PATENT ABSTRACT
A multiphase charge pump including first and second phase charge pump circuits. Each of the first and second phase charge pump circuits includes a bootstrap capacitor. The bootstrap capacitors are switchingly connected by an equalization circuit that periodically transfers charge from a discharging capacitor to a charging capacitor, thereby reducing the charge that must be externally supplied to charge the charging capacitor.

PATENT DESCRIPTION
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a charge pump, and more particularly to a charge pump adapted to be included on an integrated circuit for converting a supplied voltage to a desired voltage.  
       BACKGROUND OF THE INVENTION  
       [0002]     It is often useful to provide a voltage to a component on an integrated circuit chip that exceeds a voltage supplied to the chip. Elevated voltages are employed, for example, on DRAM integrated circuits for boosted wordline voltages and negative substrate bias voltages, and for writing and erasing EEPROMS. By generating requisite elevated voltages on the integrated circuit itself, the need for one or more external power supplies is eliminated.  
         [0003]     On an integrated circuit, inductors are more difficult to implement than capacitors. Thus, where various voltages are needed on an integrated circuit, it is advantageous to use a capacitive charge pump capable of transforming voltage without the use of inductors.  
         [0004]     Two important parameters of charge pump operation are capacity and efficiency. Capacity is a measure of how much current a pump can continuously supply. Capacity is determined in part by the size of a bootstrap capacitor and operating frequency. Efficiency is a measure of how much charge, or current, is wasted during each pump cycle. A typical prior art integrated circuit charge pump is 30-50% efficient. This translates into a loss of 2-3 milliamps of supply current for every milliamp of pump output current.  
         [0005]      FIG. 1  shows a conventional single-phase charge pump  10  adapted to receive a first voltage Vcc  12  as an input and provide a second higher voltage as an output. The single phase charge pump includes an inverter  14  having an input  16  adapted to receive an oscillating signal. Also included is a bootstrap capacitor  20  having a driving side  22  and a driven side  24 . The inverter is adapted to connect the driving side  22  of capacitor  20  alternately between VCC  12  and ground  30 . The driven side  24  of the bootstrap capacitor is operatively connected through a first diode  34  to a source of supply voltage Vcc  12 , and through a second diode  36  to a load  38 . The load  38  is operatively connected between the second diode  36  and ground  30 .  
         [0006]     Assuming ideal components, the circuit of  FIG. 1  operates as follows: at a first time, an input signal applied at input  16  of the inverter  14  is high, causing the inverter to connect the driving side  22  of capacitor  20  to ground  30 . Responsively, current flows through the first diode  34 , transporting electrical charge from the source of supply voltage Vcc  12  to the driven side  24  of the bootstrap capacitor  20 . As charge accumulates on the driven side  24  of the bootstrap capacitor  20 , voltage Vcc  12  develops across the capacitor. At a later time, the signal at the input  16  of the inverter  14  goes low. This connects the driving side  22  of the capacitor  20  to the source of supply voltage Vcc  12 . Charge flows into the driving side  22  of the capacitor  20 , and the voltage on the driven side  24  of the capacitor rises to 2 Vcc with respect to ground in response. Current flows through the second diode  36  to apply a voltage of 2 Vcc to an input  40  of the load  38 . After the voltage on the driving side of the bootstrap capacitor has risen to Vcc, the signal at the input  16  of inverter  14  transitions again causing the voltage on the driving side  22  of capacitor  20  to go to ground. The charge pump cycle is then complete. With repeated cycles, a pulsed voltage of more or less 2 Vcc can be maintained across the load.  
         [0007]     As actually constructed, the single-phase charge pump circuit of  FIG. 1  is relatively inefficient, and it produces an output voltage that varies significantly with time. Also, for non-ideal components, the output voltage is limited to two times Vcc less at least two diode threshold voltage drops (2 V t ). Accordingly, various improvements have been made to improve charge pump performance as shown for example in U.S. Pat. No. 6,294,948, the disclosure of which is incorporated herein by reference. Circuitry has been developed to bring output voltage up to two times Vcc or higher. It is also known to mutually connect the outputs of two single-phase charge pump circuits, operated out of phase with one another, to reduce ripple and achieve a more constant output voltage. Nonetheless, it is desirable to provide an improved charge pump circuit, and in particular a charge pump circuit which is more efficient than previous designs.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     The present invention includes a multiphase charge pump in which charge is shared between the respective driving sides of respective first and second bootstrap capacitors of a first phase circuit and a second phase circuit, to improve operating efficiency and lower current requirements.  
         [0009]     According to the present invention, a charge pump is provided including a first phase circuit having a first bootstrap capacitor, a second phase circuit having a second bootstrap capacitor, and an equalization circuit adapted to transfer charge between the first bootstrap capacitor and the second bootstrap capacitor. The equalization circuit is operated according to a control signal such as a timing signal. The control signal causes a transfer of charge in a manner which reduces the requirement for input current supplied by an attached power supply, and also reduces waste current. In particular, when the driving side plate of the first capacitor is at ground potential, and the driving side plate of the second capacitor is at elevated potential, the equalization circuit is switched between the two driving sides prior to connection of the first (ground potential) capacitor to Vcc  12  and prior to connection of the second (elevated potential) capacitor to ground. In this manner, charge that would have been dumped to ground from the second capacitor, and wasted, is conducted to the first capacitor, which is due to be charged. The complementary operation occurs when the driving side of the second capacitor is at ground potential and the driving side of the first capacitor is at elevated potential. This reduces current sinking and power supply requirements, and improves charge pump efficiency.  
         [0010]     These and other aspects and features of the invention will be more clearly understood from the following detailed description which is provided in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a schematic representation of a conventional single phase charge pump circuit;  
         [0012]      FIG. 2  is a schematic representation of a two phase charge pump circuit according to one aspect of the invention;  
         [0013]      FIG. 3  illustrates the timing relationships of clock signals which are used to drive the circuit of  FIG. 2 ;  
         [0014]      FIG. 4  ( 4 A and  4 B) is a schematic representation of a four phase charge pump according to one embodiment of the invention;  
         [0015]      FIG. 5  is a schematic representation of six equalization circuits adapted to transfer charge between the bootstrap capacitors of  FIGS. 4A and 4B ; and  
         [0016]      FIG. 6  is a schematic representation of a timing circuit adapted to generate timing signals to operate a four phase charge pump as shown in  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]      FIG. 2  shows a two-phase charge pump  80  according to one aspect of the invention. A first phase circuit  82  of the charge pump  80  includes a first bootstrap capacitor  84  having first  86  and second  88  terminals. The first terminal  86  of the first capacitor  84  is connected to an output  90  of a first driver circuit  92 . The first driver circuit  92  includes first  94  and second  96  complementary transistors operatively connected in series between ground  30  and Vcc  12 . The output  90  of the first driver  92  is at a mutual connection of the first  94  and second  96  transistors. The first  94  and second  96  transistors have respective first  100  and second  102  gates. The gates  100 ,  102  are connected to respective sources of respective driving signals  158 ,  159 . The second terminal  88  of the first capacitor  84  is connected through a diode-connected third transistor  104  to Vcc  12 . A fourth diode-connected transistor  114  is operatively connected between the second terminal  88  of the first capacitor  84  and a load circuit  38 .  
         [0018]     A second phase circuit  120  of the charge pump  80  includes a second bootstrap capacitor  122  having third  124  and fourth  126  terminals. The third terminal  124  of the second capacitor  122  is connected to an output  128  of a second driver circuit  130 . The second driver circuit  130  includes fifth  132  and sixth  134  complementary transistors operatively connected in series between Vcc  12  and ground  30 . The output  128  of the second driver  130  is at a mutual connection of the fifth  132  and sixth  134  transistors. The fifth  132  and sixth  134  transistors have respective third  136  and fourth  138  gates. The gates  136 ,  138  are connected to respective sources of respective driving signals  160 ,  161 . The fourth terminal  126  of the second capacitor  122  is connected through a diode-connected seventh transistor  140  to Vcc  12 . An eighth diode-connected transistor  142  is operatively connected between the fourth terminal  126  of the second capacitor  122  and the load circuit  38 . Accordingly, the respective sources of the fourth  114  and eighth  142  diode-connected transistors are mutually connected. A ninth transistor  144  is operatively connected between the respective outputs ( 90 ,  128 ) of the first  92  and second  130  drivers. The ninth transistor includes a fifth gate  146  adapted to receive an equalization signal  148 .  
         [0019]     In conjunction with  FIG. 2 ,  FIG. 3  illustrates the operation of the circuit of  FIG. 2 .  FIG. 3  graphs the control signals that control the circuit of  FIG. 2 , and the resulting voltages on the bootstrap capacitors. The control signals are configured to cyclically raise and lower the electrical potential of the driving side of each bootstrap capacitor. The voltages on the capacitors  84 ,  122  are 180 degrees out of phase with one another, as shown by signals  163 ,  164  respectively. Signals  158 ,  159 ,  160 , and  161  drive gates  100 ,  102 ,  136 , and  138  respectively. Signal  148  drives gate  146  of the equalization transistor  144 . Signals  163  and  164  show the variation with time of the electrical potentials on the respective driving sides  86 ,  124  of the first  84  and second  122  bootstrap capacitors.  
         [0020]     Assuming ideal components, the circuit of  FIG. 2  operates as follows. At an initial time t 0  the first transistor  94  is conductive in response to an applied high first gate signal  158  and the second transistor and  96  is non-conductive in response to an applied high second signal  159  so that the output  90  of the first driver  92  is at ground potential  30 . In addition, the sixth transistor  132  is non-conductive in response to an applied low third signal  160  which is the inverse of the second signal  159  and the seventh transistor  134  is conductive in response to a low fourth signal  161  which is the inverse of the first signal  158  so that the output  128  of the second driver  130  is at Vcc  12 . In response, charge flows from Vcc  12  through diode connected transistor  104  to the driven side  88  of the first bootstrap capacitor  84  to set the driven side  88  of capacitor  84  to Vcc  12 . connected seventh transistor  140  to Vcc  12 . An eighth diode-connected transistor  142  is operatively connected between the fourth terminal  126  of the second capacitor  122  and the load circuit  38 . Accordingly, the respective sources of the fourth  114  and eighth  142  diode-connected transistors are mutually connected. A ninth transistor  144  is operatively connected between the respective outputs ( 90 ,  128 ) of the first  92  and second  130  drivers. The ninth transistor includes a fifth gate  146  adapted to receive an equalization signal  148 .  
         [0021]     In conjunction with  FIG. 2 ,  FIG. 3  illustrates the operation of the circuit of  FIG. 2 .  FIG. 3  graphs the control signals that control the circuit of  FIG. 2 , and the resulting voltages on the bootstrap capacitors. The control signals are configured to cyclically raise and lower the electrical potential of the driving side of each bootstrap capacitor. The voltages on the capacitors  84 ,  122  are 180 degrees out of phase with one another, as shown by signals  163 ,  164  respectively. Signals  158 ,  159 ,  160 , and  161  drive gates  100 ,  102 ,  136 , and  138  respectively. Signal  148  drives gate  146  of the equalization transistor  144 . Signals  163  and  164  show the variation with time of the electrical potentials on the respective driving sides  86 ,  124  of the first  84  and second  122  bootstrap capacitors.  
         [0022]     Assuming ideal components, the circuit of  FIG. 2  operates as follows. At an initial time t 0  the first transistor  94  is conductive in response to an applied high first gate signal  158  and the second transistor and  96  is non-conductive in response to an applied high second signal  159  so that the output  90  of the first driver  92  is at ground potential  30 . In addition, the sixth transistor  132  is non-conductive in response to an applied low third signal  160  which is the inverse of the second signal  159  and the seventh transistor  134  is conductive in response to a low fourth signal  161  which is the inverse of the first signal  158  so that the output  128  of the second driver  130  is at Vcc  12 . In response, charge flows from Vcc  12  through diode connected transistor  104  to the driven side  88  of the first bootstrap capacitor  84  to set the driven side  88  of capacitor  84  to Vcc  12 . Simultaneously, charge flows from Vcc  12  through the seventh transistor  134  and is stored on the driving side  124  of the second bootstrap capacitor  122 . At a particular transition time t 1 , the timing circuit sets signal  158  low and signal  161  high. As a result, first  94 , second  96 , sixth  132 , and seventh  134  transistors are all non-conductive. At the same time, equalization signal  148  is applied to the gate  146  of the equalization transistor  144  causing the equalization transistor  144  to become conductive. This connects the driving side  124  of the second bootstrap capacitor  122  to the driving side  86  of the first bootstrap capacitor  84 . Consequently, charge (and energy) flows from the second bootstrap capacitor  122  to the first bootstrap capacitor  84  through the equalization transistor  144  until the voltage on the two driving side terminals  124 ,  86 , of the two bootstrap capacitors  122 ,  84  respectively, has equalized. Thereafter, at time t 2 , signal  148  transitions, the equalization transistor  144  is made non-conductive, and the second  96  and sixth  132  transistors are made conductive by the timing signals  159  and  160  respectively. Accordingly charge flows from Vcc  12  through the second transistor  96  to the driving side  86  of the first bootstrap capacitor  84 . This causes the potential of the driven side  88  of the first bootstrap capacitor  84  to rise to approximately 2 Vcc. At the same time, signal  160  turns on transistor  132  bringing the potential of the driving side  124  of capacitor  122  to ground. Current flows responsively through diode connected transistor  140  from VCC  12  to the driven side  126  of the second bootstrap capacitor  122 , charging that capacitor to a potential of VCC  12 . Thereafter, at time t 3  signals  159  and  160  again transition, making transistors  96  and  132  nonconductive. Simultaneously, signal  148  transitions making the equalization transistor  144  conductive, and allowing charge to flow from the driving side  86  of the first bootstrap capacitor  84  to the driving side  124  of the second bootstrap capacitor  122 . As shown in respective signals  163  and  164 , the charges on the two capacitors vary accordingly until the potential of the two driving sides  86 ,  124  is substantially equal. Then, at time t 4  signals  148 ,  158  and  161  transition. This turns off the equalization transistor  144 , and turns on transistors  94  and  134  respectively. Consequently, the respective outputs  90 ,  128  of drivers  92 ,  130  go to ground and VCC respectively. With the turning on of transistor  134 , the driving side  124  of the second bootstrap capacitor  122  is elevated to VCC  12 , and the driven side  126  of capacitor  122 , previously at VCC  12 , is raised to 2 VCC. This voltage (2 Vcc) is applied to the load  38  by way of diode connected transistor  142 , supplementing in counter-phase, the previous application of voltage by the first phase circuit  82 . At time t 5  the entire cycle begins again as signals  148 ,  161  and  158  again toggle, turning on the equalization transistor  144  and turning off transistors  134  and  94  respectively. This cycle is repeated continuously in order to provide a more or less steady supply of current through transistors  114  and  142  to the load  38 .  
         [0023]     As noted, the operation of the two-phase charge pump described above was presented as if the circuit were prepared using ideal components. An embodiment of the same circuit using non-ideal components would yield an output voltage across the load of less than two times the input voltage due to, among other things, the diode drop (of one threshold voltage Vth) across the diode connected transistors  114  and  142 .  
         [0024]     A circuit that overcomes this limitation and outputs a voltage equal to two times input voltage or more can be produced by using a respective ancillary charge pump circuit as part of each charge pump phase circuit.  
         [0025]     A four phase charge pump circuit which utilizes an ancillary charge pump circuit and charge sharing among the bootstrap capacitors is shown in  FIGS. 4A and 4B . Equalization circuits for equalizing capacitor charge are labeled A-F, and shown as block elements on  FIGS. 4A and 4B . These same circuits, correspondingly labeled A-F, are shown in further detail in  FIG. 5 .  
         [0026]     Four charge pump phase circuits are shown in FIGS.  4 A and  4 B: a first phase circuit  202 , a second phase circuit  204 , a third phase circuit  206 , and a fourth phase circuit  208 . In the first phase charge pump circuit  202 , a first bootstrap capacitor  210  includes first  212  (driving side) and second  214  (driven side) terminals. In the same fashion described above with respect to the two phase charge pump, the bootstrap capacitor  210  is driven on its driving side  212  by a driver circuit  218 . During discharge, capacitor  210  transfers charge through an equalization circuit B  300  to a bootstrap capacitor  250  of phase circuit  206 . The illustrated embodiment includes an ancillary pump circuit  229  associated with the first phase charge pump circuit  202 . This ancillary pump circuit  229  includes a first ancillary capacitor  231  having third  232  (driving side) and fourth  234  (driven side) terminals. The third terminal  232  is operatively connected to an output  236  of a second driver circuit  238 . As would be understood by one of skill in the art, the ancillary pump circuit  229  serves to elevate the output voltage supplied to a load  38  by applying an elevated voltage to a gate  230  of an output transistor  228 . During operation, the voltage on the driven side  214  of bootstrap capacitor  210  rises above VCC  12 . In response, current flows through diode connected transistor  239 , and charges the driven side  234  of ancillary capacitor  231  to a voltage above VCC. Thereafter, the driving side  232  of ancillary capacitor  231  is raised from ground potential to VCC  12 . The result is that the voltage applied to the gate  230  of the output transistor  228  is high enough to allow the full voltage on the driven side  214  of bootstrap capacitor  210  to reach the input of the load  38 . The first ancillary capacitor  231  also serves to supply additional charge to the load  38  by transferring charge through transistor  240  to the driven side  214  of capacitor  210  when the voltage thereon falls below a design threshold.  
         [0027]     The circuitry of the first phase circuit  202  is duplicated in the second  204 , third  206 , and fourth  208  phase circuits and the operation of these additional phase circuits is the same as that of circuits  202  and  229  as would be clear to one of skill in the art in light of the description provided above. The outputs of each of the four phase circuits  202 ,  204 ,  206 ,  208  are mutually connected at an output node VP 2   242 . In each phase circuit, a primary charge pump with a primary bootstrap capacitor is supplemented by an ancillary charge pump with an ancillary bootstrap capacitor. The respective ancillary charge pump serves to provide an elevated voltage to a gate of an output transistor of the primary charge pump, and to supply charge to the output of the primary charge pump via a pair of bridge transistors once the voltage on the driven side of the respective primary bootstrap capacitor begins to fall. According to one aspect of the invention, as shown, charge is shared between bootstrap capacitors  231  and  286  through an equalization circuit D  301 , between capacitors  231  and  280  through an equalization circuit C  303 , between capacitors  280  and  260  through an equalization circuit E  305 , between capacitors  260  and  286  through an equalization circuit F  307 , and between capacitors  287  and  291  through an equalization circuit A  309 .  
         [0028]     As shown in  FIG. 5 , each of the equalization circuits A-F includes a logic gate  304 , an inverter  306 , and a transistor  308 . The logic gate receives a plurality of control signals at a respective plurality of inputs, and switches the equalization transistor on and off accordingly.  
         [0029]      FIG. 6  shows circuitry, adapted to receive an input periodic clock signal  402  (osc) and generate a plurality of output signals. The output signals are adapted to control a four-phase charge pump as illustrated in  FIGS. 4A, 4B  and  5 . In a first portion  400 , the circuitry receives a clock signal  402  (osc) at a clock input  410  of a first flip-flop  412 . The first flip-flop outputs a first signal C 0  that is inverted to form C 0 _ and fed back into an input  416  of the first flip-flop. The C 0 _ signal is also used to control a second flip-flop  418  via a Nor gate  420 . The second flip-flop produces a C 1  signal that is inverted to form C 1 _. C 0 _ and C 1—  are combined through a Nor gate  422  with osc  402  to control a third flip-flop  424 . In similar fashion, the output of the third flip-flop  424  is inverted and combined with C 0 _ C 1 _ and osc  402  to control a fourth flip-flop  426 . As a result, four signals C 0 _ through C 3 _ of progressively longer period are generated.  
         [0030]     Of the eight signals thus generated, C 0 , C 1 , C 2 , C 3 , C 0 _, C 1 _, and C 2 _ are applied to combinational logic  430 , as illustrated, to produce eight control signals A 0 P, A 2 N, A 0 N, A 2 P, A 1 P, A 3 N, A 1 N, and A 3 P that control the respective drivers driving the four respective bootstrap capacitors  210 ,  287 ,  250 , and  291  as shown in  FIGS. 4A and 4B .  
         [0031]     Similarly, the four signals C 0 _, C 1 _, C 2 _, and C 3 _ produced by circuit portion  400  are applied to combinational logic  440  to produce 12 control signals B 0 P, B 0 P_, B 0 N, B 1 P, B 1 P_, B 1 N, B 2 P, B 2 P_, B 2 N, B 3 P, B 3 P_, and B 3 N that control the four inverters driving the four respective ancillary bootstrap capacitors  231 ,  280 ,  260 , and  286  as shown in  FIGS. 4A and 4B .  
         [0032]     Together, the timing circuit  400  and the two combinational logic circuit portions  430  and  440  serve to operate the four phase circuits, including ancillary circuits, of the charge pump of  FIGS. 4A and 4B  such that the potentials on the bootstrap capacitors, as seen at the mutual VP 2  output  242  of the four phase circuits, are at respective phase angles of 0, 90, 180, and 270 degrees.  
         [0033]     It is understood that other charge pump phase circuits, could also be designed using the equalization charge sharing techniques described herein. More generally, while preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, deletions, substitutions, and other modifications can be made without departing from the spirit or scope of the invention. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the appended claims.