Abstract:
A variable impedance control circuit for a semiconductor device reduces susceptibility to power supply variations and improves impedance matching by utilizing the same power supply for portions of the array driver and for the transistor arrays used for impedance matching.

Description:
This application claims priority from Korean patent application 99-32546 filed Aug. 9, 1999 in the name of Samsung Electronics Co., Ltd., which is herein incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     Semiconductor devices utilize output circuits called off-chip driver circuits for transmitting signals from the semiconductor device to other devices though transmission lines such as those formed on printed circuit boards. An example of an off-chip driver circuit is disclosed in U.S. Pat. No. 5,440,258. For optimum performance, the output impedance of the off-chip driver should be matched to the impedance of the transmission line. Therefore, semiconductor devices often utilize variable impedance control circuits to control the impedance of off-chip driver circuits. A variable impedance control circuit typically utilizes an extra pin to adjust or “trim” the impedance of the off-chip driver circuit. 
     For example, in a semiconductor device having a high speed transceiver logic (HSTL) interface, a variable impedance control circuit senses the value of a resistor connected to the extra pin and trims the impedance of an off-chip driver circuit responsive to the value of the resistor. In such a semiconductor device, the main power supply voltage VDD is typically applied to the main circuitry of the device (e.g., a memory cell array) while a reduced power supply voltage VDDQ is applied to the output circuitry. 
     However, because the supply voltage VDD is used as the operational voltage for driving the variable impedance control circuit and the off-chip driver circuit, the impedance control circuit and the off-chip driver circuit are susceptible to changes in the level of the supply voltage VDD, thereby causing impedance mismatches. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention is a variable impedance control circuit comprising: an impedance matching transistor array operated by a first supply voltage; and an array driver coupled to the impedance matching transistor array, wherein the array driver includes one or more first internal elements operated by the first supply voltage and one or more second internal elements operated by a second supply voltage. 
     Another aspect of the present invention is a semiconductor device comprising: an off-chip driver circuit having a transistor array operated by a first supply voltage; and an output buffer having an array driver coupled to the transistor array, wherein the array driver includes one or more first internal elements operated by the first supply voltage and one or more second internal elements operated by a second supply voltage. 
     An additional aspect of the present invention is a variable impedance control circuit comprising: an off-chip driver having first and second transistor arrays; an array driver coupled to the off-chip driver and adapted to drive the first and second transistor arrays responsive to control code data; and a control code data generator coupled to the array driver and adapted to generate the control code data, wherein the control code data generator is operated from a first supply voltage; wherein the array driver includes drive elements coupled to the first and second transistor arrays, and the drive elements and the first and second transistor arrays are operated from a second supply voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a prior art variable impedance control circuit and off-chip driver circuit. 
     FIG. 2 is a schematic diagram showing details of some of the components of FIG.  1 . 
     FIG. 3 is a schematic diagram of an embodiment of an array driving unit and transistor arrays in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a block diagram of a prior art variable impedance control circuit and off-chip driver circuit. The variable impedance control circuit includes a ZQ detecting unit  100 , a comparator  110 , an up/down counter  120 , an upper code selector  130 , and a ZQ driver  140 . The off-chip driver circuit includes a DOUT buffer  150  having an array driver (not shown) and an off-chip driver  160 . 
     A general procedure for trimming the output impedance of the off-chip driver circuit of FIG. 1 will now be described. For an HSTL interface, an extra pin is utilized for trimming the desired value of the output impedance within the range of about 35 to 70 ohms (Ω). An external resistor  50  having resistance value RZQ, which is 5 times greater than the desired output impedance value, is connected to the extra pad ZQPAD  40 . P channel and N channel MOS transistor arrays  10 - 1  and  20 - 1  preset the reference voltage REF 10  of node NO 1  to half of the output supply voltage (VDDQ/ 2 ). The comparator  110  compares the reference voltage REF 10  of the node NO 1  to the pad voltage VZQ of the node NO 2  and outputs an up/down signal U/D in response to the comparison. The up/down counter  120  increases or decreases the control code data CTQx in response to up/down signal U/D. The array driver  30  generates drive signals L 2  and L 3  which selectively drive the NMOS and PMOS transistors in arrays  10 - 1 ,  20 - 1 , and  10 - 2  responsive to the control code data CTQx so as to equalize the reference voltage REF 10  with the pad voltage VZQ at pad  40 . 
     When the reference voltage REF  10  and the PAD voltage VZQ are equalized at VDDQ/ 2 , the turn-on resistance of the PMOS array  10 - 2  is equalized with the resistance RZQ of the external resistor  50 . As the PMOS transistors of the PMOS array  10 - 1  and the NMOS transistors of the NMOS array  20 - 1  are turned on, corresponding transistors in the arrays  10 - 3  and  20 - 2  in the off-chip driver  160  are turned on through the upper code selector  130 , the ZQ driver  140  and the DOUT buffer  150  so as to match the output impedance. As many transistors of the PMOS array  10 - 3  and NMOS array  20 - 2  in the off-chip driver  160  are turned on as those of the PMOS array  10 - 1  and NMOS array  20 - 1  in the ZQ detecting unit are turned. This is because the PMOS arrays  10 - 1  and  10 - 3  are constructed with the same internal structure, and the NMOS arrays  20 - 1  and  20 - 2  are constructed with the same internal structure. 
     When impedance matching is achieved, the upper code selector  130  selects an upper portion of the control code data CTQx. When the off-chip driver  160  is in its high impedance state, the ZQ driver  140  generates impedance control code data CZQX in response to the selected control code data CTQx and outputs it to the DOUT buffer  150 . Thus, the previous impedance control code data CZQx is refreshed. The refreshed impedance control code data CZQx enables the DOUT buffer  150  responsive to the control code data. The comparator  110 , up/down counter  120 , upper code selector  130 , and ZQ driver  140  are collectively referred to as a control code data generator. 
     The DOUT buffer  150  receives memory cell data DLAT,DLATB and generates the pull-up output data DOUx and pull-down output data DODx, which are output to the off-chip driver  160  to turn on the relevant transistors of the PMOS and NMOS arrays  10 - 3 ,  20 - 2 . Therefore, final output data is supplied through the output pad DQ from the transistors selected in the off-chip driver  160 . Thus, the output data signal DQ is driven by the off-chip driver  160  which has a variable output impedance that is matched to the transmission line responsive to the value of the external resistor  50 . 
     Ideally, this causes the output data to be transmitted without distortion. However, because the operational voltage of the array driver  30  of the ZQ detecting unit and the array driver (not shown) of the DOUT buffer  150  is different from that for the PMOS arrays  10 - 1 ,  10 - 2 ,  10 - 3  and NMOS arrays  20 - 1 ,  20 - 2 , it is difficult to accurately control the output impedance. This will be explained in more detail with reference to FIG.  2 . 
     FIG. 2 is a schematic diagram showing details of an array driving unit  30 - 1 , which is part of the array driver  30  of FIG. 1, and also showing more details of the transistor arrays  10 - 1  and  20 - 1  of FIG.  1 . Referring to FIG. 2, one array driving unit  30 - 1  of the array driver  30  is constructed with three CMOS inverters IN 1 , IN 2 , IN 3  where the supply voltage VDD is used as the operational voltage. The three CMOS inverters IN 1 , IN 2 , IN 3  are each constructed from a P channel MOS field effect transistor (PMOSFET) and an N channel MOS field effect transistor (NMOSFET). The input signal to the inverters IN 1 , IN 3  is a control code data signal CTQ 1 , which is one of control code data signals CTQx. The inverters IN 2 , IN 3  output the PMOS driving control signal CTQP 1  and the NMOS driving control signal CTQN 1 , respectively, which are further transmitted to the gate terminal of PMOS transistor P 1  of the PMOS array  10 - 1  and the gate terminal of NMOS transistor N 1  of the NMOS array  20 - 1 , respectively. The inverters IN 2  and IN 3 , and any other logic elements that might be used in their place, are referred to as drive elements because they drive the transistors in the transistor arrays. 
     However, in FIG. 2, the sources of the plurality of PMOS transistors P 1 , P 2 , . . . , P 6  in the PMOS array  10 - 1  are commonly connected to the second supply voltage, that is, the output supply voltage terminal VDDQ (about 1.5 volt), and the drains of the transistors are commonly connected to the reference voltage terminal REF 10 . The gates of P 1 , P 2 , . . . , P 6  have different lengths. PMOS driving control signals CTQP 1 , CTQP 2 , . . . , CTQP 6  are applied to the gates of the corresponding PMOS transistors P 1 , P 2 , . . . , P 6 . Only a single one of the array driving units  30 - 1  is shown in FIG. 2, but the PMOS driving control signals CTQP 2 , CTQP 3 , . . . , CTQP 6  are transmitted from other identically constructed array drivers  30 - 1 . 
     Similarly, the NMOS array  20 - 1  is constructed with a plurality of NMOS transistors N 1 , N 2 , . . . , N 6  having their sources commonly connected to the second grounding voltage, that is, the output supply voltage terminal VSSQ, and their drains commonly connected to the reference voltage terminal REF 10 . Again, the gates of N 1 , N 2 , . . . , N 6  have different lengths. 
     NMOS driving control signals CTQP 1 , CTQP 2 , . . . , CTQP 6  are applied to the gates of the corresponding NMOS transistors N 1 , N 2 , . . . , N 6 . The NMOS driving control signals CTQN 2 , CTQN 2 , . . . , CTQN 6  are relevantly transmitted from other array drivers which are identical to the array driver  30 - 1 . In FIG. 1, the PMOS arrays  10 - 2 ,  10 - 3  and the NMOS array  20 - 2  have the same structure as the PMOS array  10 - 1  and the NMOS array  20 - 1  illustrated in FIG.  2 . 
     As shown in FIG. 2, the first supply voltage VDD (about 3.3 volts or 2.5 volts) is used as the operational voltage for the three CMOS inverters IN 1 , IN 2 , IN 3  of the array driver  30 - 1  of the ZQ detecting unit  30 , while the second supply voltage VDDQ (about 1.5 volts) is used as the operational voltage for the PMOS transistors P 1 , P 2 , . . . , P 6  of the PMOS array  10 - 1 . In addition, the first supply voltage VDD (about 3.3 volts or 2.5 volts) is used as the operational voltage for the array driver (not shown) of the DOUT buffer  150 , while the second supply voltage VDDQ (about 1.5 volt) is used as the operational voltage for the PMOS arrays  10 - 2 ,  10 - 3 . Therefore, the absolute value of the voltages Vgsp  1 ,  2 , . . . ,  6  between the gates and sources of PMOS transistors P 1 , P 2 , . . . , P 6  equals the second supply voltage VDDQ at the maximum, while the absolute value of voltages Vgsn 1 ,  2 , . . . ,  6  between the gates and sources of the NMOS transistors N 1 , N 2 , . . . , N 6  equals the first supply voltage VDD at the maximum. Therefore, if there is any change in the first supply voltage VDD, there may also be a change in the voltage between the gates and sources of the NMOS transistors N 1 , N 2 , . . . , N 6  and a further change in the level of the reference voltage terminal. In other words, changes can occur in the level of the pad voltage VZQ in FIG. 1, so that the turn-on resistance of the PMOS array  10 - 2  differs from the resistance of the external resistor  50 . Thus, changes in the supply voltage VDD result in impedance mismatches, thereby causing distortion and transmission errors in the output signals. 
     As described above, impedance mismatching has frequently occurred in the prior art because the impedance control circuit and off-chip driver are directly influenced by changes in the supply voltage VDD. Therefore, in order to solve the major problem in the prior art, an embodiment of the present invention is presented in FIG.  3 . 
     FIG. 3 is a schematic diagram of an embodiment of an array driving unit  30 - 2  and transistor arrays  10 - 1  and  20 - 1  of a ZQ detector constructed in accordance with the present invention. As shown in FIG. 3, the internal terminals of the circuit are constructed in the same manners as those shown in FIG.  2 . However, the second supply voltage VDDQ is used as the operational voltage for inverters IN 2 , IN 3  of array driving unit  30 - 2 . Similarly, array driving units in an array driver (not shown) in a DOUT buffer of a semiconductor device in accordance with the present invention also includes internal elements which are separately operated from the first and second supply voltages. The transistor arrays  10 - 1 ,  10 - 2 ,  10 - 3  are constructed with internal elements operated by the second supply voltage. 
     An advantage of the present invention is that it can be implemented without requiring any additional circuit elements. 
     In FIG. 3, the second supply voltage VDDQ becomes the absolute value of the voltage Vgsp 1 ,  2 , . . . ,  6  between the gates and sources of the PMOS transistors P 1 , P 2 , . . . , P 6 , respectively at the maximum. VDDQ also becomes the absolute value of the voltage Vgsn 1 ,  2 , . . . ,  6  between the gates and sources of the NMOS transistors N 1 , N 2 , . . . , N 6  at the maximum. Even if there is a change in the first supply voltage VDD, it does not influence the voltage between gates and sources of the NMOS transistors N 1 , N 2 , . . . , N 6  of the array  20 - 1  because they are part of an independent structure which is not influenced by the first supply voltage VDD. Therefore, even if there is a change in the first supply voltage VDD, the level of the reference voltage REF  10  does not change, so there is no change in the level of the pad voltage. Thus, the turn-on resistance and the external resistance  50  remain at the same value in spite of changes in the supply voltage VDD, and impedance matching can be achieved and maintained so as to prevent or minimize any error in the transmission of output signals. 
     Therefore, there is an advantage in the present invention in that an impedance control circuit and an off-chip driver circuit of a semiconductor device such as a static RAM are not directly influenced by changes in supply voltage VDD, thereby minimizing or preventing impedance mismatches and stabilizing data output operations of a semiconductor memory device constructed in accordance with the present invention. Furthermore, there is another advantage in the present invention in that no additional elements are included in the circuit for impedance matching of the semiconductor device, thereby improving performance and reducing power consumption. 
     While the invention has been described in terms of a preferred embodiment, those skilled in the art will recognize that the invention can be practiced with modifications within the spirit and scope of the appended claims. For instance, other logic elements like AND or NAND gates can be replaced for the inverters of the array driver. The transistor arrays can be constructed with other circuit elements for the same or similar functions. We claim all modifications and variations coming within the spirit and scope of the following claims.