Abstract:
A voltage stabilizer of an embedded flash memory to modulate an input voltage VDD with a wide range of variation to a fixed voltage as an output. The voltage at the bit line of the selected memory cell can be fixed to avoid error access. The voltage stablizer of the embedded flash memory performs a voltage range inspection using a voltage inspector. Comparing to a standard value, an input voltage higher or lower than the standard value is output from a first terminal or a second terminal, respectively. The input voltage output from the first or second terminals is then stabilized to output a fixed voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the priority benefit of Taiwan application serial no. 89124860, filed Nov. 23, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates in general to a voltage stabilizer. More particularly, the invention relates to a voltage stabilizer of an embedded flash memory.  
           [0004]    2. Description of the Related Art  
           [0005]    During the access of a flash memory, methods to indicate high threshold voltage and low threshold voltage are different. In one conventional method, a reference memory cell near a bit line voltage is compared with a selected memory cell. As shown in FIG. 1, a structure to compare the reference memory cell with the selected memory cell is illustrated. The structure comprises a bit line decoder  10 , a word line decoder  12 , a memory cell  14 , a current-to-voltage converter  16 , a reference word line  18 , a reference memory cell  20 , a reference voltage  22  and a voltage sense amplifier  24 .  
           [0006]    An output of the bit line decoder  10  is coupled to a drain of the memory cell  14 . An output of the word line decoder  12  is coupled to a gate of the memory cell  14 . A source of the memory cell  14  is coupled to a ground voltage Vss. The output of the bit line decoder  10  is further coupled to the current-to-voltage converter  16 . A gate of the reference memory cell  20  at the other side is coupled to the reference word line  18 . A drain of the reference memory cell  20  is coupled to another bit line decoder (not shown), and a source thereof is coupled to the ground voltage Vss. A drain of the reference memory cell  20  is coupled to the reference voltage  22 . That is, both the drain of the reference memory cell  22  and the current-to-voltage converter  16  are coupled to the voltage sense amplifier  24 .  
           [0007]    The above structure is used to detect the Vt distribution of memory cells on a chip, so as to trace the problems in fabrication process and to maintain a correct access. However, the structure is restricted by the variation range of the VDD. When the variation of the VDD exceeds ±10%, the word line voltage dependent on the VDD has a significant variation. Thus, the reference voltage bias node applied to the voltage sense amplifier  24  is shifted to cause an error access.  
         SUMMARY OF THE INVENTION  
         [0008]    The invention provides a voltage stabilizer of an embedded flash memory. After receiving and processing an input voltage, a fixed voltage is output.  
           [0009]    The stabilizer of the embedded flash memory comprises a voltage inspector, an annular oscillator, a frequency band interstitial voltage and stabilized clock generator, a switching controller, a charge pump, an NMOS transistor, a first resistor, a second resistor, a comparator, a PMOS transistor, a first capacitor and a second capacitor.  
           [0010]    The voltage inspector receives a voltage to perform a range inspection, so as to select a value higher or lower than a standard value. When the value is higher than the standard value, the input voltage is output from a first output terminal. When the value is lower than the standard value, the input voltage is output from a second output terminal.  
           [0011]    The annular oscillator generates a clock signal. The frequency band interstitial voltage and stabilized clock generator is coupled to the annular oscillator and the voltage inspector to generate a stabilized clock signal after receiving the clock signal, and to output a frequency band interstitial voltage to the voltage inspector as a power supply.  
           [0012]    The switching controller is coupled to the first output terminal of the voltage inspector. When a voltage is input, the switching controller is conducted to output the fixed voltage to the final output terminal. The charge pump is coupled to the second output terminal of the voltage inspector, the frequency band interstitial voltage and stabilized clock generator to receive the stabilized clock signal. When the input voltage is lower than the standard value, the input voltage is received and charged to a fixed voltage. The fixed voltage is output from the output terminal.  
           [0013]    The NMOS transistor has a gate coupled to the second output terminal of the voltage inspector to receive the input voltage and a source coupled to the ground voltage. The first resistor has one terminal coupled to a drain of the NMOS transistor, and the other terminal coupled to one terminal of the second resistor. The other terminal of the second resistor is coupled to the final output terminal. The comparator comprises a first input terminal, a second input terminal, a third input terminal and an output terminal. The first input terminal is to receive the frequency band interstitial voltage, the second input terminal is coupled between the first and the second resistors, and the third input terminal is coupled to an output terminal of the charge pump to control the operation of the comparator. A gate of the PMOS transistor is coupled to the output terminal of the comparator. A source of the PMOS transistor is coupled to the output terminal of the charge pump. A drain of the PMOS transistor is coupled to the final output terminal. The first capacitor C 1  is coupled between the source of the PMOS transistor and the ground voltage. The second capacitor C 2  is coupled between the final output terminal and the ground voltage.  
           [0014]    The frequency band interstitial voltage is 1.25 V. The resistance ratio of the first resistor R 1  and the second resistor R 2  is 1:3.  
           [0015]    Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 shows a conventional structure to compare a reference memory cell near a bit line voltage with a selected memory cell;  
         [0017]    [0017]FIG. 2 shows an embodiment of a VCC 5  stabilizer of an embedded flash memory to provide a power source for a word line decoder; and  
         [0018]    [0018]FIG. 3 shows the application of the stabilizer of the embedded flash memory to the bit line of a selected memory cell. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    As mentioned above, as the embedded flash memory is restricted by the variation limit of VDD±10%, the bias node of the voltage sense amplifier  24  is shifted to cause error access. Therefore, in the invention, a voltage stabilizer of the embedded flash memory is designed to generate a fixed voltage range as shown in FIG. 2. The structures which have been illustrated in FIG. 1 are not repeated here. In FIG. 2, the VCC 5  stabilized voltage generator  25 , that is, the voltage stabilizer of the embedded flash memory provided by the invention, and the potential shifter  26  are added. As the VCC 5  generator  25  constantly generates a fixed voltage VCC 5  to the potential shifter  26 , accompanied by the output voltage of the word line decoder  27 , the low voltage is directly output from the potential shifter  26  to the gate of the memory cell  28 . Similarly, as the VCC 5  generator  25  generates the fixed voltage VCC 5 , the high voltage is output from the potential shifter  26  to the gate of the memory cell  28 .  
         [0020]    Referring to FIG. 2 and FIG. 3, a detailed description of the fixed voltage VCC 5  generated by the VCC 5  generator  25 , that is, the stabilizer of the embedded flash memory, is given as follows.  
         [0021]    The stabilizer of the embedded flash memory comprises a voltage inspector  30 , an annular oscillator  32 , a frequency band interstitial voltage and stabilized clock generator  34 , a switching controller  36 , a charge pump  38 , an NMOS transistor  40 , a first resistor  42 , a second resistor  44 , a comparator  46 , a PMOS transistor  48 , a first capacitor  50  and a second capacitor  52 .  
         [0022]    During the operation, an input voltage VDD with a wide variation range (for example, ranging from about 2.4 V to about 5.5 V) is input to the voltage inspector  30 . A voltage range inspection is performed with a determined standard value (for example, 4.5 V in this embodiment). When the input voltage VDD is higher than the standard value, that is, 5.5≧VDD≧4.5, the input voltage VDD is referred as a normal voltage and output from a first output terminal  54  of the voltage inspector  30 . When the input voltage VDD is lower than the standard value, that is 4.5≧VDD≧2.4, the input voltage is referred as a low voltage and output from a second output terminal  56  of the voltage inspector  30 . Meanwhile, the annular oscillator  32  generates a clock signal Clock to the frequency band interstitial voltage and stabilized clock generator  34  which is coupled to the annular oscillator  32 . After receiving the clock signal Clock, a stabilized clock signal CLK 25  is generated and connected to the voltage inspector  30  to provide a frequency band interstitial voltage Vbg as a power source. The frequency band interstitial voltage Vbg is fixed as 1.25.  
         [0023]    When the input voltage VDD is lower than 5.5 V and higher than 4.5 V, the input voltage VDD is output from the first output terminal  54  to the switching controller  36 . The switching controller  36  is thus conducted to directly output a fixed voltage Vc to a final output terminal VCC 5 . When the input voltage VDD is lower than 4.5 V and higher than 2.4 V, the input voltage VDD is output to the charge pump  38 . With the operation of the stabilized clock signal CLK 25  input from the frequency band interstitial voltage clock generator  34 , the input voltage VDD lower than 4.5 V is charged to a sufficient high voltage (larger than 5 V). A fixed voltage VCC 5  of about 4.75 V±5% is output from the output terminal  60 . In this embodiment, the fixed voltage has a stablized range between about 2.4 V and about 5.6 V and the variation according to temperature is about 50 ppm/° C.  
         [0024]    In addition, the gate of the NMOS transistor  40  also receives the input voltage VDD output from the second output terminal  56  of the voltage inspector  30 . The source of the NMOS transistor  40  is coupled to a ground voltage, and a drain of the NMOS transistor  40  coupled to one terminal of the first resistor (R 1 )  42  that has the other terminal coupled to one terminal of the second resistor (R 2 )  44 . The other terminal of the second resistor  44  is coupled to a final output terminal VCC 5 . The above first resistor (R 1 )  42  and the second resistor (R 2 )  44  are 1:3.  
         [0025]    The comparator  46  comprises a first input terminal  62  to receive the frequency band interstitial voltage Vbg, a second input terminal  64  coupled between the first and the second resistors  42  and  44 , and a third input terminal  66  coupled to an output terminal  60  of the charge pump  38 . A gate of the PMOS transistor  48  is coupled to the output terminal  68  of the comparator  46 . A drain of the PMOS transistor  48  is coupled to the output terminal  60  of the charge pump  38 . A source of the PMOS transistor  48  is coupled to the final output terminal VCC 5 . The first capacitor  50  is coupled between the source of the PMOS transistor  48  and the ground voltage. The second capacitor  52  is coupled between the final output terminal VCC 5  and the ground voltage.  
         [0026]    When the input voltage VDD is lower than 4.5 V and higher than 2.4 V, the high voltage output from the output terminal  60  of the charge pump  38  is a power supply for operation of the comparator  46 . Meanwhile, the input voltage VDD is fed into the gate of the NMOS transistor  40  to conduct the NMOS transistor  40 . The second capacitor  52  is charged to the fixed voltage Vc (about 4.75 V). When the voltage VCC 5  is too low (&lt;4.5 V), the discharge is performed from the second resistor (R 2 )  44  and the first resistor (RI)  42 . The voltage at the second output terminal  64  between the first resistor  42  and the second resistor  44  is lower then 1.2 V. The frequency band interstitial voltage Vbg is fixed as 1.25 V. The output of the comparator  46  is maintained at “0”. As a result, the PMOS transistor  48  is conducted. The output voltage of the output terminal  60  of the charge pump  38  is pulled up and output to the final terminal VCC 5 . When the voltage VCC 5  is high (&gt;4.75 V), the second output terminal  64  has a voltage higher than 1.2 V. The output of the comparator  46  is “1”. As a result, the PMOS transistor  48  is turned off. The charging process to the capacitor C 2  is stopped. Therefore, the voltage VCC 5  is maintained at the fixed voltage (about 4.75 V±5%). If the input voltage is raised to higher than the normal voltage 4.5 V, the second output terminal  56  stops outputting the input voltage, so that the NMOS transistor  40  can not be conducted, and the input voltage is not supplied to the charge pump  38  for operation. The above process is no longer performed. In contrast, the process is performed via the first terminal  54 . The voltage VCC 5  is directly supplied by the fixed voltage Vc to save power consumption when the normal voltage source is not operating.  
         [0027]    In the above voltage stabilizer of the embedded flash memory, the VDD with a large variation is output within a fixed voltage range, according to whether it is lower or higher than a fixed voltage. To further depict the invention, a description, FIG. 2, combining the voltage stabilizer of the embedded flash memory with the structure as shown in FIG. 1 is given here. The difference between FIG. 1 and FIG. 2 is the addition of the VCC 5  generator  25  and the potential shifter  26 . Since the VCC 5  generator  25  constantly generates a fixed voltage to the potential shifter  26 , the word line decoder  27  outputs a voltage (low voltage 0 and high voltage VDD). The low voltage 0 is directly output from the potential shifter  26  to the gate of the memory cell  28 . The high voltage is the Voltage VCC 5  generated by the VCC 5  generator and is output from the potential shifter  26  to the gate of the memory cell  28 .  
         [0028]    According to the above, the voltage stabilizer of the embedded flash memory modulates a voltage VDD with a significant variation to a fixed voltage to be output. The voltage received at the bit line is thus fixed to avoid error access.  
         [0029]    Other embodiments of the invention will appear to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.