Abstract:
A refresh circuit for individually performing a refresh operation to a plurality of banks included in a semiconductor memory device, includes: a perbank counter for generating a refresh bank address according to a refresh pulse signal and a perpank command; a perbank multiplexer for selectively outputting one of the refresh bank address and a data access bank address in response to the perbank command; and an internal command state machine for generating a refresh command in response to the refresh pulse signal and the perbank command.

Description:
FIELD OF INVENTION  
       [0001]     The present invention relates to a refresh circuit for use in a semiconductor memory device; and, more particularly, to a refresh circuit for individually performing a refresh operation to banks included in a semiconductor memory device.  
       DESCRIPTION OF PRIOR ART  
       [0002]     Since a dynamic random access memory (DRAM) is a volatile memory device, the DRAM requires a refresh operation in order to hold a data stored in a memory cell. In case of a synchronous DRAM (SDRAM), a plurality of banks are included in the SDRAM and each bank includes memory cells in a matrix form corresponding to a row address and a column address. If each of four banks has 8K numbers of row addresses, four row addresses are activated for a single refresh operation. Accordingly, when 8K numbers of refresh operations are performed, the refresh operation is completed, i.e., all the memory cells are refreshed.  
         [0003]     However, as the SDRAM is highly integrated, the number of memory cells connected to a single row address is increased and, thus, a power consumption for the refresh operation is increased.  
         [0004]     Meanwhile, recently, for reducing a load due to the refresh operation, a piled refresh is performed. For instance, if 8 banks are included in a semiconductor memory device, four bank are firstly refreshed and, then, the other four banks are refreshed. That is, bank  0  to bank  3  are firstly activated and, then, bank  4  to bank  7  are activated after a lapse of a row to row activated delay (tRRD). However, in this case, a refresh cycle time (tRFC) is increased since the tRRD is added to the tRFC.  
       SUMMARY OF INVENTION  
       [0005]     It is, therefore, an object of the present invention to provide a refresh circuit for reducing a peak current required for a refresh operation without increasing a refresh cycle time.  
         [0006]     In accordance with an aspect of the present invention, there is provided a refresh circuit for individually performing a refresh operation to a plurality of banks included in a semiconductor memory device, including: a perbank counter for generating a refresh bank address according to a refresh pulse signal and a perpank command; a perbank multiplexer for selectively outputting one of the refresh bank address and a data access bank address in response to the perbank command; and an internal command state machine for generating a refresh command in response to the refresh pulse signal and the perbank command.  
         [0007]     In accordance with another aspect of the present invention, there is provided a refresh method for performing a refresh operation to a selected bank among a plurality of banks included in a semiconductor memory device, including the steps of: generating a refresh bank address according to a refresh pulse signal and a perpank command; selectively outputting one of the refresh bank address and a data access bank address in response to the perbank command; and generating a refresh command in response to the refresh pulse signal and the perbank command. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments taken in conjunction with the accompanying drawings, in which:  
         [0009]      FIG. 1  is a block diagram showing a refresh circuit for individually refreshing banks included in a semiconductor memory device in accordance with a preferred embodiment of the present invention;  
         [0010]      FIG. 2  is an operational timing diagram of a counter;  
         [0011]      FIG. 3  is a schematic circuit diagram showing the perbank lockout unit shown in  FIG. 1 ;  
         [0012]      FIG. 4  is a timing diagram showing an operation of the perbank lockout unit shown in  FIG. 3 ;  
         [0013]      FIG. 5  is a schematic circuit diagram showing the internal command state machine shown in  FIG. 1 ; and  
         [0014]      FIGS. 6A  to  6 C are operational timing diagrams according to an operation mode. 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0015]     Hereinafter, a refresh circuit in accordance with the present invention will be described in detail referring to the accompanying drawings.  
         [0016]      FIG. 1  is a block diagram showing a refresh circuit for individually refreshing banks included in a semiconductor memory device in accordance with a preferred embodiment of the present invention.  
         [0017]     As shown, the refresh circuit includes a perbank counter  110 , a perbank multiplexer  120 , an internal command state machine  130  and a perbank lockout unit  140 . Herein, it is assumed that the semiconductor memory device includes four banks.  
         [0018]     At a normal operation mode, when an external address signal including an external bank address signal BA&lt; 0 : 1 &gt; and an external sub address signal A&lt; 0 :n&gt; is inputted, the perbank multiplexer  120  passes the external bank address signal BA&lt; 0 : 1 &gt; in response to a perbank command Perbank and a sub multiplexer  170  passes the external sub address signal A&lt; 0 :n&gt; in response to the perbank command Perbank.  
         [0019]     A decoder  180  decodes the external bank address signal BA&lt; 0 : 1 &gt; and the external sub address signal A&lt; 0 :n&gt; passed by the perbank multiplexer  120  and the sub multiplexer  170  respectively in order to activate corresponding address of all the banks. Herein, for instance, the perbank command Perbank may be inputted from a mode register set circuit, or the perbank command Perbank can be directly inputted from a control circuit employed for controlling the semiconductor memory device.  
         [0020]     Meanwhile, when a refresh pulse signal REFP and the perbank command Perbank are inputted to the perbank counter  110 , the perbank counter  110  outputs an internal address signal. Herein, the internal address signal includes an internal bank address signal RcntBA&lt; 0 : 1 &gt; and an internal sub address signal Rcnt&lt; 0 :n&gt;. That is, the perbank counter  110  outputs a first and a second bank address signals RcntBA 0  and RcntBA 1  to the perbank multiplexer  120  and outputs the refresh pulse signal REFP to a sub refresh counter  160 .  
         [0021]     Therefore, the perbank multiplexer  120  selects only one of the four banks according to a combination of a first bank address signal RcntBA 0  and a second bank address signal RcntBA 1 . Also, the sub refresh counter  160  sequentially outputs the internal sub address signal Rcnt&lt; 0 :n&gt; and the sub multiplexer  170  selectively outputs the internal sub address signal Rcnt&lt; 0 :n&gt; according to the refresh pulse signal REFP. The internal command state machine  130  generates an active signal Act or a precharge signal Pcg according to external commands, i.e., a row address strobe (RAS) signal, a column address strobe (CAS) signal and a write enable (WE) signal.  
         [0022]     Each addresses, which is enabled as the sequentially inputted internal sub address signal Rcnt&lt; 0 :n&gt; is decoded, is activated or precharged according to the active signal Act and the precharge signal Pcg.  
         [0023]     Herein, according to a first embodiment of the present invention, by setting the first and the second bank address signals RcntBA 0  and RcntBA 1  as the uppermost bit and the secondly uppermost bit respectively, the banks can be refreshed as shown in the following sequence: address  0  of bank  0 -&gt;address  1  of bank  0 -&gt; . . . -&gt;address n of bank  0 -&gt;address  0  of bank  1 -&gt;address  1  of bank  1 -&gt; . . . and so on.  
         [0024]     According to another embodiment of the present invention, by setting the first and the second bank address signals RcntBA 0  and RcntBA 1  as the downmost bit and the secondly downmost bit respectively, the banks can be refreshed as shown in the following sequence: address  0  of bank  0 -&gt;address  0  of bank  1 -&gt;address  0  of bank  2 -&gt;address  0  of bank  3 -&gt;address  1  of bank  0 -&gt;address  1  of bank  1 -&gt; . . . and so on (refer to an operational timing diagram of a counter shown in  FIG. 2 ).  
         [0025]      FIG. 3  is a schematic circuit diagram showing the perbank lockout unit  140  shown in  FIG. 1 , and  FIG. 4  is a timing diagram showing an operation of the perbank lockout unit  140 .  
         [0026]     The perbank lockout unit  140  serves to prevent another operation from being performed when a refresh operation is performed according to the external address signal or the internal address signal. For instance, even though another command is generated when the refresh operation is performed by activating an address  0 , the perbank lockout unit  140  intercepts the command so that the refresh operation can be stably performed. That is, a forbiddance signal (forbidden) is enabled when the refresh pulse signal REFP is inputted. The forbiddance signal (forbidden) is disabled after a lapse of a refresh cycle time.  
         [0027]      FIG. 5  is a schematic circuit diagram showing the internal command state machine  130  shown in  FIG. 1 .  
         [0028]     At a perbank operation, in response to a power-up signal pwrup, bank address signals badd&lt; 0 &gt; and badd&lt; 1 &gt; generated by the perbank multiplexer  120 , the forbiddance signal generated by the perbank lockout unit  140 , the refresh pulse signal REFP, the perbank signal Perbank, commands for an active operation and commands for a precharge operation, the internal command state machine  130  outputs an active command signal and a precharge command signal to only a bank which is selected according to a bank address signal.  
         [0029]     A node A has a logic high level in response to the power-up signal pwrup and the node A has a logic low level in response to the commands for an active operation. Herein, a refresh precharge signal Rpcg 0  is a pulse signal activated after a predetermined delay time tRAS is passed since an activation of an active command signal. The predetermined delay time tRAS is a required delay time for activating a row address strobe signal. Therefore, if an active command signal ACT 0  is activated, a precharge command signal PCG 0  is activated after the predetermined delay time.  
         [0030]      FIGS. 6A  to  6 C are operational timing diagrams according to an operation mode.  
         [0031]      FIG. 6A  is a timing diagram at a normal mode. As shown, a predetermined bank is precharged after the predetermined bank is activated.  
         [0032]      FIG. 6B  is a timing diagram at a refresh mode according to the prior art. As shown, the active operation and the precharge operation are performed to all the banks.  
         [0033]      FIG. 6C  is a timing diagram at a refresh mode according to the present invention. As shown, the active operation and the precharge operation are performed to a single bank.  
         [0034]     Therefore, in accordance with the present invention, a peak current required for the refresh operation can be reduced without increasing a refresh cycle time.  
         [0035]     The present application contains subject matter related to Korean patent application No. 2005-27353, filed in the Korean Patent Office on Mar. 31, 2005, the entire contents of which being incorporated herein by reference.  
         [0036]     While the present invention has been described with respect to the particular embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.