Abstract:
The present invention relates to an output driver circuit which exhibits a reduced variation in the slew rate of an output signal thereof, irrespective of a variation in temperature occurring during a process carried out by a semiconductor memory device, to which the output driver circuit is applied, or a variation in temperature caused by the operation characteristics of the semiconductor memory device, while exhibiting excellent operation characteristics even in a high-speed operation mode thereof.

Description:
[0001]     This application relies for priority upon Korean Patent Application No. 2004-0025059 filed on Apr. 12, 2004, the contents of which are herein incorporated by reference in their entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an output driver circuit, and more particularly to an output driver circuit which is capable of preventing an abrupt variation in the slew rate thereof when a variation in resistance and supply voltage occurs due to a variation in process and temperature conditions, so that it is widely usable on output stages of high-speed interface circuits.  
         [0004]     2. Description of the Related Art  
         [0005]     Semiconductor devices use an output driver circuit to output internal data from a chip thereof to the outside of the chip through an output terminal, that is, an output pad.  
         [0006]     Such an output driver circuit includes a push-pull driver. One important function of such a push-pull driver is to control the slew rate of an output signal.  
         [0007]     “Slew rate” means a variation rate of the voltage level of an output signal, so that it may be considered as a gradient of voltage to time. Such a slew rate may be a rising slew rate or a falling slew rate. The rising slew rate represents the gradient of an output voltage, the level of which is transited from a low level to a high level. On the other hand, the falling slew rate represents the gradient of an output voltage, the level of which is transited from a high level to a low level. At a higher slew rate, the gradient of the output voltage is sharper. In other words, the level of the output voltage is abruptly varied within a short time. Where an output push-pull driver exhibits a high slew rate, large noise current is generated. For this reason, such an output push-pull driver is unsuitable for the output stage of a high-speed interface circuit.  
         [0008]      FIG. 1  illustrates a configuration of a conventional output driver circuit. Operation of the conventional driver circuit and problems involved therewith will be described with reference to  FIG. 1 .  
         [0009]      FIG. 1  shows an example in which the conventional output driver circuit is applied to a DRAM. As shown in  FIG. 1 , the output driver circuit includes a pre-driver including a CMOS transistor composed of a PMOS transistor P 1  and an NMOS transistor N 1 , and adapted to perform a switching operation in response to an input data signal IN, and another CMOS transistor composed of a PMOS transistor P 2  and an NMOS transistor N 2 , and adapted to perform a switching operation in response to the input data signal IN. The output driver circuit also includes a PMOS transistor P 0  serving as a pull-up element adapted to perform a switching operation in response to a signal at an output node up between the PMOS transistor P 1  and the NMOS transistor N 1 , and an NMOS transistor N 0  serving as a pull-down element adapted to perform a switching operation in response to a signal at an output node dn between the PMOS transistor P 2  and the NMOS transistor N 2 .  
         [0010]     The conventional output driver circuit further includes a means for controlling the slew rate of an output signal thereof. In the illustrated case, the slew rate control means comprises resistors R 1  and R 2  included in the pre-driver. In accordance with this configuration, when the input data signal IN has a high level, both the PMOS transistors P 1  and P 2  are turned off, whereas both the NMOS transistors N 1  and N 2  are turned on. In this state, discharging of current is carried out. As a result, the potential at the output node up, that is, voltage Vup, is lowered to a level Vss in accordance with a time constant t 1  (t 1 =(R 1 +R_N 1 )×C_up). Here, “C_up” represents a parasitic capacitance generated at the output nod up, and “R_N 1 ” represents an on-resistance of the NMOS transistor N 1  in an ON state thereof. That is, the voltage Vup is lowered, as expressed by the following Expression: 
 
 Vup=Vdd×e−t/t   1   [Expression 1]
 
         [0011]     In accordance with Expression 1 associated with discharging of current, the PMOS transistor P 0 , which is a driving element, drives the voltage at an output terminal of the output driver circuit to a level Vdd. Thus, the resultant output signal has a certain slew rate.  
         [0012]     Similarly, when the input data signal IN has a low level, both the NMOS transistors N 1  and N 2  are turned off, whereas both the PMOS transistors P 1  and P 2  are turned on. In this state, discharging of current is carried out. As a result, the potential at the output node dn, that is, voltage Vdn, is raised to the level Vdd in accordance with a time constant t 2  (t 2 =(R 2 +R_P 2 )×C_dn). Here, “C_down” represents a parasitic capacitance generated at the output nod dn, and “R_P 2 ” represents an on-resistance of the PMOS transistor P 2  in an ON state thereof. That is, the voltage Vdn is raised, as expressed by the following Expression: 
 
 Vdn=Vdd ×(1− e   −t/t2 )  [Expression 2]
 
         [0013]     In accordance with the Expression associated with charging of current, the NMOS transistor N 0 , which is a driving element, drives the voltage at the output terminal of the output driver circuit to a level Vss. Thus, the resultant output signal has a certain slew rate.  
         [0014]     However, the conventional output driver circuit has a problem in that the slew rate characteristics thereof are unstable. That is, the semiconductor memory device, to which the output driver circuit is applied, exhibits a high temperature variation during a process carried out thereby. In particular, the semiconductor memory device exhibits an increase in temperature during a high-speed operation thereof. In this case, the resistors included in the output driver circuit exhibit a variation in resistance because they are sensitive to a variation in temperature. For this reason, a variation in temperature causes each resistor to exhibit a variation in resistance, thereby varying the time constants t 1  and t 2 , which are expressed in Expressions 1 and 2. As a result, respective voltages at the nodes up and dn vary. Thus; the conventional output driver circuit exhibits an abrupt variation in slew rate in accordance with a temperature variation occurring in the associated semiconductor memory device. Such a slew rate variation adversely affects the signal characteristics of the semiconductor memory device.  
       SUMMARY OF THE INVENTION  
       [0015]     Therefore, an object of the invention is to provide an output driver circuit which exhibits a reduced variation in the slew rate of an output signal thereof, irrespective of a variation in temperature occurring during a process carried out by a semiconductor memory device, to which the output driver circuit is applied, or a variation in temperature caused by the operation characteristics of the semiconductor memory device, while exhibiting excellent operation characteristics even in a high-speed operation mode thereof.  
         [0016]     In accordance with one aspect, the present invention provides an output driver circuit comprising: a pull-up driver adapted to output a supply voltage to an output terminal; a pull-down driver adapted to output a ground voltage to the output terminal; a first pull-up pre-driver adapted to be enabled in response to a data input signal, and to pull up, in an enable state thereof, a gate of the pull-up driver to a level of the supply voltage, using a first charge current controlled by a first current supplied from a current supply unit; a first pull-down pre-driver adapted to be enabled in response to the data input signal, and to pull down, in an enable state thereof, the gate of the pull-up driver to a level of the ground voltage, using a first discharge current controlled by a second current supplied from the current supply unit; a second pull-up pre-driver adapted to be enabled in response to the data input signal, and to pull up, in an enable state thereof, a gate of the pull-down driver to the level of the supply voltage, using a second charge current controlled by the first current; and a second pull-down pre-driver adapted to be enabled in response to the data input signal, and to pull down, in an enable state thereof, the gate of the pull-down driver to the level of the ground voltage, using a second discharge current controlled by the second current.  
         [0017]     The first pull-up pre driver may comprise a first transistor adapted to allow the first current to flow between a supply voltage source and the current supply unit, and a second transistor adapted to compose a current mirror, together with the first transistor; to pull up the gate of the pull-up driver, using the first charge current, which has a predetermined magnitude proportional to the first current.  
         [0018]     The first pull-down pre-driver may comprise a first transistor adapted to allow the second current to flow between the current supply unit and a ground, and a second transistor adapted to compose a current mirror, together with the first transistor, to pull down the gate of the pull-up driver, using the first discharge current, which has a predetermined magnitude proportional to the second current.  
         [0019]     Where the first pull-up pre-driver comprises a first transistor adapted to allow the first current to flow between a supply voltage source and the current supply unit, and a second transistor adapted to compose a current mirror, together with the first transistor, to pull up the gate of the pull-up driver, using the first charge current, which has a predetermined magnitude proportional to the first current, the first pull-down pre-driver comprises a third transistor adapted to allow the second current to flow between the current supply unit and a ground, and a fourth transistor adapted to compose a current mirror, together with the third transistor, to pull down the gate of the pull-up driver, using the first discharge current, which has a predetermined magnitude proportional to the second current.  
         [0020]     The second pull-up pre-driver may comprise a first transistor adapted to allow the first current to flow between a supply voltage source and the current supply unit, and a second transistor adapted to compose a current mirror, together with the first transistor, to pull up the gate of the pull-down driver, using the second charge current, which has a predetermined magnitude proportional to the first current.  
         [0021]     The second pull-down pre-driver may comprise a first transistor adapted to allow the second current to flow between the current supply unit and a ground, and a second transistor adapted to compose a current mirror, together with the first transistor, to pull down the gate of the pull-down driver, using the second discharge current, which has a predetermined magnitude proportional to the second current.  
         [0022]     Where the second pull-up pre-driver comprises a first transistor adapted to allow the first current to flow between a supply voltage source and the current supply unit, and a second transistor adapted to compose a current mirror, together with the first transistor, to pull up the gate of the pull-down driver, using the second charge current, which has a predetermined magnitude proportional to the first current, the second pull-down pre-driver may comprise a third transistor adapted to allow the second current to flow between the current supply unit and a ground, and a fourth transistor adapted to compose a current mirror, together with the third transistor, to pull down the gate of the pull-down driver, using the second discharge current, which has a predetermined magnitude proportional to the second current.  
         [0023]     Where the first pull-up pre-driver comprises a first transistor adapted to allow the first current to flow between a supply voltage source and the current supply unit, and a second transistor adapted to compose a current mirror, together with the first transistor, to pull up the gate of the pull-up driver, using the first charge current, which has a predetermined magnitude proportional to the first current, and the first pull-down pre-driver comprises a third transistor adapted to allow the second current to flow between the current supply unit and a ground, and a fourth transistor adapted to compose a current mirror, together with the third transistor, to pull down the gate of the pull-up driver, using the first discharge current, which has a predetermined magnitude proportional to the second current, the second pull-up pre-driver may comprise a fifth transistor adapted to allow the first current to flow between the supply voltage source and the current supply unit, and a sixth transistor adapted to compose a current mirror, together with the fifth transistor, to pull up the gate of the pull-down driver, using the second charge current, which has a predetermined magnitude proportional to the first current. In this case, the second pull-down pre-driver may also comprise a seventh transistor adapted to allow the second current to flow between the current supply unit and the ground, and an eighth transistor adapted to compose a current mirror, together with the seventh transistor, to pull down the gate of the pull-down driver, using the second discharge current, which has a predetermined magnitude proportional to the second current.  
         [0024]     The output driver circuit may further comprise a compensation capacitor connected to the gate of the pull-down driver and the ground, and adapted to compensate for a parasitic capacitance difference between the gate of the pull-up driver and the gate of the pull-down driver.  
         [0025]     The current supply unit may comprise a plurality of current-dividing transistors adapted to divide, into current of a predetermined magnitude, current flowing between a first node, which is maintained at a level corresponding to a reference voltage, and the ground, and then to supply the divided current to the first node, a ninth transistor adapted to allow a third current to flow therethrough in a magnitude corresponding to that of the divided current in response to a voltage at respective gates of the current-dividing transistors, a tenth transistor adapted to compose a current mirror, together with the ninth transistor, to output, as the second current, a current having a predetermined magnitude proportional to the third current, an eleventh transistor adapted to supply the third current from the ninth transistor to the ground, and a twelfth transistor adapted to compose a current mirror, together with the eleventh transistor, to output, as the first current, a current having a predetermined magnitude proportional to the third current.  
         [0026]     In accordance with another aspect, the present invention provides an output driver circuit comprising: a pull-up driver adapted to output a supply voltage to an output terminal; a pull-down driver adapted to output a ground voltage to the output terminal; a first pre-driver including a first pull-up unit adapted to pull up a gate of the pull-up, driver to a level of the supply voltage in response to a first voltage having a predetermined level, and a first pull-down unit adapted to pull down the gate of the pull-up driver to a level of the ground voltage in response to a second voltage having a predetermined level; and a second pre-driver including a second pull-up unit adapted to pull up a gate of the pull-down driver to the level of the supply voltage in response to the first voltage, and a second pull-down unit adapted to pull down the gate of the pull-down driver to the level of the ground voltage in response to the second voltage.  
         [0027]     The first pre-driver may further include first and second switching units adapted to switch respective operations of the first pull-up unit and first pull-down unit in response to a predetermined data input signal.  
         [0028]     The second pre-driver may further include first and second switching units adapted to switch respective operations of the second pull-up unit and second pull-down unit in response to a predetermined data input signal.  
         [0029]     The output driver circuit may further comprise a compensation capacitor connected to the gate of the pull-down driver and a ground, and adapted to compensate for a parasitic capacitance difference between the gate of the pull-up driver and the gate of the pull-down driver. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]     The above objects, and other features and advantages of the present invention will become more apparent after reading the following detailed description when taken in conjunction with the drawings, in which:  
         [0031]      FIG. 1  is a circuit diagram illustrating a configuration of a conventional output driver circuit;  
         [0032]      FIG. 2  is a circuit diagram illustrating an output driver circuit according to a first embodiment of the present invention;  
         [0033]      FIG. 3  is a circuit diagram illustrating a reference current generating circuit included in a current supply unit used in the output driver circuit according to the first embodiment of the present invention;  
         [0034]      FIG. 4  is a circuit diagram illustrating a configuration of the current supply unit used in the output driver circuit according to the first embodiment of the present invention;  
         [0035]      FIG. 5  is a circuit diagram illustrating an output driver circuit according to a second embodiment of the present invention; and  
         [0036]      FIG. 6  is a diagram depicting a characteristic waveform of the output driver circuit according to the first embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]      FIG. 2  is a circuit diagram illustrating an output driver circuit according to a first embodiment of the present invention.  
         [0038]     As shown in  FIG. 2 , the output driver circuit includes a PMOS transistor M 10  as a pull-up driver adapted to output a supply voltage Vdd to an output terminal OUT, and an NMOS transistor M 20  as a pull-down driver adapted to output a ground voltage Vss to the output terminal OUT. The output driver circuit also includes a first pull-up pre-driver  110  adapted to be enabled in response to a data input signal IN, and to pull up, in an enable state thereof, a gate up of the PMOS transistor M 10  to the level of the supply voltage Vdd in accordance with a first charge current flowing therethrough under the control of a current ISRCN supplied from a current supply unit, and a first pull-down pre-driver  120  adapted to be enabled in response to the data input signal IN, and to pull down, in an enable state thereof, the gate up of the PMOS transistor M 10  to the level of the ground voltage Vss in accordance with a first discharge current flowing therethrough under the control of a current ISRCP supplied from the current supply unit. The output driver circuit further includes a second pull-up pre-driver  130  adapted to be enabled in response to the data input signal IN, and to pull up, in an enable state thereof, a gate dn of the NMOS transistor M 20  to the level of the supply voltage Vdd in accordance with a second charge current flowing therethrough under the control of the current ISRCN, and a second pull-down pre-driver  140  adapted to be enabled in response to the data input signal IN, and to pull down, in an enable state thereof, the gate dn of the NMOS transistor M 20  to the level of the ground voltage Vss in accordance with a second discharge current flowing therethrough under the control of the current ISRCP.  
         [0039]     Operation of the output driver circuit according to the first embodiment of the present invention will now be described in detail with reference to  FIG. 2 .  
         [0040]     In accordance with a transition of the input data signal IN from a low level to a high level, an inverse signal INB to the input data signal IN is transited from a high level to a low level. In this state, switches G 11  and G 12 , which are coupled to the inverse signal INB, are turned off. On the other hand, switches G 13  and G 14 , which are also coupled to the inverse signal INB, are turned on. Accordingly, PMOS transistors M 11  and M 12 , which are connected to respective switches G 11  and G 12  while composing a current mirror of the first pull-up pre-driver  110 , are floated, so that they are inactive. On the other hand, NMOS transistors M 13  and M 14 , which are connected to respective switches G 13  and G 14  while composing a current mirror of the first pull-down pre-driver  120 , are turned on. As a result, the current ISRCP supplied from the current supply unit flows to the ground Vss through the NMOS transistor M 13 . Meanwhile, current of a predetermined magnitude corresponding to that of the current ISRCP flows through the NMOS transistor M 14 , which shares a gate thereof with the NMOS transistor M 13  to compose the current mirror of the first pull-down pre-driver  120 . Accordingly, charges stored in the gate up of the PMOS transistor M 10  are discharged, thereby causing the PMOS transistor M 10  to turn on. In this case, the magnitude of discharge current is determined in accordance with a size ratio between the NMOS transistors M 13  and M 14 . Therefore, where the NMOS transistor M 14  has a size corresponding to, for example, N times the size of the NMOS transistor M 13 , the discharge current magnitude corresponds to “N×ISRCP”. The time taken for the gate up of the PMOS transistor M 10  to be completely discharged in this case is determined in accordance with the magnitude of the current ISRCP, the parasitic capacitance generated at the gate up of the PMOS transistor M 10 , and the charged voltage at the gate up of the PMOS transistor M 10 . Also, the time taken for the PMOS transistor M 10  to turn on is determined in accordance with a variation in potential at the gate up of the PMOS transistor M 10  depending on the discharge current magnitude. For example, where the NMOS transistor M 14  has a size corresponding to N times the size of the NMOS transistor M 13 , the potential at the gate up of the PMOS transistor M 10  can be expressed as follows: 
 
 Vup=Vdd −( I   —   M   14 / C   —   up )× t  
 
 I   —   M   14 = N×ISRCP   [Expression 3]
 
         [0041]     (“Vup” represents a voltage at the gate up, “I_M 14 ” represents the magnitude of current flowing through the NMOS transistor M 14 , and “C_up” represents a parasitic capacitance at the gate up. Where “Vup” approximates to zero, the operating region of the NMOS transistor M 14  is changed from a saturation region to a triode region because the source-drain voltage of the NMOS transistor M 14  approximates to zero, so that the current I_M 14  approximates to zero.)  
         [0042]     On the other hand, when the input data signal IN is transited from a low level to a high level, the second pull-up pre-driver  130  and second pull-down pre-driver  140  operate as follows. That is, switches G 21  and G 22 , which are coupled to the inverse signal INB, are turned off. On the other hand, switches G 23  and G 24 , which are also coupled to the inverse signal INB, are turned on. Accordingly, PMOS transistors M 21  and M 22 , which are connected to respective switches G 21  and G 22  while composing a current mirror of the second pull-up pre-driver  130 , are inactive. On the other hand, NMOS transistors M 23  and M 24 , which are connected to respective switches G 23  and G 24  while composing a current mirror of the second pull-down pre-driver  140 , are turned on. As a result, the current ISRCP supplied from the current supply unit flows to the ground Vss through the NMOS transistor M 23 . Meanwhile, current of a predetermined magnitude corresponding to that of the current ISRCP flows through the NMOS transistor M 24 . Accordingly, charges stored in the gate dn of the NMOS transistor M 20  are discharged, thereby causing the NMOS transistor M 20  to turn off. In this case, the magnitude of discharge current is determined in accordance with a size ratio between the NMOS transistors M 23  and M 24 . Therefore, where the NMOS transistor M 24  has a size corresponding to, for example, M times the size of the NMOS transistor M 23 , the discharge current magnitude corresponds to “M×ISRCP”. The time taken for the gate dn of the NMOS transistor M 20  to be completely discharged in this case is determined in accordance with the magnitude of the current ISRCP, the parasitic capacitance generated at the gate dn of the NMOS transistor M 20 , and the charged voltage at the gate dn of the NMOS transistor M 20 . Also, the time taken for the NMOS transistor M 20  to turn off is determined in accordance with a variation in potential at the gate dn of the NMOS transistor M 20  depending on the discharge current magnitude. For example, where the NMOS transistor M 24  has a size corresponding to N times the size of the NMOS transistor M 23 , the potential at the gate dn of the NMOS transistor M 20  can be expressed as follows: 
 
 Vdn=Vdd −( I   —   M   24 / C   —   dn )× t  
 
 I   —   M   24 = M×ISRCP   [Expression 4]
 
         [0043]     (“Vdn” represents a voltage at the gate dn, “I_M 24 ” represents the magnitude of current flowing through the NMOS transistor M 24 , and “C_dn” represents a parasitic capacitance at the gate dn. Where “Vdn” approximates to zero, the operating region of the NMOS transistor M 24  is changed from a saturation region to a triode region because the source-drain voltage of the NMOS transistor M 24  approximates to zero, so that the current I_M 24  approximates to zero.)  
         [0044]     Thus, the turning-on time of the PMOS transistor M 10  and the turning-off time of the NMOS transistor M 20  can be controlled by the current ISRCP, which has a predetermined magnitude. Accordingly, it is possible to stably control a slew rate according to rising of an output voltage, that is, a rising slew rate, determined in accordance with the turning-on time of the PMOS transistor M 10  and the turning-off time of the NMOS transistor  20 , using the current ISRCP.  
         [0045]     When the input data signal IN is transited from a high level to a low level, the switches G 11  and G 12  are turned on, whereas the switches G 13  and G 14  are turned off. Accordingly, the PMOS transistors M 11  and M 12  turn on, whereas the NMOS transistors M 13  and M 14  turn off. As a result, the current ISRCN flows from the supply voltage source Vdd to the current supply unit via the PMOS transistor M 11 . Meanwhile, current of a predetermined magnitude corresponding to that of the current ISRCN flows through the PMOS transistor M 12 , which composes the current mirror of the first pull-down pre-driver  120 , together with the PMOS transistor M 11 . Accordingly, the gate up of the PMOS transistor M 10  is charged, thereby causing the PMOS transistor M 10  to turn off. In this case, the magnitude of charge current is determined in accordance with a size ratio between the PMOS transistors M 11  and M 12 . Therefore, where the PMOS transistor M 12  has a size corresponding to, for example, N times the size of the PMOS transistor M 11 , the magnitude of charge current through the PMOS transistor M 12  corresponds to “N×ISRCN”. The time taken for the gate up of the PMOS transistor M 10  to be completely charged in this case is determined in accordance with the magnitude of the current ISRCN, and the parasitic capacitance generated at the gate up of the PMOS transistor M 10 . Also, the time taken for the PMOS transistor M 10  to turn off is determined in accordance with a variation in potential at the gate up of the PMOS transistor M 10  depending on the charge current magnitude. For example, where the PMOS transistor M 12  has a size corresponding to N times the size of the PMOS transistor M 11 , the potential at the gate up of the PMOS transistor M 10  can be expressed as follows: 
 
 Vup =( I   —   M   12 / C   —   up )× t  
 
 I   —   M   12 = N×ISRCN   [Expression 5]
 
         [0046]     (“I_M 12 ” represents the magnitude of current flowing through the PMOS transistor M 12 . Where “Vup” approximates to “Vdd”, the operating region of the PMOS transistor M 12  is changed from a saturation region to a triode region because the source-drain voltage of the PMOS transistor M 12  approximates to zero, so that the current I_M 12  approximates to zero.)  
         [0047]     On the other hand, when the input data signal IN is transited from a high level to a low level, the second pull-up pre-driver  130  and second pull-down pre-driver  140  operate as follows. That is, the switches G 21  and G 22  turn on, whereas the switches G 23  and G 24  turn off. Accordingly, the PMOS transistors M 21  and M 22  of the second pull-down pre-driver  140  turn on, whereas the NMOS transistors M 23  and M 24  turn off. As a result, the current ISRCN flows from the supply voltage source Vdd to the current supply unit through the PMOS transistor M 21 . Meanwhile, current of a predetermined magnitude corresponding to that of the current ISRCN flows through the PMOS transistor M 22 , which composes the current mirror of the second pull-up pre-driver  130 , together with the PMOS transistor M 21 . Accordingly, the gate dn of the NMOS transistor M 20  is discharged, thereby causing the NMOS transistor M 20  to turn on. In this case, the magnitude of charge current is determined in accordance with a size ratio between the PMOS transistors M 21  and M 22 . Therefore, where the PMOS transistor M 22  has a size corresponding to, for example, M times the size of the PMOS transistor M 21 , the charge current magnitude corresponds to “M×ISRCN”. The time taken for the gate dn of the NMOS transistor M 20  to be completely charged in this case is determined in accordance with the magnitude of the current ISRCN, and the parasitic capacitance generated at the gate dn of the NMOS transistor M 20 . Also, the time taken for the NMOS transistor M 20  to turn on is determined in accordance with a variation in potential at the gate dn of the NMOS transistor M 20  depending on the charge current magnitude. For example, where the PMOS transistor M 22  has a size corresponding to N times the size of the PMOS transistor M 21 , the potential at the gate dn of the NMOS transistor M 20  can be expressed as follows: 
 
 Vdn =( I   —   M   22 / C   —   dn )× t  
 
 I   —   M   22 = M×ISRCN   [Expression 6]
 
         [0048]     (“I_M 22 ” represents the magnitude of current flowing through the PMOS transistor M 22 . Where “Vdn” approximates to “Vdd”, the operating region of the PMOS transistor M 22  is changed from a saturation region to a triode region because the source-drain voltage of the PMOS transistor M 22  approximates to zero, so that the current I_M 22  approximates to zero.)  
         [0049]     Thus, the turning-off time of the PMOS transistor M 10  and the turning-on time of the NMOS transistor M 20  can be controlled by the current ISRCN, which has a predetermined magnitude. Accordingly, it is possible to stably control a slew rate according to falling of an output voltage, that is, a falling slew rate, determined in accordance with the turning-off time of the PMOS transistor M 10  and the turning-on time of the NMOS transistor  20 , using the current ISRCN.  
         [0050]     As apparent from the above description, the output driver circuit according to the first embodiment of the present invention can effectively control the slew rate of an output voltage, using current of a predetermined magnitude supplied from the current supply unit. Accordingly, it is possible to obtain stable slew rate characteristics even under severe conditions of the semiconductor memory device such as a variation in temperature and high-speed operation.  
         [0051]     Meanwhile, the output driver circuit should exhibit the same pull-up and pull-down abilities at the output stage thereof. To this end, the PMOS transistor M 10 , which is a pull-up driver, should have a size corresponding to about 2 times that of the NMOS transistor M 20 , which is a pull-down driver. In this case, however, the parasitic capacitance generated at the gate up of the pull-up driver, also corresponds to about 2 times the parasitic capacitance generated at the gate dn of the pull-down driver. Such a parasitic capacitance difference may cause different ON/OFF timings. In order to avoid such a phenomenon, the output driver circuit according to the first embodiment of the present invention may further include a compensation capacitor C 10  connected between the gate dn of the NMOS transistor M 20  and the ground Vss, as shown in  FIG. 2 .  
         [0052]      FIG. 3  illustrates a reference current generating circuit included in the current supply unit used in the output driver circuit according to the first embodiment of the present invention.  FIG. 4  illustrates a configuration of the current supply unit used in the output driver circuit according to the first embodiment of the present invention. Now, the configuration and operation of the current supply unit, which supplies the current ISRCP and the current ISRCN in accordance with the first embodiment of the present invention, will be described with reference to  FIGS. 3 and 4 .  
         [0053]     The current supply unit includes a plurality of current-dividing PMOS transistors M 31 , M 32 , . . . , M 3 N adapted to divide, into current of a predetermined magnitude, current flowing between a node A, maintained at a level corresponding to a reference voltage Vref, and the ground Vss, and then to supply the divided current having the predetermined magnitude to the node A. The current supply unit also includes a PMOS transistor M 41  adapted to allow a current I 20  to flow therethrough in a magnitude corresponding to that of the divided current in response to a voltage VRP at respective gates of the PMOS transistors M 31 , M 32  . . . , M 3 N, a PMOS transistor M 42  adapted to share a gate thereof with the PMOS transistor M 41 , and to output, as the current ISRCP, a mirror current having a predetermined magnitude proportional to the current I 20 , an NMOS transistor M 43  adapted to supply the current I 20  from the PMOS transistor M 41  to the ground Vss, and an NMOS transistor M 44  adapted to share a gate thereof with the NMOS transistor M 43 , and to output, as the current ISRCN, a mirror current having a predetermined magnitude proportional to the current I 20 .  
         [0054]     Operation of the current supply unit having the above described configuration will now be described.  
         [0055]     First, the node A is maintained at the level of the reference voltage Vref in accordance with an operation of an operational amplifier, which is coupled to the reference voltage Vref and node A at respective inputs thereof. Accordingly, current I 10 , which flows from the node A to the ground Vss through a resistor R the node A, has a magnitude of “Vref/R” Switches SW 1  to SWN, which are connected to respective PMOS transistors M 31 , M 32  . . . , M 3 N, are controlled by respective digital signals applied thereto, in order to turn on/off respective PMOS transistors M 31 , M 32 , . . . , M 3 N. By controlling the number of the switches to be switched on, It is possible to control the magnitude of current flowing through ones of the PMOS transistors M 31 , M 32 , . . . , M 3 N turned on in accordance with the switched-on switches. For example, where the number of the switched-on ones of the switches SW 1  to SWN is n, the n turned-on ones of the PMOS transistors M 31 , M 32 , . . . , M 3 N allow current to flow therethrough. In this case, each turned-on PMOS transistor serves as a current distributor for distributing the current I 10 , which has a predetermined magnitude. Accordingly, if the PMOS transistors M 31 , M 32 , . . . , M 3 N are the same size, then the current supplied to each of the n PMOS transistors has a magnitude of “Vref/nR”. Also, the voltage level VRP at the gate of each turned-on PMOS transistor has a value capable of allowing the current having the above magnitude to flow.  
         [0056]     In the case of  FIG. 4 , accordingly, current  20  having a predetermined magnitude corresponding to the voltage VRP flows through the PMOS transistor M 41 . If the PMOS transistor M 41  has the same size as the PMOS transistors M 31 , M 32 , . . . , M 3 N, current having a magnitude of “Vref/nR” flows therethrough. Also, a mirror current corresponding to the current flowing through the PMOS transistor M 41  flows through the PMOS transistor M 42 . Accordingly, the current, which has a magnitude proportional to that of the current I 20  in the size ratio between the PMOS transistors M 41  and M 42 , is supplied, as the current ISRCP, to the output driver circuit according to the first embodiment of the present invention through the PMOS transistor M 42 . Here, if the PMOS transistors M 31 , M 32 , . . . , M 3 N, M 41 , and M 42  have the same size, the current ISRCP has a magnitude corresponding to “Vref/nR”.  
         [0057]     In the case of  FIG. 4 , the current I 20  flows through the NMOS transistor M 43  when a switch G 40  connected between the PMOS transistor M 41  and the NMOS transistor M 43  is switched on. Also, current having a predetermined magnitude proportional to the current I 20  flows through the NMOS transistor M 44  sharing the gate thereof with the NMOS transistor M 43 . Accordingly, this current is supplied to the output driver circuit according to the first embodiment of the present invention. In this case, the magnitude of the current ISRCN is determined in accordance with the size ratio between the NMOS transistors M 43  and M 44 . For example, where the NMOS transistors M 43  and M 44  have the same size, the current ISRCN has a magnitude corresponding to “Vref/nR”.  
         [0058]      FIG. 6  illustrates a characteristic waveform of the output driver circuit according to the first embodiment of the present invention. Referring to  FIG. 6 , it can be seen that the current flowing through the PMOS transistor M 10  and NMOS transistor M 20  and signals outputted from the PMOS transistor M 10  and NMOS transistor M 20  are controlled in a linear manner.  
         [0059]     Thus, the output driver circuit according to the first embodiment of the present invention can obtain stable slew rate characteristics without any considerable influence by conditions of the semiconductor memory device such as a variation in temperature and high-speed operation, by controlling the slew rate of an output signal thereof in accordance with the current ISRCP and current ISRCN supplied from the current supply unit. The current ISRCP and current ISRCN can be controlled by controlling the number of turned-on ones of the switches SW 1  to SWN included in the reference current generating circuit, and thus, controlling the voltage VRP.  
         [0060]      FIG. 5  is a circuit diagram illustrating an output driver circuit according to a second embodiment of the present invention. As shown in  FIG. 5 , the output driver circuit includes a PMOS transistor M 50  adapted to output a supply voltage Vdd to an output terminal OUT, and an NMOS transistor M 60  adapted to output a ground voltage Vss to the output terminal OUT. The output driver circuit also includes a first pre-driver  210  composed of a PMOS transistor M 51  adapted to pull up a gate up of the PMOS transistor M 50  to the level of the supply voltage Vdd in response to a predetermined voltage Vr 1 , and an NMOS transistor M 52  adapted to pull down a gate dn of the PMOS transistor M 50  to the level of the ground voltage Vss in response to a predetermined voltage Vr 2 . The output driver circuit further includes a second pre-driver  220  composed of a PMOS transistor M 61  adapted to pull up a gate up of the PMOS transistor M 60  to the level of the supply voltage Vdd in response to the voltage Vr 1 , and an NMOS transistor M 62  adapted to pull down a gate dn of the NMOS transistor M 60  to the level of the ground voltage Vss in response to the voltage Vr 2 .  
         [0061]     Operation of the output driver circuit according to the second embodiment of the present invention will now be described in detail.  
         [0062]     In accordance with a transition of an input data signal IN from a low level to a high level, gates G 51  and G 61  are turned off, whereas gates G 52  and G 62  are turned on. Accordingly, charges stored in the gate up of the PMOS transistor M 50  are discharged through the NMOS transistor M 52 . As a result, the voltage level at the gate up of the PMOS transistor M 50  is transited to the level of the ground voltage Vss, thereby causing the PMOS transistor M 50  to turn on. Accordingly, the PMOS transistor M 50  outputs, to the output terminal OUT, a signal having a level corresponding to the supply voltage Vdd. Also, charges stored in the gate dn of the NMOS transistor M 60  are discharged through the NMOS transistor M 62 . As a result, the voltage level at the gate dn of the NMOS transistor M 60  is transited to the level of the ground voltage Vss, thereby causing the NMOS transistor M 60  to turn off. In this case, the magnitude of discharge current flowing through each of the NMOS transistors M 52  and M 62  is controlled by the voltage Vr 2 . Also, the discharge time is determined in accordance with the magnitude of the discharge current, the parasitic capacitance generated at the associated gate up or dn, and the charged voltage at the associated gate up or dn. Also, the time taken for the PMOS transistor M 50  to turn on or the time taken for the NMOS transistor M 60  to turn off is determined in accordance with a variation in potential at the associated gate up or dn depending on the discharge current magnitude.  
         [0063]     Thus, the turning-on time of the PMOS transistor M 50  and the tuning-off time of the NMOS transistor M 60  can be controlled by the voltage Vr 2  having a predetermined level. Accordingly, it is possible to stably control a slew rate according to rising of an output voltage, that is, a rising slew rate, determined in accordance with the turning-on time of the PMOS transistor M 50  and the turning-off time of the NMOS transistor  60 , using the voltage Vr 2 .  
         [0064]     On the other hand, when the input data signal IN is transited from a high level to a low level, the gates G 51  and G 61  are turned on, whereas the gates G 52  and G 62  are turned off. Accordingly, the gate up of the PMOS transistor M 50  is charged with current supplied from the supply voltage source Vdd through the PMOS transistor M 51 . As a result, the voltage level at the gate up of the PMOS transistor M 50  is transited to the level of the supply voltage Vdd, thereby causing the PMOS transistor M 50  to turn off. Also, the gate dn of the NMOS transistor M 60  is charged with current supplied from the supply voltage source Vdd through the PMOS transistor M 61 . As a result, the voltage level at the gate dn of the NMOS transistor M 60  is transited to the level of the supply voltage Vdd, thereby causing the NMOS transistor M 60  to turn on. Accordingly, the NMOS transistor M 60  outputs, to the output terminal OUT, a signal having a level corresponding to the ground voltage Vss. In this case, the magnitude of charge current flowing through each of the PMOS transistors M 51  and M 61  is controlled by the voltage Vr 1 . Also, the charge time is determined in accordance with the magnitude of the charge current, and the parasitic capacitance generated at the associated gate up or dn. Also, the time taken for the PMOS transistor M 50  to turn off or the time taken for the NMOS transistor M 60  to turn on is determined in accordance with a variation in potential at the associated gate up or dn depending on the charge current magnitude.  
         [0065]     Thus, the turning-off time of the PMOS transistor M 50  and the turning-on time of the NMOS transistor M 60  can be controlled by the voltage Vr 1  having a predetermined level. Accordingly, it is possible to stably control a slew rate according to falling of an output voltage, that is, a falling slew rate, determined in accordance with the turning-off time of the PMOS transistor M 50  and the turning-on time of the NMOS transistor  60 , using the voltage Vr 1 .  
         [0066]     Each of the voltages Vr 1  and Vr 2  may be supplied from a separate voltage source. In particular, for the voltage Vr 1  adapted to control the magnitude of charge current, the voltage VRP used in the current supply unit according to the first embodiment of the present invention may be used. For the voltage Vr 2  adapted to control the magnitude of discharge current, the voltage VRN generated in the current supply unit may be used.  
         [0067]     Meanwhile, the output driver circuit should exhibit the same pull-up and pull-down abilities at the output stage thereof. To this end, the PMOS transistor M 50 , which is a pull-up driver, should have a size corresponding to about 2 times that of the NMOS transistor M 60 , which is a pull-down driver. In this case, however, the parasitic capacitance generated at the gate up of the pull-up driver also corresponds to about 2 times the parasitic capacitance generated at the gate dn of the pull-down driver. Such a parasitic capacitance difference may cause different ON/OFF timings. In order to avoid such a phenomenon, the output driver circuit according to the second embodiment of the present invention may further include a compensation capacitor C 20  connected between the gate dn of the NMOS transistor M 60  and the ground Vss, as shown in  FIG. 5 .  
         [0068]     Thus, the output driver circuit according to the second embodiment of the present invention can effectively control the slew rate of an output signal, using a relatively simple circuit configuration, because the turning-on and turning-off times of the PMOS transistor M 50  and NMOS transistor M 60  adapted to determine the slew rate of the output signal are controlled, based on the voltages Vr 1  and Vr 2 , each of which has a predetermined level. It is also possible to prevent an increase in the current of the output driver circuit.  
         [0069]     As apparent from the above description, the present invention provides an output driver circuit which uses a current or voltage supplied from a current supply unit exhibiting a small output variation, to control a slew rate at an output stage thereof, thereby being capable of preventing an abrupt variation in the slew rate even when a variation in resistance and supply voltage occurs due to a variation in process and temperature conditions. Accordingly, the output driver circuit can be widely used on output stages of high-speed interface circuits.  
         [0070]     Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.