Abstract:
A semiconductor memory device prevents deterioration of refresh operation caused by sensing noise and a driving method thereof. First pull-down and second pull-down voltages which are different from each other are as a pull-down voltage of a bit line sense amplifier. The first and the second pull-down voltages are used in different driving periods to protect data from noises caused by another memory bank. A driving period can be separated into an initial sensing period, wherein large currents are consumed and significant noise is generated, and a subsequent stable period. The driving period can be separated into a pre-precharge period and a post-precharge period.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a semiconductor memory device; and, more particularly, to a scheme controlling a bit line sense amplifier of the semiconductor memory device.  
       DESCRIPTION OF RELATED ARTS  
       [0002]     Scaling down of line width and cell size of a semiconductor memory chip proceeds continuously. Accordingly, as an external supply voltage VDD becomes low, modification of the architecture of the semiconductor memory device operating under low-power system is required.  
         [0003]     Most semiconductor memory chips are provided with an internal voltage generating circuit generating internal voltage from the external supply voltage VDD and supplied with a predetermined voltage required by internal circuits therein. Particularly, in a semiconductor memory device including a bit line sense amplifier such as a DRAM, a core voltage VCORE is used for sensing cell data. The core voltage VCORE has a voltage level corresponding to a digital data ‘1’.  
         [0004]     When a word line selected by a low address is activated, data stored in a plurality of memory cells connected to the word line are respectively transmitted to plural bit line pairs. Bit line sense amplifiers, each coupled to a respective bit line pair, sense and amplify a voltage difference between a bit line and a bit line bar of each bit line pair. If thousands of bit line sense amplifiers operate, a large amount of current is consumed when the core voltage VCORE is supplied to a pull-up supply line, so-called RTO line, coupled to plural bit line sense amplifiers. As the level of the core voltage VCORE is lower with the trend of lower external supply voltage VDD, amplifying a large amount of cell data rapidly using the core voltage VCORE can cause overload.  
         [0005]     A method of over-driving a bit line sense amplifier is used to solve the problem. The over-driving method drives the RTO line of the bit line sense amplifier with a higher voltage than the core voltage VCORE, usually the VDD at an initial operation of the bit line sense amplifier, after data stored in the memory cell is transmitted to the bit line pair.  
         [0006]      FIG. 1  is a block diagram of a conventional DRAM core performing an over-driving operation.  
         [0007]     Referring to  FIG. 1 , the DRAM core includes a memory cell  400 , a bit line sense amplifier  300 , a supply line driver  200  and a sense amplifier controller  100 , whether performing over-driving operation or not.  
         [0008]     Though not shown in  FIG. 1 , a bit line divider, a bit line equalizer/precharger and a column selector are coupled to the bit line sense amplifier  300 .  
         [0009]     The memory cell  400  includes a first NMOS transistor N 1  and a capacitor C 1 . The first NMOS transistor N 1 , connected between a bit line BL or a bit line bar BLB and a storage node SN, has a gate connected in a word line WL. The capacitor C 1  is connected between the storage node SN and a cell plate voltage VCP.  
         [0010]     The bit line sense amplifier  300  has a latch structure wherein two pull-up PMOS transistors P 3  and P 4  are cross connected with two pull-down NMOS transistors N 2  and N 3 . On the other side, the two pull-up PMOS transistors P 3  and P 4  are connected with the RTO line. And the two pull-down NMOS transistors N 2  and N 3  are connected with a pull-down supply line, so-called a SB line. When a predetermined voltage is supplied to the SB and RTO lines in response to an enable signal, the bit line sense amplifier senses a fine voltage difference between both the bit line BL and the bit line bar BLB having charges in common. The bit line sense amplifier amplifies one of the bit line pair to a ground voltage VSS level and the other of the bit line pair to the core voltage VCORE level.  
         [0011]     The supply line driver  200  includes a first PMOS transistor P 1  for an over-driving operation and PMOS and NMOS transistors P 2  and N 4  for a normal-driving operation. The first PMOS transistor P 1  supplies the RTO line with the external supply voltage VDD in the response to a first RTO line driving signal SAP 1 B. The second PMOS transistor P 2  supplies the RTO line with the core voltage VCORE in the response to a second RTO line driving signal SAP 2 B. The fourth NMOS transistor N 4  supplies the SB line with the ground voltage VSS in the response to a SB line driving signal SAN.  
         [0012]      FIG. 2  is a schematic circuit diagram of the sense amplifier controller  100  shown in  FIG. 1 .  
         [0013]     As shown, the conventional controller  100  includes an first inverter IV 0  receiving an active command signal ACT, a pull-up PMOS transistor P 11  controlled by an output of the first inverter IV 0 , a pull-down NMOS transistor N 11  controlled by a precharge command signal PCG, an inverter latch and a second delay  30 . The inverter latch, containing sixth and seventh inverters IV 5  and IV 6 , is connected between the pull-up transistor P 11  and the pull-down transistor N 11 . The second delay  30  delays a falling edge of an output of the inverter latch by a second delay time tDelay 2 .  
         [0014]     The conventional controller  100  further includes a falling pulse generator  10 , a cross-coupled NAND latch consisting of first and second NAND gates ND 1  and ND 2 , a first delay  20  and inverters IV 1  to IV 4 . The falling pulse generator  10  generates a pulse in response to a falling edge of a signal A_sig, an output of the second delay  30 . The cross-coupled NAND latch receives an output of the falling pulse generator  10  as a set signal and an output of the fifth inverter IV 4  as a reset signal. The first delay  20  delays an output of the cross-coupled NAND latch by a first delay time tDelay 1 . The fifth inverter IV 4  inverts an output of the first delay  20 . The second inverter IV 1  receives an output of the cross-coupled NAND latch and the third inverter IV 2  receives a signal C_sig, an output of the second inverter IV 1 . The fourth inverter IV 3  receives an output of the third inverter IV 2  and outputs the first RTO line driving signal SAP 1 B.  
         [0015]     In addition, the conventional controller  100  includes third and fourth delays  40  and  50 , three inverters IV 7  to IV 9 , a third NAND gate ND 3  and an inverter chine consisting of inverters IV 10  to IV 12 . The third delay  40  delays a rising edge of the signal A_sig by a third delay time tDelay 3 . The fourth delay  50  delays a falling edge of a signal B_sig, an output of the third delay  40  by a fourth delay time tDelay 4 . The eighth inverter IV 7  receives an output of the fourth delay  50 . The third NAND gate ND 3  receives a signal D_sig, an output of the eighth inverter IV 7  and the signal C_sig. The ninth inverter IV 8  receives an output of the third NAND gate ND 3 . The tenth inverter IV 9  receives an output of the ninth inverter IV 8  and outputs the second RTO line driving signal SAP 2 B. The delay chain receives the signal B sig and outputs the SB line driving signal SAN.  
         [0016]      FIG. 3  is a timing diagram for illustrating an operation of the sense amplifier controller  100  shown in  FIG. 2 . As shown, an output of the inverter latch falls in response to activation of the active command signal ACT and rises in response to activation of the precharge command signal PCG.  
         [0017]     The second delay  30  delays a falling edge of the output of the inverter latch and outputs the signal A_sig delayed from the activation of the active command signal ACT by the second delay time tDelay 2 . The third delay  40  delays a rising edge of the signal A_sig and outputs the signal B_sig delayed from the activation of the precharge command signal PCG by the third delay time tDelay 3 .  
         [0018]     The falling pulse generator  10 , the first delay  20 , the first and the fourth inverters IV 1  and IV 4  and the cross-coupled NAND latch output the signal C_sig having tDelay 1  time window from a falling edge of the signal A_sig.  
         [0019]     The fourth delay  50  and the seventh inverter IV 7  output the signal D_sig by inverting after delaying a falling edge of the signal B_sig by the fourth delay time tDelay 4 . The signal D is transited within the tDelay 1  time window of the output C (tDelay 1 &gt;tDelay 4 ).  
         [0020]     The first RTO line driving signal SAP 1 B represents an over-driving period which continues for the first delay time tDelay 1  after delayed from an active point as the second delay time tDelay 2 . The second RTO line driving signal SAP 2 B represents a normal-driving period which continues from non-activation of the first RTO line driving signal SAP 1 B to the point delayed from a precharge point by the third delay time tDelay 3 . The SB line driving signal SAN is activated to high logic level during the over-driving operation and the normal-driving operation responsive to the first and the second RTO line driving signals SAP 1 B and SAP 2 B. Delay times by logic gates are not considered to simplify understanding.  
         [0021]      FIG. 4  is a timing diagram for illustrating an operation of the conventional DRAM core shown in  FIG. 1 .  
         [0022]     When the active command is input and the word line WL is enabled, the storage node SN and the bit line are coupled. There is a fine voltage difference between the bit line BL and the bit line bar BLB.  
         [0023]     Continuously, in the event that the bit line sense amplifier  300  is enabled, the over-driving PMOS transistor P 1  and the normal-driving PMOS/NMOS transistors P 2 /N 4  drive the RTO line and the SB line with the first and the second RTO line driving signals SAP 1 B/SAP 2 B and the SB line driving signal SAN.  
         [0024]     After data amplified through the over-driving and the normal-driving operations are rewritten, the word line WL and the bit line sense amplifier  300  are disabled when the precharge command is input. The bit line pair BL and BLB is equalized/precharged to half of the core voltage VCORE.  
         [0025]      FIG. 5  is a waveform for illustrating sensing noise occurring in the conventional DDR core.  
         [0026]     In the conventional DDR core, all banks are supplied with the pull-up/pull-down voltages of the bit line sense amplifier  300  in common. That is, all banks use the core voltage VCORE and the ground voltage VSS in common. Accordingly, as illustrated in  FIG. 5 , sensing noise is caused in a bank by an active operation of another bank when a word line WL of the bank is disabled by the precharge command and the cell NMOS transistor N 1  turns off. The data damaged by the sensing noise are stored on the storage node SN. Accordingly, data retention time decrease and each cell cannot perform a refresh operation appropriately.  
       SUMMARY OF THE INVENTION  
       [0027]     It is, therefore, an object of the present invention to provide a semiconductor memory device prohibiting malfunction such as data loss due to a sensing noise and a driving method thereof.  
         [0028]     In accordance with an aspect of the present invention, there is provided a semiconductor memory device with a plurality of banks, including a bit line sense amplifier for sensing and amplifying data applied on a bit line pair, and a supply line driver for supplying pull-up and pull-down supply lines of the bit line sense amplifier with a pull-up voltage and a pull-down voltages, wherein the pull-down voltage is adjusted according to an operation period.  
         [0029]     In accordance with another aspect of the present invention, there is provided a semiconductor memory device with a plurality of banks, including, a bit line sense amplifier for sensing and amplifying the data applied on the bit line pair, a pull-up driver for driving the pull-up supply line of the bit line sense amplifier with the pull-up voltage in response to a pull-up driving signal, a first pull-down driver for driving the pull-down supply line of the bit line sense amplifier with the first pull-down voltage in response to a first pull-down driving signal activated in the first driving period, a second pull-down driver for driving the pull-down supply line with the second pull-down voltage different from the first pull-down voltage in response to a second pull-down driving signal activated in the second driving period, and a driving controller for generating the pull-up driving signal and the first and the second pull-down driving signals in response to active and precharge command signals.  
         [0030]     In accordance with further aspect of the present invention, there is provided a driving method of semiconductor memory device with a plurality of banks, including driving the pull-up supply line of the bit line sense amplifier with the pull-up voltage and the pull-down supply line of the bit line sense amplifier with the first pull-down voltage, and driving the pull-up supply line of the bit line sense amplifier with the pull-up voltage and pull-down supply line of the bit line sense amplifier with the second pull-down voltage different from the first pull-down voltage.  
         [0031]     In the present invention, first and second separate pull-down supply voltages are used as a pull-down supply voltage of a bit line sense amplifier. By using the first and the second pull-down supply voltages in different operation periods, data are secured from noises generated by another bank operation. The operation periods are separated into an initial sensing period and subsequent stable period. Large currents are consumed and larger noises are generated in initial sensing period. Also the operation periods are separated into pre and post precharge periods.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]     The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings, in which:  
         [0033]      FIG. 1  is a block diagram of a conventional DRAM core performing an over-driving operation;  
         [0034]      FIG. 2  is a schematic circuit diagram of a sense amplifier controller shown in  FIG. 1 ;  
         [0035]      FIG. 3  is a timing diagram for illustrating an operation of the sense amplifier controller shown in  FIG. 2 ;  
         [0036]      FIG. 4  is a timing diagram for illustrating an operation of the conventional DRAM core shown in  FIG. 1 ;  
         [0037]      FIG. 5  is a waveform diagram for illustrating sensing noise occurring in the conventional DDR core;  
         [0038]      FIG. 6  is a block diagram of a DRAM core in accordance with the present invention;  
         [0039]      FIG. 7  is a schematic circuit diagram of a sense amplifier controller shown in  FIG. 6  in accordance with a first embodiment;  
         [0040]      FIG. 8  is a timing diagram for illustrating an operation of the sense amplifier controller shown in  FIG. 7 ;  
         [0041]      FIG. 9  is a timing diagram for illustrating an operation of the DRAM core shown in  FIG. 6  in accordance with the first embodiment;  
         [0042]      FIG. 10  is a waveform diagram for illustrating prevention of sensing noise in accordance with the first embodiment.  
         [0043]      FIG. 11  is a schematic circuit diagram of the sense amplifier controller shown in  FIG. 6  in accordance with a second embodiment;  
         [0044]      FIG. 12  is a timing diagram for illustrating an operation of the sense amplifier controller shown in  FIG. 11 ;  
         [0045]      FIG. 13  is a timing diagram for illustrating an operation of the DRAM core shown in  FIG. 6  in accordance with the second embodiment; and  
         [0046]      FIG. 14  is a waveform diagram for illustrating prevention of sensing noise in accordance with the second embodiment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0047]     Hereinafter, a semiconductor memory device having a shared bit line sense amplifier scheme and a driving method thereof in accordance with exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.  
         [0048]      FIG. 6  is a block diagram of a DRAM core in accordance with the present invention;  
         [0049]     Referring to  FIG. 6 , the DRAM core comprises a memory cell  400 , a bit line sense amplifier  300 , a supply line driver  700  and a sense amplifier controller  600 .  
         [0050]     The memory cell  400  and the bit line sense amplifier  300  are similar to corresponding elements of the conventional scheme described above. The supply line driver  700  and the sense amplifier controller  600  are designed differently.  
         [0051]     The supply line driver  700  includes an over-driving PMOS transistor P 21 , a normal-driving PMOS transistor P 22 , a first driving NMOS transistor N 24  and a second driving NMOS transistor N 25 . The over-driving PMOS transistor P 21  supplies a RTO line with an external supply voltage VDD in response to a first RTO line driving signal SAP 1 B. The normal-driving PMOS transistor P 22  supplies the RTO line with a core voltage VCORE in response to a second RTO line driving signal SAP 2 B. The first driving NMOS transistor N 24  drives a SB line with a first ground voltage VSS 1  in response to a first SB line driving signal SAN 1 . The second driving NMOS transistor N 25  drives the SB line with a second ground voltage VSS 2  in response to a second SB line driving signal SAN 2 .  
         [0052]     The first ground voltage VSS 1  and the second ground voltage VSS 2  are mutually distinguished and independent voltages, and the first SB line driving signal SAN 1  and the second SB line driving signal SAN 2  are activated in respectively different periods.  
         [0053]     When an active command is input in precharge condition, an active command signal ACT is activated as a logic high level. Accordingly, a word line WL is enabled and a cell NMOS transistor N 1  turns on. Data charge stored in a cell capacitor C 1  is transferred to a bit line. When the word line is enabled, one of the bit line pair BL and BLB and a storage node SN is coupled to transfer datacharge and, thereby, develop a fine voltage difference between the bit line BL and bit line bar BLB.  
         [0054]     When the bit line sense amplifier  300  is enabled thereafter, the RTO line driving signal SAP 1 B is activated as a logic low level. And the first SB line driving signal SAN 1  is activated as a logic high level. Accordingly, an over-driving PMOS transistor P 21  supplies the RTO line with the eternal supply voltage VDD. The first driving NMOS transistor N 24  drives the SB line with the first ground voltage VSS 1 .  
         [0055]     The over-driving operation is completed, and then the over-driving PMOS transistor P 21  turns off. As the second RTO line driving signal SAP 2 B is activated as a logic low level, the normal-driving PMOS transistor P 22  supplies the RTO line with the core voltage VCORE.  
         [0056]     The SB line is driven with the first ground voltage VSS 1  by the first SB line driving signal SAN 1  activated as a logic high level in an initial operation for sensing and amplifying data. After a certain time when amplified data is stable, the first SB line driving signal SAN  1  is non-activated as a logic low level. The second SB line driving signal SAN 2  is activated as a logic high level and the SB line is driven with the second ground voltage VSS 2   
         [0057]     Because the second RTO line driving signal SAP 2 B and the second SB line driving signal SAN 2  are activated when amplified data are stable, the times for activation may be similar, although the times for activation of two signals have no relation.  
         [0058]      FIG. 7  is a schematic circuit diagram of the sense amplifier controller  600  shown in  FIG. 6  in accordance with a first embodiment.  
         [0059]     Referring to  FIG. 7 , the sense amplifier controller  600  is provided with a RTO line driving signal generator  630  and a SB line driving signal generator  620 . The RTO line driving signal generator  630  generates the first and the second RTO line driving signals SAP 1 B and SAP 2 B in response to the active command signal ACT and the precharge command signal PCG. The SB line driving signal generator  620  generates the first and the second SB line driving signals SAN 1  and SAN 2  in response to an output of The RTO line driving signal generator  630 .  
         [0060]     The RTO line driving signal generator  630  has the same architecture generating the first and the second RTO line driving signals SAP 1 B and SAP 2 B as the conventional generator (Referring to  FIG. 2 ).  
         [0061]     The SB line driving signal generator  620  is provided with a cross-coupled NAND latch consisting of NAND gates ND 5  and ND 6 , three inverters IV 21  to IV 23  and a NAND gate ND 4 . The cross-coupled NAND latch receives an output of a falling pulse generator  10  in the RTO line driving signal generator  630  as a set signal and its own output inverted/delayed by a fifth delay  60  having a fifth delay time tDelay 5  and a fourteenth inverter IV 24  as a reset signal. The twelfth inverter IV 22  receives an output of the cross-coupled NAND latch and the thirteenth inverter IV 23 , receiving a signal F_sig, an output of the twelfth inverter IV 22 , outputs the first SB line driving signal SAN 1 . The fourth NAND gate ND 4  receives the signal F_sig and the signal D_sig, the output of the eighth inverter IV 7  in the RTO line driving signal generator  630 . The eleventh inverter IV 21  inverts an output of the fourth NAND gate ND 4  and outputs the second SB line driving signal SAN 2 .  
         [0062]      FIG. 8  is a timing diagram for illustrating an operation of the sense amplifier controller  600  shown in  FIG. 7 ;  
         [0063]     The wave pattern of signals A_sig to D_sig and the first and the second RTO line driving signals SAP 1 B and SAP 2 B are the same as described with respect to  FIG. 3 . The signal F_sig has a similar wave pattern as the signal C_sig. However, the first delay time tDelay 1  and the fifth delay time tDelay 5  are different. According to pulse widths of the first RTO line driving signal SAP 1 B and the first SB line driving signal SAN 1 , the delay times are settled appropriately.  
         [0064]      FIG. 9  is a timing diagram for illustrating an operation of the DRAM core shown in  FIG. 6  in accordance with the first embodiment.  
         [0065]     Referring to  FIG. 9 , amplifying operation such as the over-driving operation is performed by the first RTO line driving signal SAP 1 B and the first SB line driving signal SAN 1  in initial sensing and amplifying period after bit line sense amplifier is enabled. When the first and the second SB line driving signals SAN 1  and SAN 2  are transited, pull-down voltage of the bit line sense amplifier is converted from the first ground voltage VSS 1  into the second ground voltage VSS 2 .  
         [0066]     The first ground voltage VSS 1  is raised by noises from initial amplifying, and becomes stable soon thereafter. The core voltage VCORE is also unstable when the first and the second RTO line driving signal SAP 1 B and SAP 2 B are transited, but becomes stable soon thereafter.  
         [0067]      FIG. 10  is a waveform for illustrating prevention of sensing noise in accordance with the first embodiment.  
         [0068]     As shown in  FIG. 10 , a word line WL of the corresponding bank is disabled at the beginning of the precharge operation. At the same time, active operation of another bank begins and sensing noises are generated by another bank. In the present invention, no influence of the sensing noises is caused, because a SB line of a bit line sense amplifier is driven with the first ground voltage VSS 1  in the active operation while a SB line of a bit line sense amplifier is driven with the second ground voltage VSS 2  in a subsequent operation.  
         [0069]      FIG. 11  is a schematic circuit diagram of the sense amplifier controller shown in  FIG. 6  in accordance with a second embodiment.  
         [0070]     The sense amplifier controller  600 A also is provided with a RTO line driving signal generator  630 A and a SB line driving signal generator  640 A. The RTO line driving signal generator  630 A generates the first and the second RTO line driving signals SAP 1 B and SAP 2 B in response to the active command signal ACT and the precharge command signal PCG. The SB line driving signal generator  640 A generates the first and the second SB line driving signals SAN 1  and SAN 2  in response to an output of The RTO line driving signal generator  630 A.  
         [0071]     The RTO line driving signal generator  630 A has the same architecture generating the first and the second RTO line driving signals SAP 1 B and SAP 2 B as the conventional scheme (Referring to  FIG. 2 ) and the first embodiment (Referring to  FIG. 7 ).  
         [0072]     The SB line driving signal generator  640 A is provided with a cross-coupled NAND latch consisting of NAND gates ND 8  and ND 9 , six inverters IV 30  to IV 35  and a fourth NAND gate ND 7 . The thirteenth inverter IV 32  receives the prechage command signal PCG. The cross-coupled NAND latch receives an output of the thirteenth inverter IV 32  as a set signal and its own output inverted/delayed by a fifth delay  70  having a delay time: tDelay 6 ) and a sixteenth inverter IV 35  as a reset signal. The fourteenth inverter IV 33  receives an output of the cross-coupled NAND latch and the fifth inverter IV 34  receiving a signal H_sig, an output of the fourteenth inverter IV 33 , outputs the second SB line driving signal SAN 2 . The eleventh inverter IV 30  receives the signal B_sig. The fourth NAND gate ND 7  performs a logic NAND operation to a signal G_sig, an output of the eleventh inverter IV 30 , and the signal H. The twelfth inverter IV 31 , receiving an output of the fourth. NAND gate ND 7 , outputs the first SB line driving signal SAN 1 .  
         [0073]     The precharge command signal PCG is used to generate the first and the second SB line driving signals SAN 1  and SAN 2  in the second logic embodiment. The first and the second SB line driving signals SAN 1  and SAN 2  are transited in response to the precharge command signal PCG.  
         [0074]      FIG. 12  is a timing diagram for illustration an operation of the sense amplifier controller  600 A shown in  FIG. 11 ;  
         [0075]     The wave pattern of signals A_sig to D_sig and the first and the second RTO line driving signals SAP 1 B and SAP 2 B correspond with the explanation of  FIG. 3 . Similar to the first logic embodiment, the first SB line driving signal SAN 1  is activated to a logic high level by using the signal B_sig.  
         [0076]     However, an inactivation time of the first SB line driving signal SAN 1  and an activation time of the second SB line driving signal SAN 2  are determined by not the delay time, but the precharge command signal PCG in the second embodiment of the present invention. The fifth delay  70  in the SB line driving signal generator  640 A determines a pulse width of the second SB line driving signal SAN 2 . Accordingly, the delay time is determined to meet inactivation time of the second RTO line driving signal SAP 2 B appropriately.  
         [0077]      FIG. 13  is a timing diagram for illustrating operation of the DRAM core shown in  FIG. 6  in accordance with the second embodiment. And  FIG. 14  is a waveform for illustrating prevention of sensing noise in accordance with the second embodiment.  
         [0078]     Referring to  FIGS. 13 and 14 , the over-driving operation is performed in response to the first SB and the first RTO line driving signals SAN 1  and SAP 1 B at the initial sensing and amplifying period after the bit line sense amplifier is enabled. Thereafter, the normal-driving operation is performed in response to the first SB and the second RTO line driving signals SAN 1  and SAP 2 B. The bit line sense amplifier is driven in the response to the second SB and the second RTO line driving signals SAN 2  and SAP 2 B after the precharge command signal is input.  
         [0079]     The pull-down power of the bit line sense amplifier is converted from the first ground voltage VSS 1  to the second ground voltage VSS 2  according to the precharge command signal. When a word line WL of the corresponding bank is disabled at the beginning of the precharge operation, another bank generates the sensing noises. While a SB line of a bit line sense amplifier in the corresponding bank is driven with the second ground voltage VSS 2 , a SB line of a bit line sense amplifier in another bank is driven with the first ground voltage VSS 1 . The corresponding data access can be performed regardless of the sensing noises occurring in other banks.  
         [0080]     The present invention is applicable to not only above embodiment but also various methods.  
         [0081]     For example, besides the over driving operation driving a RTO line with a normal driver and an over driver, another embodiment for driving a RTO line with a normal driver and driving a normal driver supply line with an over driver is possible in the present invention. Using supply voltage VDD as over driving voltage and using core voltage VCORE as normal driving voltage are additional alternatives.  
         [0082]     In addition, besides first and second ground voltages, other base voltages could be used for pull down voltage of a SB line.  
         [0083]     The present invention is efficient to prevent sensing noises of one bank from affecting another bank by dividing an operation period of a bit line sense amplifier into plural sections and supplying different voltage in each section. Particularly, the present invention stabilizes supply voltage in precharge operation and prevents data loss in a memory cell. Accordingly, data retention time increases and refresh operation is improved.  
         [0084]     The present application contains subject matter related to Korean patent applications Nos. 10-2005-0091685 and 10-2006-0050041, filed in the Korean Patent Office on Sep. 29, 2005 and Jun. 2, 2006 respectively, the entire contents of which are incorporated herein by reference.  
         [0085]     While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.