TECHNICAL FIELD  
         [0001]    The present invention relates generally to voltage generation circuits, and, more particularly, to controlling the voltage developed by a voltage generation circuit.  
         BACKGROUND OF THE INVENTION  
         [0002]    Voltage generation circuits are utilized in many integrated circuits to generate voltages required for proper operation of the integrated circuit. For example, in a semiconductor memory device such as a dynamic random access memory (DRAM) a supply voltage VCC is applied to the device and a voltage generation circuit within the memory device generates a pumped voltage VCCP having a value greater than the supply voltage. In a DRAM, the pumped voltage VCCP is utilized, for example, in driving word lines of a memory-cell array when accessing rows of memory cells contained in the array, as will be appreciated by those skilled in the art. The value of the pumped voltage VCCP is greater than the supply voltage VCC so that capacitors in the memory cells may be charged to the supply voltage, as will once again be understood by those skilled in the art.  
           [0003]    [0003]FIG. 1 is a functional block diagram and schematic illustrating a conventional voltage generation circuit  100  that may be utilized in a DRAM to generate a pumped voltage VCCP having a value greater than an applied supply voltage VCC. The voltage generation circuit  100  includes an oscillator  102  that generates an oscillator signal OSC in response to an enable signal EN applied by a Schmitt Trigger comparator  104 . The oscillator  102  clocks the OSC signal when the EN signal is active and does not clock the OSC signal when the EN signal is inactive, instead maintaining the OSC signal either high or low. The OSC signal is applied to clock a charge pump circuit  106  which, in response to the OSC signal, generates the pumped voltage VCCP. More specifically, when the OSC signal clocks the charge pump circuit  106 , the circuit turns ON and charges a load capacitor  108  to thereby develop the pumped voltage VCCP and drive a load resistance  109 . When the OSC signal does not clock the charge pump circuit  106 , the circuit turns OFF and stops charging the load capacitor  108 . The detailed operation and circuitry for forming the oscillator  102  and charge pump circuit  106  are well understood by those skilled in the art, and thus, for the sake of brevity, these components will not be described in further detail.  
           [0004]    The pumped voltage VCCP is applied through a diode-coupled PMOS transistor  110  and a level shifting circuit  112  to develop a pump feedback voltage VPF that is applied to a first input of the Schmitt Trigger comparator  104 . The diode-coupled transistor  110  functions as a level shifter to reduce the value of the pumped voltage VCCP and ensure proper common-mode operation of the Schmitt Trigger comparator  104 , as will be appreciated by those skilled in the art. The level shifting circuit  112  reduces the voltage from the diode-coupled transistor  110  by an offset voltage VOFF, which has a value determined, in part, by the desired value of the pump feedback voltage VPF. A current source  114  causes a desired current to flow through the diode-coupled transistor  110  and level shifting circuit  112  so that the feedback voltage VPF having the desired value is developed on the first input of the Schmitt Trigger comparator  104 . A second input of the Schmitt Trigger comparator  104  receives a reference voltage VREF that is developed by a diode-coupled PMOS transistor  116  and a current source  118  coupled in series between the supply voltage VCC and ground. The diode-coupled transistor  116  functions as a level shifter to reduce the value of the supply voltage VCC and provide for proper common mode operation of the Schmitt Trigger comparator  104 , as will be appreciated by those skilled in the art. The current source  118  causes a desired current to flow through the diode-coupled transistor  116  to develop the reference voltage VREF on the second input of the Schmitt Trigger comparator  104 .  
           [0005]    The voltage generation circuit  100  further includes over voltage protection components that attempt to limit the value of the pumped voltage VCCP as the supply voltage VCC increases. The overvoltage protection components include an overvoltage detector  120  that monitors the supply voltage VCC and develops an overvoltage signal OV having a value that is a function of the monitored supply voltage. The overvoltage signal OV is applied to an NMOS transistor  122  that is connected in series with a current source  124  and coupled between the second input of the Schmitt Trigger comparator  104  and ground. When the overvoltage signal OV has a sufficient magnitude, the transistor  122  turns ON causing current to flow through the transistor and current source  124  to ground. The transistor  122  and current source  124  together form a current limiting circuit  126  that operates during an overvoltage mode of the circuit  100 , as will be described in more detail below. The overvoltage signal OV is further applied to a voltage clamping circuit  128  formed by an NMOS transistor  130  and diode-coupled transistor  132  coupled between the output of the charge pump  106  and the supply voltage VCC. When the overvoltage signal OV as a sufficient magnitude, the transistor  130  turns ON allowing current to flow through the diode-coupled transistor  132  and transistor to the supply voltage VCC to thereby clamp the pumped voltage VCCP.  
           [0006]    During normal operation of the voltage generation circuit  100 , the supply voltage VCC has a predetermined value and the overvoltage detector  120  drives the overvoltage signal OV sufficiently low to turn OFF the transistors  122  and  130 . Thus, during normal operation the current limiting circuit  126  and clamping circuit  128  do not affect operation of the voltage generation circuit  100 . In operation, the oscillator  102  applies the OSC signal to clock the charge pump  106  which, in turn, develops the pumped voltage VCCP. The pumped voltage VCCP is fed back through the diode-coupled transistor  110  and level shifting circuit  112  to develop the pump feedback voltage VPF. At this point, the current flowing through the diode-coupled transistor  116  as determined by the current source  118  develops the reference voltage VREF. As long as the pump feedback voltage VPF is less than the reference voltage VREF, the comparator drives the EN signal active, causing the oscillator  102  to clock the charge pump  106 .  
           [0007]    As the charge pump  106  operates, the pumped voltage VCCP increases to a point where the pumped voltage fed back through the diode-coupled transistor  110  and level shifting circuit  112  causes the pump feedback voltage VPF to exceed the reference voltage VREF. When the pump feedback voltage VPF is greater than the reference voltage VREF, the Schmitt Trigger comparator  104  deactivates the EN signal causing the oscillator  102  to stop clocking the charge pump  106  which, in turn, turns OFF. The charge pump  106  remains OFF until the pumped voltage VCCP discharges through a load resistance  109  and drops to a value causing the pump feedback voltage VPF to once again become less than the reference voltage VREF. When this occurs, the Schmitt Trigger comparator  104  once again activates the EN signal causing the oscillator  102  to clock the charge pump  106 , which turns ON to once again begin charging the pumped output voltage VCCP.  
           [0008]    When the supply voltage VCC increases, the overvoltage detector  120 , current limiting circuit  126 , and clamping circuit  128  operate in combination to limit the value of the pumped voltage VCCP. As the supply voltage VCC increases, the reference voltage VREF likewise increases, meaning that the pumped voltage VCCP similarly increases to thereby increase the feedback voltage VPF until it equals the increased reference voltage. When the supply voltage VCC exceeds a predetermined value, the overvoltage detector  120  activates the overvoltage signal OV, turning ON the transistors  122  and  130 . When the transistor  130  turns ON, the pumped voltage VCCP is limited to a value above the supply voltage VCC determined by a small voltage drop across the transistor  130  plus the voltage drop across the diode-coupled transistor  132 . Similarly, the turned ON transistor  122  and current source  124  attempt to sink current in parallel with the current source  118  to increase the voltage across transistor  116  and thereby limit the increase in the value of the reference voltage VREF. Ideally, the reference voltage VREF tracks the supply voltage VCC until the supply voltage exceeds the predetermined value which activates the overvoltage detector  120 . This maintains a constant difference between the supply voltage VCC and the pumped voltage VCCP until the supply voltage exceeds the predetermined value. Ideally, once the supply voltage VCC exceeds the predetermined value, the reference voltage VREF is held constant, causing the pumped feedback voltage VPF to become greater than the reference voltage, which causes the Schmitt Trigger comparator  104  to deactivate the EN signal to thereby deactivate the oscillator  102  and turn OFF the charge pump  106 . As will now be explained in more detail, the voltage generation circuit  100  does not, however, operate in this ideal manner.  
           [0009]    The supply voltage VCC may increase, for example, during burn-in of an integrated circuit containing the voltage generation circuit  100 . Typically, during burn-in the supply voltage VCC is increased to stress components contained within the integrated circuit, as will be understood by those skilled in the art. FIG. 2 is a graph illustrating the values of the pumped voltage VCCP, reference voltage VREF, and the overvoltage signal OV in the voltage generation circuit  100  as the supply voltage VCC increases. In the example of FIG. 2, the values of the supply voltage VCC and pumped voltage VCCP are initially two and three volts, respectively. At a time T 1 , the supply voltage VCC begins to increase and the pumped voltage VCCP and reference voltage VREF similarly begin increasing as illustrated. At this point, the overvoltage detector  120  is monitoring the supply voltage VCC but has not activated the overvoltage signal OV. Until a time T 2 , the reference voltage VREF tracks the supply voltage VCC to maintain a constant difference between the supply voltage and the pumped voltage VCCP. At the time T 2 , the overvoltage signal OV goes active, turning ON the current limiting circuit  126  and clamping circuit  128 . Notwithstanding the activation of the circuits  126 ,  128 , it is seen that the pumped voltage VCCP and the reference voltage VREF continue increasing after the time T 2 . This is true because due to physical limitations, such as heat dissipation and size limitations when forming components of the current source  124 , the current limiting circuit  126  cannot sink enough current to limit the value of the reference voltage VREF as the supply voltage VCC increases. As a result, as the supply voltage VCC increases the pumped voltage VCCP and reference voltage VREF likewise increase.  
           [0010]    In the voltage generation circuit  100 , the pumped voltage VCCP may become so great as the supply voltage VCC increases that components of the integrated circuit containing the voltage generation circuit may be damaged. For example, the pumped voltage VCCP may exceed the breakdown voltages of various devices such as MOS transistors formed within the integrated circuit. Moreover, it should be noted that the clamping circuit  128  must dissipate what may be significant amounts of power as the pumped voltage VCCP increases and thus the voltage generation circuit  100  consumes wasted power and generates unwanted heat during the burn-in process.  
           [0011]    There is a need for a voltage generation circuit that reliably limits the value of the pumped voltage as the supply voltage increases.  
         SUMMARY OF THE INVENTION  
         [0012]    A method and circuit control the value of generated voltage derived from a supply voltage as the value of the supply voltage varies, such as during burn-in of an integrated circuit. According to one aspect of the present invention, a voltage generation circuit includes a generator circuit that receives a supply voltage and has a reference node. The generator circuit develops an output voltage from the supply voltage and the output voltage has a value that is a function of a reference voltage applied on the reference node. A coupling circuit is coupled to the reference node and also receives the supply voltage. The coupling circuit operates in response to a voltage control signal to vary an electronic coupling of the supply voltage to the reference node which thereby adjusts the value of the reference voltage. A voltage sensing circuit receives the reference voltage and develops the voltage control signal that is applied to the coupling circuit in response to the reference voltage. The coupling circuit controls coupling of the supply voltage to the reference node, adjusting the value of the reference voltage to control the output voltage of the voltage generation circuit. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a functional block diagram and schematic illustrating a conventional voltage generation circuit.  
         [0014]    [0014]FIG. 2 is a graph illustrating the effect of an increasing supply voltage on an output voltage and several other signals in the voltage generation circuit of FIG. 1.  
         [0015]    [0015]FIG. 3 is a functional block diagram and schematic of a voltage generation circuit according to one embodiment of the present invention.  
         [0016]    [0016]FIG. 4 is a graph illustrating the effect of an increasing supply voltage on an output voltage and several other signals in the voltage generation circuit of FIG. 3.  
         [0017]    [0017]FIG. 5 is a functional block diagram of a memory device including the voltage generation circuit of FIG. 3.  
         [0018]    [0018]FIG. 6 is a functional block diagram of a computer system including the memory device of FIG. 5. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    [0019]FIG. 3 is a functional block diagram and schematic of a voltage generation circuit  300  according to one embodiment of the present invention. The voltage generation circuit  300  limits the value of a generated pumped voltage VCCP as an applied supply voltage VCC increases so that components within an integrated circuit containing the voltage generation circuit are not damaged, as will be explained in more detail below. In the following description, certain details are set forth to provide a sufficient understanding of the invention. However, it will be clear to one skilled in the art that the invention may be practiced without these particular details. In other instances, well-known circuits, control signals, timing protocols, and software operations have not been shown in detail in order to avoid unnecessarily obscuring the invention.  
         [0020]    The voltage generation circuit  300  includes a Schmitt Trigger comparator  302 , oscillator  304 , and charge pump circuit  306 , which operate in the same manner as previously described for the corresponding components in the voltage generation circuit  100  of FIG. 1. For the sake of brevity, these components will not again be described in detail. A more detailed description of a charge pump circuit is provided in U.S. Pat. No. 6,160,723 to Liu entitled “Charge Pump Circuit Including Level Shifters for Threshold Voltage Cancellation and Clock Signals Boosting, and Memory Device Using Same,” and in U.S. patent application Ser. No. 09/256,972 to Liu entitled “Method and Circuit for Regulating the Output Voltage from a Charge Pump Circuit, and Memory Device Using Same” filed on Feb. 24, 1999, both of which are incorporated herein by reference.  
         [0021]    A diode-coupled PMOS transistor  308  and current source  310  develop a control signal  312 . The control signal  312  is applied to a gate of an NMOS transistor  314  that is coupled in series with a current source  316  between the supply voltage VCC and ground, and the transistor  314  develops a pump feedback voltage VPF in response to the control signal. The pump feedback voltage VPF is applied to one input of the Schmitt Trigger comparator  302 . A coupling circuit  315  is formed by a diode-coupled PMOS transistor  317 , an NMOS transistor  318 , and a current source  320 , which operate in combination to develop a reference voltage VREF in response to an overvoltage control signal OVC, with the reference voltage being applied to a second input of the Schmitt Trigger comparator  302 . In response to the OVC signal, the transistor  318  adjusts the current through the diode-coupled transistor  317  to control the value of the reference voltage VREF.  
         [0022]    The diode-coupled transistors  308  and  317  are matched, as are the current sources  310  and  320 , which provides common-mode level shifting of the pump feedback voltage VPF and reference voltage VREF. In the embodiment of FIG. 3, the transistor  314  is a long-channel device that develops a voltage between control signal  312  and voltage VPF of about 1.5 volts, which determines the difference between the supply voltage VCC and pumped voltage VCCP, as will be appreciated by those skilled in the art.  
         [0023]    A voltage sensing circuit  322  develops the OVC signal in response to the reference voltage VREF. The voltage sensing circuit  322  includes an NMOS transistor  324 , a diode-coupled NMOS transistor  326 , and a current source  328  that operate in combination to develop a control signal  330  in response to the reference voltage VREF. More specifically, the transistor  324  adjusts the current through the diode-coupled transistor  326  in response to the reference voltage VREF to control the value of the control signal  330 . The control signal  330  is applied to a gate of an NMOS transistor  332  that is coupled in series with a current source  334  between the supply voltage VCC and ground. In response to the control signal  330 , the transistor  332  controls the value of the OVC signal applied to the transistor  318 . Thus, the voltage sensing circuit  322  forms a feedback circuit that adjusts the value of the OVC signal in response to the reference voltage VREF to and thereby control the value of the reference voltage.  
         [0024]    During normal operation of the voltage generation circuit  300 , the oscillator  304  clocks the charge pump  306  which, in turn, charges a load capacitor  336  to develop the pumped voltage VCCP across the load capacitor. In response to the pumped voltage VCCP, the diode-coupled transistor  308  and current source  310  develop the control signal  312  that is applied to the transistor  314  which, in turn, develops the pump feedback voltage VPF applied to the Schmitt Trigger comparator  302 . At the same time, the coupling circuit  315  and voltage sensing circuit  322  operate combination to develop the voltage reference VREF that is applied to the Schmitt Trigger comparator  302 . During normal operation, the transistor  332  is turned OFF, causing the OVC signal to go to approximately the supply voltage VCC and turning ON the transistor  318 . In this situation, the value of the reference voltage VREF is determined by a small voltage drop (less than the threshold voltage of the transistor  332 ) across the current source  328  plus the voltage drop across the diode-coupled transistor  326  plus the threshold voltage of the transistor  324 .  
         [0025]    In the normal operation mode, as long as the pump feedback voltage VPF is less than the reference voltage VREF, the Schmitt Trigger comparator  302  enables the oscillator  304  which, in turn, clocks the charge pump  306  so that the charge pump continues charging the capacitor  336  to increase the value of the pumped voltage VCCP. When the pumped voltage VCCP reaches a value causing the pump feedback voltage VPF to become greater than the reference voltage VREF, the Schmitt Trigger comparator  302  disables the oscillator  304  which, in turn, stops clocking the charge pump  306 . At this point, capacitor  336  begins to discharge through a load resistance  337 . When the voltage VPF once again becomes less than the reference voltage VREF the Schmitt Trigger comparator  302  activates the oscillator  304  to clock the charge pump  306  to charge the load capacitor  336  and increase the pumped voltage VCCP.  
         [0026]    The operation of the voltage generation circuit  300  in an overvoltage mode, which occurs when the supply voltage VCC increases such as may occur during burn-in of an integrated circuit (not shown) containing the voltage generation circuit, will now be explained in more detail with reference to FIGS. 3 and 4. FIG. 4 illustrates the values for the pumped voltage VCCP, the supply voltage VCC, the overvoltage control signal OVC, the reference voltage VREF, and the control signal  330  during operation of the voltage generation circuit  300  in the overvoltage mode. Although not shown in FIG. 4 to simplify the figure, the pumped voltage VCCP has ripple due to the hysteresis of the Schmitt Trigger comparator  302 . As will now be explained in more detail, during the overvoltage mode the coupling circuit  315  and voltage sensing circuit  322  operate in combination to the to limit the value of the pumped voltage VCCP. More specifically, as the supply voltage VCC increases, the voltage on the gate and drain of the diode-coupled transistor  317  increases, and this increased voltage is applied through the transistor  318  to increase of the reference voltage VREF. In response to the increased reference voltage VREF, the Schmitt Trigger comparator  302  activates the oscillator  304  which, in turn, causes the charge pump  306  to increase the pumped voltage VCCP until the pump feedback voltage VPF once again equals the increased reference voltage. In FIG. 4, at a time a TO the supply voltage VCC begins increasing and the pumped voltage VCCP and reference voltage VREF likewise increase in response to the increasing supply voltage.  
         [0027]    At a time a T 1 , the control signal  330  begins increasing from a value of approximately zero volts in response to the increasing reference voltage VREF and corresponding increase in current through the transistor  324 , diode-coupled transistor  326 , and current source  328 . At this point, note that the overvoltage control signal OVC also increases and approximately equals the supply voltage VCC since the transistor  332  is turned OFF. The control signal  330  continues increasing along with the other signals until a time T 2 , when the magnitude of the control signal equals approximately the threshold voltage of the transistor  332 . In response to the control signal  330 , the transistor  332  turns ON at the time T 2 , causing current to flow through the current source  334  and the transistor and controlling the value of the overvoltage control signal OVC as illustrated in FIG. 4 When the value of the overvoltage signal OVC is limited at the time T 2 , the value of the reference voltage VREF is limited to the sum of the threshold voltages of transistors  324 ,  326 , and  332 . As a result, the increases in the supply voltage VCC no longer increase the reference voltage VREF. After the time T 2 , the pumped voltage VCCP no longer increases and is thus limited to prevent damage to components (not shown) in the integrated circuit (not shown) containing the voltage generation circuit  300 . Moreover, the power consumption of the charge pump  306  does not increase after the time T 2  notwithstanding further increases in the supply voltage VCC.  
         [0028]    [0028]FIG. 5 is a block diagram of a memory device  500  including the voltage generation circuit  300  of FIG. 3. The voltage generation circuit  300  applies the pumped voltage VCCP to a memory-cell array  502  contained in the memory device  500 , and may also apply the pumped voltage to other components in the memory device. In the memory-cell array  502 , the pumped voltage VCCP is applied, for example, to word lines (not shown) to access corresponding rows of memory cells (not shown), as will be understood by those skilled in the art. The memory device  500  further includes an address decoder  504 , a control circuit  506 , and read/write circuitry  508 , all of which are conventional and known in the art. The address decoder  504 , control circuit  506 , and read/write circuitry  508  are all coupled to the memory-cell array  502 . In addition, the address decoder  504  is coupled to an address bus, the control circuit  506  is coupled to a control bus, and the read/write circuitry  508  is coupled to a data bus.  
         [0029]    In operation, external circuitry (not shown) provides address, control, and data signals on the respective busses to the memory device  500 . During a read cycle, the external circuitry provides a memory address on the address bus and control signals on the control bus. In response to the memory address on the address bus, the address decoder  504  provides a decoded memory address to the memory-cell array  502  while the control circuit  506  provides control signals to the memory-cell array in response to the control signals on the control bus. The control signals from the control circuit  506  control the memory-cell array  502  to provide data to the read/write circuitry  508 . The read/write circuitry  508  then provides this data on the data bus for use by the external circuitry. During a write cycle, the external circuitry provides a memory address on the address bus, control signals on the control bus, and data on the data bus. Once again, the address decoder  504  decodes the memory address on the address bus and provides a decoded address to the memory-cell array  502 . The read/write circuitry  508  provides the data on the data bus to the memory-cell array  502  and this data is stored in the addressed memory cells in the memory-cell array under control of the control signals from the control circuit  506 . The memory device  500  may be a dynamic random access memory (DRAM), synchronous DRAM (SDRAM), double-data-rate (DDR) DRAM, packetized memory device such as an SLDRAM or RAMBUS device, or other type of memory device as well. Moreover, the voltage generation circuit  300  may be placed integrated circuits other than memory devices.  
         [0030]    [0030]FIG. 6 is a block diagram of a computer system  600  which uses the memory device  500  of FIG. 5. The computer system  600  includes computer circuitry  602  for performing various computing functions, such as executing specific software to perform specific calculations or tasks. In addition, the computer system  600  includes one or more input devices  604 , such as a keyboard or a mouse, coupled to the computer circuitry  602  to allow an operator to interface with the computer system. Typically, the computer system  600  also includes one or more output devices  606  coupled to the computer circuitry  602 , such output devices typically being a printer or a video terminal. One or more data shortage devices  608  are also typically coupled to the computer circuitry  602  to store data or retrieve data from external storage media (not shown). Examples of typical data storage devices  608  include hard and floppy disks, tape cassettes, and compact disk read only memories (CD-ROMs). The computer circuitry  602  is typically coupled to the memory device  500  through a control bus, a data bus, and an address bus to provide for writing data to and reading data from the memory device.  
         [0031]    It is to be understood that even though various embodiments and advantages of the present invention have been set forth in the foregoing description, the above disclosure is illustrative only, and changes may be made in detail, and yet remain within the broad principles of the invention. For example, some of the components described above may be implemented using either digital or analog circuitry, or a combination of both, and also, where appropriate, may be realized through software executing on suitable processing circuitry. Also, the conductivity types of the devices, such as NMOS and PMOS transistors, may also be varied as required by particular applications, as will be understood by those skilled in the art. Therefore, the present invention is to be limited only by the appended claims.
A method and circuit control the value of generated voltage derived from a supply voltage as the value of the supply voltage varies, such as during burn-in of an integrated circuit. A voltage generation circuit includes a generator circuit that receives a supply voltage and has a reference node and develops an output voltage from the supply voltage, the output voltage having a value that is a function of a reference voltage applied on the reference node. A coupling circuit receives the supply voltage and operates in response to a voltage control signal to vary an electronic coupling of the supply voltage to the reference node to thereby adjust the value of the reference voltage. A voltage sensing circuit develops the voltage control signal that is applied to the coupling circuit in response to the reference voltage.