BACKGROUND  
       [0001]     The present invention relates to high-voltage switching circuits of nonvolatile memory devices and more particularly, to a high-voltage switching circuit shortening a boosting time of a drive signal activating a high-voltage switch.  
         [0002]     In nonvolatile memory devices such as flash memories or EEPROMs, a high voltage (VPP) is needed for programming or erasing operations relative to other memory devices. The high voltage is internally generated and has a higher voltage than an external power source voltage (VCC). A high-voltage switch is required to switch to the high voltage VPP or to transfer the high voltage to a wordline. And, when the high-voltage switch includes an NMOS transistor, the high-voltage switch needs a voltage higher than the high voltage VPP that is applied to a gate of the NMOS transistor. For this, a boosting circuit for boosting the gate of the NMOS transistor may be required for the high-voltage switch.  
         [0003]      FIG. 1  is a circuit diagram illustrating a conventional high-voltage switching circuit, including a high-voltage switch  11 , a boosting enable unit  12 , and a high-voltage switching booster  13 .  
         [0004]     Referring to  FIG. 1 , the high-voltage switch  11  includes an NMOS transistor N 1  that transfers the high voltage VPP as an output signal VPPOUT, activated by a drive signal (i.e., a signal of a node NB) provided from the high-voltage switching booster  13 .  
         [0005]     The boosting enable unit  12  includes inverters IV 1  and IV 2 , and an NMOS transistor N 2 , activating the high-voltage switching booster  13  in response to an enable signal EN.  
         [0006]     The high-voltage switching booster  13  is provided to boost the drive signal (i.e., the signal of the node NB) for activating the high-voltage switch  11 , high-voltage switching booster  13  including two capacitors Ca and Cb, and two NMOS transistors N 3  and N 4 . NMOS transistor N 2  is configured in the form of diode connection. The capacitors Ca and Cb respond respectively to clock signals CK and CKB, where clock pulse CKB is the inverse of clock pulse CK.  
         [0007]      FIG. 2  illustrates waveforms of opposite clock signals CK and CKB, and  FIGS. 3A through 3C  illustrate an operation of the high-voltage switching booster  13  shown in  FIG. 1 . Hereinafter, the operation of the high-voltage switching booster  13  will be described with reference to  FIGS. 2 , and  3 A through  3 C.  
         [0008]     The simplest way for boosting a signal or a node is to utilize the clock signals CK and CKB. As shown in  FIG. 3A , one electrode of the capacitor Ca is coupled to the clock signal CK while the other electrode of the capacitor Ca is connected to the node NA. One electrode of the capacitor Cb is coupled to the clock signal CKB while the other electrode of the capacitor Cb is connected to the node NB. Thus, the nodes NA and NB are changed whenever the clock signals CK and CKB vary. Here, Cs represents a parasitic capacitance of the node NB.  
         [0009]     First, as shown in  FIG. 3A , when the clock signal CK is 0V while the clock signal CKB is VCC (refer to  FIG. 2 ), the capacitor Ca does not conduct a pumping operation while the capacitor Cb conducts a pumping operation. Then, the NMOS transistor N 3  is turned on while the NMOS transistor N 4  is turned off. Thus, the node NB is boosted up to Vb through the pumping operation by the capacitor Ca. Then node NB decreases to Vb−Vth 1  by a threshold voltage Vth 1  of the NMOS transistor N 3  because node NB is connected to the gate of the NMOS transistor N 3 .  
         [0010]     Next, as shown in  FIG. 3B , if the clock signal CK turns to Vcc from 0V while the clock signal CKB turns to 0V from Vcc, the capacitor Ca starts to pump charges while the capacitor Cb does not conduct the pumping operation. Then, the NMOS transistor N 3  is turned off while the NMOS transistor N 4  is turned on. Thus, the node NA is boosted up to Vb−Vth 1 +Vcc from Vb−Vth 1  by the pumping operation. As a gate of the NMOS transistor N 4  is coupled to the node NA, the node NB decreases to Vb−Vth 1 +Vcc−Vth 2  by a threshold voltage Vth 2  of the NMOS transistor N 4 .  
         [0011]     And, as shown in  FIG. 3C , if the clock signal CK turns to 0V from Vcc while the clock signal CKB turns to Vccc from 0V, the capacitor Cb starts to pump charges again while the capacitor Ca does not conduct the pumping operation. Then, the NMOS transistor N 3  is turned on while the NMOS transistor N 4  is turned off. Thus, the node NB is boosted up to Vb−Vth 1 +Vcc−Vth 2 +rVcc, where r=Cb/(Cb+Cs), from Vb−Vth 1 +Vcc+Vth 2  by the pumping operation.  
         [0012]     Here, the maximum voltage gain at the node NB is Vb−Vth 1 +Vcc−Vth 2 +rVcc. A practical voltage gain at the node NB is lower than the maximum gain because the NMOS transistor N 4  is diode-coupled therein. Thereby, a gate voltage of NMOS transistor N 4  is affected from the voltage of node NA, node A decreasing when the voltage level of the node NA falls down.  
         [0013]     Accordingly, problems may occur when a voltage level transferred to the node NB, i.e., a charge amount (a source voltage of NMOS transistor N 4 ), becomes lower as time progresses as illustrated in  FIG. 4 .  
         [0014]     As a result, a time for transferring the high voltage VPP as an output signal by the high-voltage switch  11  increases as shown in  FIG. 5 , which increases even more as the power source voltage Vcc becomes lower.  
       SUMMARY OF THE INVENTION  
       [0015]     The present invention is directed to a high-voltage switching circuit shortening a boosting time of a drive signal for activating a high-voltage switch, reducing a time for transferring a high voltage as an output signal by the high-voltage switch.  
         [0016]     In one embodiment, a high-voltage switching circuit comprises: a high-voltage switch configured to transfer a high voltage; a pumping circuit configured to boost signals of first, second, and third nodes by conducting pumping operations in response to a plurality of clock signals; and a drive signal transmission circuit configured to boost the signal of the second node at a constant rate while maintaining a voltage level of the third node regardless of variation of a voltage level at the first node and transfer the boosted signal of the second node to the high-voltage switch, activating the high-voltage switch.  
         [0017]     In this embodiment, the high-voltage switching circuit also comprises a boosting enable circuit configured to activate the pumping circuit and the signal transmission circuit.  
         [0018]     In this embodiment, the pumping circuit comprises: a first pumping unit configured to boost the first node in response to a first clock signal; a second pumping unit configured to boost the second node in response to a second clock signal; and a third pumping unit configured to boost the third node in response to a third clock signal.  
         [0019]     In this embodiment, the drive signal transmission circuit comprises: first and second switching units coupled between a high voltage and the second node in series; and a third switching unit configured to maintain a voltage level of the third node at a constant level regardless of variation of a voltage level at the first node by isolating the first and third node from each other.  
         [0020]     In this embodiment, the drive signal transmission circuit comprises: a first switching unit coupled between a high voltage and the first node, having a gate coupled to the second node; a second switching unit coupled between the first node and the second node, having a gate coupled to the third node; and a third switching unit coupled between the first node and the third node, having a gate coupled to the second node.  
         [0021]     In this embodiment, the third switching unit separates the first node from the third node, maintaining a voltage level of the third node on a constant level regardless of variation of a voltage level at the first node.  
         [0022]     In this embodiment, the plurality of clock signals includes: a first clock signal enabled in a first pulse period; a second clock signal disabled in a second pulse period while the first clock signal is active (e.g., logic high), the second pulse period being narrower than the first pulse period; and a third clock signal disabled in a third pulse period while the first clock signal is active and the second clock signal is inactive (e.g., logic low), the third pulse period being narrower than the second pulse period.  
         [0023]     In another embodiment of the present invention, a high-voltage switching circuit comprises: a high-voltage switch configured to transfer a high voltage; a high-voltage switching booster configured to boost a signal for driving the high-voltage switch in response to a plurality of clock signals; and a boosting enable circuit configured to activate the high-voltage switching booster. The high-voltage switching booster comprises: a pumping circuit configured to boost signals of first, second, and third nodes by conducting pumping operations in response to the plurality of clock signals; and a drive signal transmission circuit configured to maintain a voltage level of the third node at a constant level regardless of variation of a voltage level at the first node and transfer the boosted drive signal to the high-voltage switch. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated herein and constitute a part of this specification. The drawings illustrate example embodiments of the present invention and, together with the description, serve to explain principles of the present invention. In the drawings:  
         [0025]      FIG. 1  is a circuit diagram illustrating a conventional high-voltage switching circuit;  
         [0026]      FIG. 2  is a diagram illustrating waveforms of clock signals operating in the high-voltage switching circuit of  FIG. 1 ;  
         [0027]      FIGS. 3A through 3C  are diagrams illustrating an operation of the high-voltage switching circuit of  FIG. 1 ;  
         [0028]      FIG. 4  is a timing diagram illustrating waveforms of signals at nodes NA and NB in the high-voltage switching circuit of  FIG. 1 ;  
         [0029]      FIG. 5  is a timing diagram illustrating time points outputting a high voltage VPPOUT through the high-voltage switching circuit of  FIG. 1 ;  
         [0030]      FIG. 6  is a circuit diagram illustrating a high-voltage switching circuit in accordance with one embodiment of the present invention;  
         [0031]      FIG. 7  is a diagram illustrating waveforms of clock signals operating in the high-voltage switching circuit of  FIG. 6 ;  
         [0032]      FIGS. 8A through 8D  are diagrams illustrating an operation of the high-voltage switching circuit of  FIG. 6 ;  
         [0033]      FIG. 9  is a timing diagram illustrating waveforms of signals at nodes NA, NB, and NC in the high-voltage switching circuit of  FIG. 6 ; and  
         [0034]      FIG. 10  is a timing diagram illustrating time points outputting a high voltage VPPOUT through the high-voltage switching circuit of  FIG. 6 . 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0035]     Embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be constructed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numerals refer to like elements throughout the specification.  
         [0036]     Hereinafter, an exemplary embodiment of the present invention will be described in conjunction with the accompanying drawings.  
         [0037]      FIG. 6  is a circuit diagram illustrating a high-voltage switching circuit in accordance with one embodiment of the present invention.  
         [0038]     Referring to  FIG. 6 , the high-voltage switch circuit  100  includes a high-voltage switch  110 , a boosting enable circuit  120 , and a high-voltage switching booster  130 .  
         [0039]     The high-voltage switch  110  includes an NMOS transistor N 1  configured to transfer a high voltage VPP as an output signal VPPOUT.  
         [0040]     The boosting enable circuit  120  includes inverters IV 1  and IV 2  and an NMOS transistor N 2 , activating the high-voltage switching booster  130  in response to an enable signal EN. The inverters, IV 1  and IV 2 , act as a buffer and output the enable signal EN, and the NMOS transistor N 2  is turned on by a power source voltage Vcc and transfers the enable signal EN to the high-voltage switching booster  130 .  
         [0041]     The high-voltage switching booster  130  includes three NMOS transistors N 11 ˜N 13  and three capacitors C 1 ˜C 3 . Here, Cs represents a parasitic capacitance. One electrode of the capacitor C 1  is coupled to the node NA and the other electrode of the capacitor C 1  is coupled to a clock signal CK 1 . One electrode of the capacitor C 2  is coupled to the node NB and the other electrode of the capacitor C 2  is coupled to a clock signal CK 2 . One electrode of the capacitor C 3  is coupled to the node NC and the other electrode of the capacitor C 3  is coupled to a clock signal CK 3 . The NMOS transistor N 11  is coupled between the terminal of the high voltage VPP and the node NA, responding to a signal from the node NB through its gate. The NMOS transistor N 12  is coupled between the node NA and the node NB, whose gate is coupled to the electrode of the capacitor C 3 . The NMOS transistor N 13  is coupled between the node NA and the node NC, responding to a signal from the node NB through its gate.  
         [0042]     NMOS transistor N 13  of the high-voltage switching circuit  100  shown in  FIG. 6  is able to maintain a gate voltage of the NMOS transistor N 12  at a constant level.  
         [0043]      FIG. 7  is a diagram illustrating waveforms of clock signals, CK 1 , CK 2 , and CK 3 , operating in the high-voltage switching circuit of  FIG. 6 , in which periods, t 1 , t 1 ′, t 2 , and t 2 ′, of the clock signals CK 1 , CK 2 , and CK 3  may be controllable with values over 0. The clock signal CK 1  is enabled (e.g., set to logic high) in a first pulse period. The clock signal CK 2  is disabled (e.g., set to logic low) in a second pulse period, which is narrower than the first pulse period, while the clock signal CK 1  is active (e.g., remains logic high). The clock signal CK 3  is enabled in a third pulse period, which is narrower than the second pulse period, while the clock signal CK 1  is active and the clock signal CK 2  is inactive (e.g., remains logic low).  
         [0044]      FIGS. 8A through 8D  are diagrams illustrating an operation of the high-voltage switching booster  130  shown in  FIG. 6 . Hereinafter, the method of maintaining the gate voltage of the NMOS transistor N 12  at a constant level will be described with reference to  FIGS. 7 , and  8 A through  8 D.  
         [0045]     First, as illustrated in  FIG. 8A , when the clock signal CK 1  is 0V, the clock signal CK 3  is 0V, and the clock signal CK 2  is Vcc (refer to  FIG. 7 ), the capacitors C 1  and C 3  do not conduct pumping operations while the capacitor C 2  conducts a pumping operation. As a result, NMOS transistors N 11  and N 13  are turned on and NMOS transistor N 12  is turned off. Thus, the node NB is boosted up to Vb by the pumping operation with the capacitor C 2 , while the voltage at node NA decreases to Vb−Vth 1  by a threshold voltage Vth 1  of the NMOS transistor N 11  because the node NB is coupled to the gate of the NMOS transistor N 11 . The voltage at node NC decreases to Vb−Vth 3  by a threshold voltage Vth 3  of the NMOS transistor N 13  because the node NB is coupled to the gate of the NMOS transistor N 13 .  
         [0046]     Next, as illustrated in  FIG. 8B , when the clock signal CK 1  turns to Vcc from 0V, the clock signal CK 3  remains at 0V, and the clock signal CK 2  remains at Vcc (refer to  FIG. 7 ), the capacitor C 1  conducts a pumping operation while the capacitor C 3  stops pumping and the capacitor C 2  remains pumping. Then, the NMOS transistors N 11  and N 13  maintain their off state while the NMOS transistor N 12  maintains its off state. Thus, the voltage at node NA is boosted to Vb−Vth 1 +Vcc from Vb−Vth 1  while the node NC maintains the voltage level of Vb−Vth 3  and the node NB maintains the voltage level of Vb.  
         [0047]     As illustrated in  FIG. 8C , when the clock signal CK 1  remains at Vcc, the clock signal CK 3  returns to 0V after being set to Vcc for a predetermined time, and the clock signal CK 2  turns to 0V from Vcc (refer to  FIG. 7 ), the capacitor C 1  keeps pumping and the capacitor C 3  conducts its pumping operation for a predetermined time (e.g., while CK 3  is active) while the capacitor C 2  stops pumping. Then, the NMOS transistor N 11  maintains its on state while the NMOS transistor N 12  is turned on for a time. The NMOS transistor N 13  is turned off. Thus, the node NA maintains the voltage level of Vb−Vth 1 +Vcc while the voltage at node NC is boosted to Vb−Vth 3 +Vcc for a predetermined time (the gate voltage of the NMOS transistor N 12  maintains the voltage level of Vb−Vth 3 +Vcc without being affected from variation of a voltage level at the node NA). The voltage at node NB decreases to Vb−Vth 3 +Vcc−Vth 2  by the threshold voltage Vth 2  of the NMOS transistor N 12  because the node NC is coupled to the gate of the NMOS transistor N 12 .  
         [0048]     And next, as illustrated in  FIG. 8D , when the clock signal CK 1  remains at Vcc, the clock signal CK 3  is set to 0V, and the clock signal CK 2  turns to Vcc from 0V (refer to  FIG. 7 ), the capacitor C 1  keeps pumping while the capacitor C 3  stops pumping and the capacitor C 2  resumes its pumping operation. Then, the NMOS transistor N 11  maintains its on state while the NMOS transistor N 12  is turned off and the NMOS transistor is turned on again. Thus, the node NA maintains the voltage level of Vb−Vth 1 +Vcc while the voltage at node NB is boosted to Vb−Vth 3 +Vcc−Vth 2 +rVcc (here, r is Cb/(Cb+Cs) in consideration of parasitic capacitance). And, the voltage at node NC remains at Vb−Vth 3 +Vcc by directly transferring a drain voltage of the NMOS transistor N 13  thereto without a voltage drop because a gate voltage of the NMOS transistor N 13 , Vb−Vth 3 +Vcc−Vth 2 +rVcc, is higher than the drain voltage Vb−Vth 3 +Vcc.  
         [0049]     As stated above, by the operation of the high-voltage switching booster  130 , the gate voltage of the NMOS transistor N 12  maintains the level of Vb−Vth 3 +Vcc regardless of variation of the voltage level at the node NA because the gate of the NMOS transistor N 12 , i.e., the node NC, is separated from the drain of the NMOS, i.e., the node NA, transistor N 12  by the NMOS transistor N 13  after the step of  FIG. 8C  where the clock signal CK 3  goes to a logic high level.  
         [0050]      FIG. 9  is a timing diagram illustrating waveforms of signals at nodes NA, NB, and NC in the high-voltage switching circuit of  FIG. 6 , as a result of simulation for the high-voltage switching booster, and  FIG. 10  is a timing diagram illustrating time points outputting the high voltage output signal VPPOUT through the high-voltage switching circuit of  FIG. 6 .  
         [0051]     Referring to  FIG. 9 , a voltage level of the source of the NMOS transistor N 12 , i.e., the voltage level of the node NB (i.e., the solid black line), rises because the voltage level of the node NC, i.e., the gate voltage of the NMOS transistor N 12 , becomes high as time progresses.  
         [0052]     As illustrated in  FIG. 10 , times for outputting the high voltage signal VPPOUT are shown. The voltage VPPOUT has a boosting characteristic despite decreases in the voltage of Vcc.  
         [0053]     The present invention can transfer a high voltage faster than a conventional case although the power source voltage Vcc decreases.  
         [0054]     The present invention is advantageous in improving the efficiency of high-voltage switching operation in the application with a low power source voltage.  
         [0055]     Although the present invention has been described in connection with specific embodiments of the present invention illustrated in the accompanying drawings, it is not limited thereto. It will be apparent to those ordinary skilled in the art that various substitutions, modifications and changes may be made thereto without departing from the scope and spirit of the invention.
A high-voltage switching circuit comprises: a high-voltage switch configured to transfer a high voltage; a pumping circuit configured to boost signals of first, second, and third nodes by conducting pumping operations in response to a plurality of clock signals; and a drive signal transmission circuit configured to boost the signal of the second node at a constant rate while maintaining a voltage level of the third node regardless of variation of a voltage level at the first node and transfer the boosted signal of the second node to the high-voltage switch, activating the high-voltage switch.