 2327 of the present invention are disposed between the data output buffers 13 and 27, and the sense amplifier 2, respectively. However, they can be disposed between the sense amplifiers or between the sense amplifier 2 and a multiplexer (not shown) since the present invention can be utilized on any input circuits which receive an equalizing signal. 
     As previously mentioned, even if the equalizing levels of the first and second nodes are varied according to the process conditions, the equalizing level is adjusted to be matched to the logic threshold voltage level of the data output buffers thereby preventing the malfunctions without the overall sensing speed decreasing.