Abstract:
A multi stage voltage pumping circuit includes a plurality of voltage pumping stages each operated by a first clock signal and a second clock signal for raising a voltage level of an inputted voltage; and a plurality of charge storing means each connected to outputs of the plurality of voltage pumping stages respectively except for a last voltage pumping stage in order to store charge, wherein each of the plurality of voltage pumping stages is a cross coupled voltage doubler and an output of a previous voltage pumping stage is connected to an input of a next voltage pumping stage.

Description:
FIELD OF INVENTION 
   The present invention relates to a semiconductor device; and, more particularly, to a multi stage voltage pumping circuit capable of raising a voltage level of a power supply voltage for obtaining a high voltage. 
   DESCRIPTION OF PRIOR ART 
   Generally, an integrated semiconductor circuit includes a voltage pumping circuit for raising a voltage level of a power supply voltage supplied from an external circuit of the integrated semiconductor circuit, and, thus to obtain a required high voltage. For instance, the voltage pumping circuit is applied for generating a voltage required to operate a word line included in a dynamic random access memory (DRAM) or for generating a high voltage power supply in a flash memory. 
     FIG. 1  is a schematic circuit diagram showing a conventional multi stage voltage pumping circuit. 
   As shown, the conventional multi stage voltage pumping circuit includes a plurality of N-channel metal oxide semiconductor (NMOS) transistors M 1  to M N  connected in series, and each of the plurality of NMOS transistors M 1  to M N  is diode-connected by shorting a gate and a drain in order to control current flow from a power supply voltage VDD to an output voltage VOUT. 
   There are a plurality of connection nodes V 1  between the NMOS transistor M 1  and the NMOS transistor M 2 , V 2  between the NMOS transistor M 2  and the NMOS transistor M 3 , . . . and V N−1  between the NMOS transistor M N−1 , and the NMOS transistor M N . A plurality of pumping capacitors C 1  to C N−1  are connected to the plurality of connection nodes V 1  to V N−1  respectively. A clock signal clk is inputted to the capacitor C 1 , the capacitor C 3 , . . . and the capacitor C N−1 . An inverted signal of the clock signal clk, i.e., a clock bar signal clkb is inputted to the capacitor C 2 , the capacitor C 4 , . . . and the capacitor C N−2 . 
   Each charge stored in the plurality of pumping capacitors C 1  to C N−1  is pumped to the plurality of connection nodes V 1  to V N−1  respectively by the clock signal clk and the clock bar signal clkb, in order to raise voltage levels of the plurality of connection nodes V 1  to V N−1 . 
   A load capacitor C L  is connected to the output voltage VOUT so that the output voltage VOUT can be loaded on the load capacitor C L . 
   If the clock signal clk is in a logic LOW level and the clock bar signal clkb is in a logic HIGH level, a voltage of VDD−Vth is loaded on the node V 1 , and a charge Q 1  having amount of C 1 ×(VDD−Vth) is stored in the capacitor C 1 , wherein the Vth is a threshold voltage. 
   Thereafter, if the clock signal clk is changed into a logic HIGH level and the clock bar signal clkb is changed into a logic LOW level, the node V 1  keeps the charge Q 1  and has a voltage of VDD+(VDD−Vth) according to the electric charge conservation law. Since the NMOS transistor M 1  is diode-connected, it is turned off while V N  is greater than V N−1 . As a result, a voltage of the node V 1 , i.e., VDD+(VDD−Vth) is transferred to the node V 2  so that the node V 2  has a voltage of 2VDD−2Vth. 
   Likewise, as the clock signal clk and the clock bar signal clkb keep toggling, the output voltage VOUT becomes a required high voltage. 
   However, at each of the plurality of the NMOS transistors M 1  to M N , there occurs a voltage loss due to the threshold voltage Vth, and each of the plurality of the NMOS transistors M 1  to to M N  consumes a power of I load ×Vth×N. Therefore, the conventional multi stage voltage pumping circuit has low efficiency of transferring current. 
   SUMMARY OF INVENTION 
   It is, therefore, an object of the present invention to provide a multi stage pumping circuit for generating a required high voltage having ability of preventing unnecessary power consumption of each stage of the multi stage pumping circuit, and, thus to prevent low efficiency of transferring current. 
   In accordance with an aspect of the present invention, there is provided a multi stage voltage pumping circuit including a plurality of voltage pumping stages each operated by a first clock signal and a second clock signal for raising a voltage level of an inputted voltage; and a plurality of charge storing means each connected to outputs of the plurality of voltage pumping stages respectively except for a last voltage pumping stage in order to store charge, wherein each of the plurality of voltage pumping stages is a cross coupled voltage doubler and an output of a previous voltage pumping stage is connected to an input of a next voltage pumping stage. 
   In accordance with an another aspect of the present invention, there is provided a multi stage voltage pumping circuit including a clock signal generating means for generating a first clock signal and a second clock signal; a plurality of voltage pumping stages each operated by the first clock signal and the second clock signal for raising a voltage level of an inputted voltage; and a plurality of charge storing means each connected to outputs of the plurality of voltage pumping stages respectively except for a last voltage pumping stage in order to store charge, wherein the first clock signal is an inverted signal of the second clock signal and is not overlapped with the second clock signal, and each of the plurality of voltage pumping stages is a cross coupled voltage doubler operated by the first clock signal and the second clock signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
       FIG. 1  is a schematic circuit diagram showing a conventional multi stage voltage pumping circuit; 
       FIG. 2  is a schematic circuit diagram showing a conventional cross coupled voltage doubler; 
       FIG. 3  is a block diagram showing a multi stage voltage pumping circuit in accordance with the present invention; 
       FIG. 4  is a circuit diagram showing a non overlapping clock generator; 
       FIG. 5  is a schematic circuit diagram showing a multi stage voltage pumping circuit in accordance with a first preferred embodiment of the present invention; 
       FIG. 6  is a simulated timing diagram showing an operation of the multi stage voltage pumping circuit in accordance with the first preferred embodiment of the present invention; 
       FIG. 7  is a schematic circuit diagram showing a multi stage voltage pumping circuit in accordance with a second preferred embodiment of the present invention; and 
       FIG. 8  is a simulated timing diagram showing an operation of the multi stage voltage pumping circuit in accordance with the second preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF INVENTION 
   Hereinafter, a multi stage voltage pumping circuit in accordance with the present invention will be described in detail referring to the accompanying drawings. 
     FIG. 2  is a schematic circuit diagram showing a cross coupled voltage doubler generally used for a high voltage oscillator. 
   An operation of the cross coupled voltage doubler is described below for explaining the principle of the present invention. 
   As shown, the cross coupled voltage doubler includes a first pumping capacitor C 21 , a second pumping capacitor C 22 , a first N-channel metal oxide semiconductor (NMOS) transistor pair M 21  and M 22  and a first P-channel metal oxide semiconductor (PMOS) transistor pair M 23  and M 24 . 
   One side of the first pumping capacitor C 21  receives a clock bar signal clkb and the other side is connected to a first node V 21 . Likewise, one side of the second pumping capacitor C 22  receives a clock signal clk and the other side is connected to a second node V 22 . 
   A gate of the NMOS transistor M 21 , a drain of the NMOS transistor M 22 , a gate of the NMOS transistor M 23  and a source of the NMOS transistor M 24  are connected to the second node V 22 . Likewise, a gate of the NMOS transistor M 24 , a source of the NMOS transistor M 23 , a drain of the NMOS transistor M 21  and a gate of the NMOS transistor M 22  are connected to the first node V 21 . The first NMOS transistor pair M 21  and M 22  transfers a power supply voltage VDD to the first node V 21  and the second node V 22 . The first PMOS transistor pair M 23  and M 24  transfers voltage loaded on the first node V 21  and the second node V 22  to an output voltage VOUT. 
   As shown above, since PMOS transistors and NMOS transistors included in the cross coupled voltage doubler are cross-connected, switch gates, i.e., gates of the first NMOS transistor pair M 21  and M 22  are controlled by higher voltage level than that of VDD+Vth, i.e., voltage levels of the first and the second nodes V 21  and V 22  are 2VDD. Herein, the Vth is a threshold voltage. Therefore, it occurs no voltage loss due to a threshold voltage. Therefore, the cross coupled voltage doubler can generate the output voltage VOUT having voltage level of 2VDD from the power supply voltage VDD. 
   The multi stage voltage pumping circuit in accordance with the present invention applies the cross coupled voltage doubler for generating a required high voltage stably having high efficiency of transferring current. 
     FIG. 3  is a block diagram showing the multi stage voltage pumping circuit in accordance with the present invention. 
   As shown, the multi stage voltage pumping circuit includes N numbers of voltage pumping stages VPS 1  to VPSN connected in series between a power supply voltage VDD and an output voltage VOUT. Each of the voltage pumping stages VPS 1  to VPSN is a cross-coupled voltage doubler described in  FIG. 2  operating in response to a clock signal clk and a clock bar signal clkb. 
   There are a first charge storing unit CS 1  connected between the first voltage pumping stage VPS 1  and the second voltage pumping stage VPS 2 , a second charge storing unit CS 2  connected between the second voltage pumping stage VPS 2  and the third voltage pumping stage VPS 3 , . . . and a (N−1)th charge storing unit CS(N−1) connected between the (N−1)th voltage pumping stage VPS(N−1) and the Nth voltage pumping stage VPSN. 
   The clock bar signal clkb is an inverted signal of the clock signal clk and is not overlapped with the clock signal clk.  FIG. 4  is a circuit diagram showing a non-overlapping clock generator for generating the clock signal clk and the clock bar signal clkb. 
   As described above, since the multi stage voltage pumping circuit adopts N numbers of cross coupled voltage doublers as voltage pumping stages, there occurs no power loss due to a threshold voltage of a MOS transistor, and, thus, unnecessary power consumption and low efficiency of power transfer can be prevented. In addition, since the multi stage voltage pumping circuit stably generates a required high voltage from low voltage, the multi stage voltage pumping circuit is useful for a semiconductor memory device which consumes low voltage to be operated. 
     FIG. 5  is a schematic circuit diagram showing a multi stage voltage pumping circuit in accordance with a preferred embodiment of the present invention. 
   As shown, the multi stage voltage pumping circuit includes a first voltage pumping stage  410 A, a second voltage pumping stage  410 B, a third voltage pumping stage  410 C, a first charge storing unit  420 A and a second charge storing unit  420 B. 
   The first voltage pumping stage  410 A doubles a power supply voltage VDD and transfers the doubled power supply voltage VDD to the first charge storing unit  420 A. The second voltage pumping stage  410 B doubles voltage loaded in the first charge storing unit  420 A and transfers the doubled voltage to the second charge storing unit  420 B. Likewise, the third voltage pumping stage  410 C doubles voltage loaded in the second charge storing unit  420 B and outputs the doubled voltage as an output voltage VOUT. As a result, a voltage level of the output voltage VOUT is four times higher than that of the power supply voltage VDD. 
   The first charge storing unit  420 A includes two capacitors C 43  and C 44  connected to a first node N 41  and a second node N 42  respectively for storing voltage transferred from the first voltage pumping stage  410 A. Likewise, the second charge storing unit  420 B includes two capacitors C 47  and C 48  connected to a third node N 43  and a fourth node N 44  respectively for storing voltage transferred from the second voltage pumping stage  410 B. 
   The first voltage pumping stage  410 A includes a first NMOS transistor M 41 , a second NMOS transistor M 42 , a first PMOS transistor M 43 , a second PMOS transistor M 44 , a first pumping capacitor C 41  and a second pumping capacitor C 42 . 
   The first NMOS transistor M 41  whose gate is connected to a second pumping node PN 2  is connected between the power supply voltage VDD and a first pumping node PN 1 . The second NMOS transistor M 42  whose gate is connected to the first pumping node PN 1  is connected between the power supply voltage VDD and the second pumping node PN 2 . 
   The first PMOS transistor M 43  whose gate is connected to the second pumping node PN 2  is connected between the first pumping node PN 1  and the first node N 41 . The second PMOS transistor M 44  whose gate is connected to the first pumping node PN 1  is connected between the second pumping node PN 2  and the second node N 42 . 
   One side of the first pumping capacitor C 41  receives a clock signal clk and the other side is connected to the first pumping node PN 1 . Likewise, one side of the second pumping capacitor C 42  receives a clock bar signal clkb and the other side is connected to the second pumping node PN 2 . 
   When the first voltage pumping stage  410  is operated normally after it is initialized, an operation of the first voltage pumping stage  410 A is described below. 
   If the clock signal clk becomes in a logic HIGH level and the clock bar signal clkb becomes in a logic LOW level, the second NMOS transistor M 42  and the first PMOS transistor M 43  are turned on, and the first NMOS transistor M 41  and the second PMOS transistor M 44  are turned off. 
   At this time, the power supply voltage VDD is loaded on the second pumping node PN 2  and voltage of 2VDD is loaded on the first pumping node PN 1 . Herein, it is assumed that a parasitic capacitance for each of the first and the second NMOS transistors M 41  and M 42  and the first and the second PMOS transistors M 43  and M 44  is ignored. As a result, the voltage of 2VDD is loaded on the first node N 41  due to the first PMOS transistor M 43 . 
   Likewise, if the clock signal clk becomes in a logic LOW level and the clock bar signal clkb becomes in a logic LOW level, the second NMOS transistor M 42  and the first PMOS transistor M 43  are turned off, and the first NMOS transistor M 41  and the second PMOS transistor M 44  are turned on. 
   Therefore, the power supply voltage VDD is loaded on the first pumping node PN 1  and voltage of 2VDD is loaded on the second pumping node PN 2 . As a result, the voltage of 2VDD is loaded on the second node N 42  due to the second PMOS transistor M 44 . 
   The structure of the second voltage pumping stage  410 B and the third voltage pumping stage  410 C is the same as that of the first voltage pumping stage  410 A except that a third pumping capacitor C 45  and a fourth pumping capacitor C 46  included in the second voltage pumping stage  410 B are operated by the clock bar signal clkb and the clock signal clk respectively. 
   That is, each pumping capacitor is operated by the clock bar signal clkb or the clock signal clk having a different level from that of the pumping capacitor included in the neighboring voltage pumping stage. 
   For instance, the third pumping capacitor C 45  is operated by the clock bar signal clkb, but the first pumping capacitor C 41  and a fifth pumping capacitor C 49  are operated by the clock signal clk. 
   An operation of the second voltage pumping stage  410 B is the same as that of the first voltage pumping stage  410 A except that the second voltage pumping stage  410 B receives the voltage of 2VDD from the first charge storing unit  420 A when the first and second nodes N 41  and N 42  are in a steady state. 
   Therefore, the second voltage pumping stage  410 B generates voltage whose level is three times higher than that of the power supply voltage VDD, i.e., 3VDD. Likewise, the third voltage pumping stage  410 C generates voltage whose level is four times higher than that of the power supply voltage VDD, i.e., 4VDD. 
     FIG. 6  is a timing diagram showing a simulated result of an operation of the multi stage voltage pumping circuit. 
   As shown, a voltage level of the first pumping node PN 1  becomes two times higher than that of the power supply voltage VDD after a predetermined time, and, finally, the output voltage VOUT is generated having voltage of 4VDD. Herein, the power supply voltage VDD is set to be 1.8V and the predetermined time is about 1.50E–06 sec. 
   The output voltage VOUT should have voltage of 4VDD, i.e., 7.2V, ideally. However, the output voltage VOUT can not reach an ideal voltage of 4VDD because each MOS transistor included in the multi stage voltage pumping circuit has a parasitic capacitance and there exists a coupling between charge transferring capacitance. 
     FIG. 7  is a schematic circuit diagram showing a multi stage voltage pumping circuit in accordance with a second preferred embodiment of the present invention. 
   As shown, the multi stage voltage pumping circuit includes a first voltage pumping stage  710 A, a second voltage pumping stage  710 B and a charge storing unit  720 . Each of the first voltage pumping stage  710 A and the second voltage pumping stage  710 B is a cross coupled voltage doubler which serves to double its input voltage. 
   The first voltage pumping stage  710 A receives a power supply voltage VDD and doubles a voltage level of the power supply voltage VDD for transferring the doubled power supply voltage VDD to the charge storing unit  720 . 
   Likewise, the second voltage pumping stage  710 B doubles a voltage level of the doubled power supply voltage VDD stored in the charge storing unit  720 , and, thus to output an output voltage VOUT whose voltage level is three times higher than that of the power supply voltage VDD. 
   The charge storing unit  720  includes a first capacitor C 73  and a second capacitor C 74 . One side of the first capacitor C 73  is connected to the first node N 41  and the other side is grounded. Likewise, one side of the second capacitor C 74  is connected to the second node N 42  and the other side is grounded. 
   The first voltage pumping stage includes a first NMOS transistor M 71 , a second NMOS transistor M 72 , a first PMOS transistor M 73 , a second PMOS transistor M 74 , a first pumping capacitor C 71  and a second pumping capacitor C 72 . 
   The first NMOS transistor M 71  is connected between the power supply voltage VDD and a first pumping node PN 71 , and a gate of the first NMOS transistor M 71  is connected to a second pumping node PN 72 . The second NMOS transistor M 72  is connected between the power supply voltage VDD and the second node PN 72 , and a gate of the second NMOS transistor M 72  is connected to the first pumping node PN 71 . 
   The first PMOS transistor M 73  is connected between the first pumping node PN 71  and the first node N 41 , and a gate of the first PMOS transistor M 73  is connected to the second pumping node PN 72 . The second PMOS transistor M 74  is connected between the second pumping node PN 72  and the second node N 42 , and a gate of the second PMOS transistor M 74  is connected to the first pumping node PN 71 . 
   One side of the first pumping capacitor C 71  is connected to the first pumping node PN 71  and the other side receives a clock signal clk. Likewise, one side of the second pumping capacitor C 72  is connected to the second pumping node PN 72  and the other side receives a clock bar signal clkb. 
   As shown in  FIG. 7 , the structure of the second voltage pumping stage  710 B is the same as that of the first voltage pumping stage  710 A. 
     FIG. 8  is a timing diagram showing a simulated result of an operation of the multi stage voltage pumping circuit. 
   As shown, a voltage level of the first pumping node PN 1  becomes two times higher than that of the power supply voltage VDD after a predetermined time, and, finally, the output voltage VOUT is generated having voltage of 3VDD. Herein, the power supply voltage VDD is set to be 1.8V and the predetermined time is about 7.00E–07 sec. 
   The output voltage VOUT should have voltage of 3VDD, i.e., 5.4V, ideally. However, as shown in  FIG. 8 , the output voltage VOUT does not reach an ideal voltage of 3VDD because each MOS transistor included in the multi stage voltage pumping circuit has a parasitic capacitance and there exists a coupling between charge transferring capacitance. 
   As described above, the multi stage voltage pumping circuit in accordance with the present invention adopts N numbers of cross coupled voltage doublers as voltage pumping stages for obtaining a required high voltage whose voltage level is N times higher than that of an inputted power supply voltage. 
   Since a cross coupled voltage doubler is adopted as a voltage pumping stage, there occurs no power loss due to a threshold voltage of a MOS transistor included in a voltage pumping stage, and, thus, high efficiency of power transfer can be obtained. 
   In addition, since the multi stage voltage pumping circuit stably generates a required high voltage from a low voltage, the multi stage voltage pumping circuit can be nicely applied to a semiconductor memory device which uses a low power supply voltage. 
   The present application contains subject matter related to Korean patent application No. 2003-76257, filed in the Korean Patent Office on Oct. 30, 2003, the entire contents of which being incorporated herein by reference. 
   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.