Abstract:
A circuit for generating a trim bit signal in a flash memory device, comprises a control unit selected by a trim bit select signal and including a programmable and erasable cell, and an output unit for outputting a High level signal or a Low level signal through the trim bit signal output terminal depending on the program cell of the control unit.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a circuit for generating a trim bit signal in NAND type flash memory devices, and more particularly, to a circuit for generating a trim bit signal capable of recovering the trim bit in a package status. 
   2. Background of the Related Art 
   In the NAND type flash memory device, the trim bit signal is generated in order to improve and change characteristics of the circuit within the chip. 
     FIG. 1  illustrates a conventional trim bit block. As shown in  FIG. 1 , a plurality of trim blocks B 1 , B 2 , . . . , Bn are provided. The first trim block B 1  outputs a first trim bit signal TBIT 1 , the second trim block B 2  outputs a second trim bit signal TBIT 2  and the N th  trim block Bn outputs a N th  trim bit signal TBIT 1 . Characteristics of the circuit within the chip could be modified or improved by these trim bit signals. 
     FIG. 2  is a detailed circuit diagram of the trim block shown in FIG.  1 . If a fuse FUSE is connected, the input of the inverter INV 0  becomes a Low level due to a small current load of the PMOS transistor P 0 . Therefore, the output of the inverter INV 1  becomes a High level, so that the trim bit signal TBIT becomes the High level. 
   On the contrary, if the fuse FUSE is disconnected, the input of the inverter INV 0  becomes a High level. Accordingly, the output of the inverter INV 1  becomes a Low level, so that the trim bit signal TBIT becomes the Low level. 
   In this conventional circuit for generating the trim bit signal, a work for the fuse is performed only in a wafer status. For this reason, the trim bit could not be recovered again after the work is performed. Therefore, there is a disadvantage that the value could not be changed depending on the status after packaging. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is contrived to substantially obviate one or more problems due to limitations and disadvantages of the related art, and an object of the present invention is to provide to a circuit for generating a trim bit signal that generates the trim bit using a cell. 
   According to the present invention, the trim bit could be recovered depending on circumstances even after the package. 
   Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
   To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a circuit for generating a trim bit signal in a flash memory device according to the present invention is characterized in that it comprises a control unit selected by a trim bit select signal and including a cell that is programmable and erasable, and an output unit for outputting a High level signal or a Low level signal through the trim bit signal output terminal depending on the program cell of the control unit. 
   The control unit comprises a first inverter for inverting the trim bit select signal, a first switching device connected between the output of the first inverter and a first node and turned on by a first control signal, the cell connected between the first node and a second node and driven by a second control signal, a second switching device connected between the second node and the output of the control unit and turned on by a third control signal, and a third switching device connected between the first node and the ground and turned on by a fourth control signal. 
   In another aspect of the present invention, it is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings, in which: 
       FIG. 1  illustrates a conventional trim bit block of a flash memory device; 
       FIG. 2  is a detailed circuit diagram of the trim block shown in  FIG. 1 ; 
       FIG. 3  illustrates a circuit for generating a trim bit signal in a flash memory device according to a preferred embodiment of the present invention; and 
       FIG. 4  illustrates an example of the trim bit block to which the circuit for generating the trim bit of the present invention is applied. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     FIG. 3  illustrates a circuit for generating a trim bit signal in a flash memory device according to a preferred embodiment of the present invention. 
   Referring to  FIG. 3 , the construction of the circuit for generating the trim bit signal will be first described. 
   An inverter INV 0  for inverting a trim bit select signal TSEL is provided. A NMOS transistor NO that is driven by a control signal TS is connected between the node node 0  being the output of the inverter INV 0  and the node node 1 . A cell TCELL that is driven by a control signal TWL is connected between the node node 1  and the node node 2 . A NMOS transistor N 1  that is driven by a control signal TD is connected between the node node 2  and the node node 3 . A PMOS transistor P 0  is connected between the node node 3  and the power supply, the gate the PMOS transistor P 0  is grounded. Inverters INV 1  and INV 2  are serially connected between the node node 3  and the output TBIT. In addition, a NMOS transistor N 2  that is controlled by a control signal TN is connected between the node node 1  and the ground. 
   The operation of the circuit for generating the trim bit signal constructed above will be below described. 
   In a normal operation, if a power supply voltage (VCC) is applied to the control signal TWL, TD and TN, 0V is applied to the control signal TS and the cell TCELL is programmed, the node node 3  becomes a High level since the cell TCELL is turned off. Therefore, the output TBIT becomes a High level. 
   If the cell TCELL is an erased cell, the potential of the node node 3  becomes a Low level since the current flows into the NMOS transistor N 2 . Therefore, the output TBIT becomes the Low level. 
   In other words, the output TBIT is decided as in the case where the conventional fuse is used depending on the characteristic of the cell TCELL. 
   Meanwhile, the cell TCELL must be initially basically erased. If it is desired that the cell TCELL becomes an erase status, it is required that 0V is applied to the control signal TD, TS, TN and TWL, respectively, and an erase voltage (Vera) be applied to CPWELL of the cell TCELL. At this time, all the outputs TBIT have a Low level. 
   If it is desired that a specific bit becomes a High level, it is required that a corresponding trim bit signal TSEL be selected and the cell TCELL be programmed. This corresponds to the role to disconnect the fuse. If it is desired that the cell TCELL is programmed, it is required that the power supply voltage (VCC) is applied to the control signal TS, 0V is applied to the control signal TD and TN, a program voltage (Vpgm) is applied to the control signal TWL. In case that the cell is selected, as the node node 0  becomes 0V and the channel of the cell becomes 0V, the cell is programmed. In case that the cell is not selected, the node node 0  becomes a High level. Accordingly, as the node node 1 , the cell channel and the node node 2  are self-boosted by the control signal TWL, the cell is not programmed. At this time, programming and erasing of the cell are performed based on the FN tunneling scheme. Furthermore, the bias condition depending on respective operations is well shown in Table 1. 
   
     
       
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               TSEL 
               TN 
               TS 
               TWL 
               TD 
               CPWELL 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               Normal Operation 
               X 
               VCC 
               0 V 
               VCC 
               VCC 
               0 V 
             
             
               Trim Bit Program 
               H 
               0 V 
               VCC 
               Vpgm 
               0 V 
               0 V 
             
             
               Trim Bit Erase 
               X 
               0 V 
               0 V 
               0 V 
               0 V 
               Vera 
             
             
                 
             
           
        
       
     
   
     FIG. 4  illustrates an example of the trim bit block to which the circuit for generating the trim bit of the present invention is applied. 
     FIG. 4  shows an example in which several trim bit blocks are actually used. In the trim bit block, CPWELL, TD, TWL, TS and TN are controlled at the same time. The trim bit blocks B 1 , B 2  . . . BN are selected by the trim bit select signals TSEL 0 , TSEL 1 , . . . TSELn. Therefore, the trim bit signals TBIT 0 , TBIT 1 , . . . TBITn are outputted. 
   As described above, according to the present invention, important parameters could be controlled that decide the characteristics of the chip even after the package. Therefore, the present invention has advantageous effects that it can improve the characteristics of the chip and thus improve the productivity. 
   The forgoing embodiments are merely exemplary and are not to be construed as limiting the present invention. The present teachings can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art.