Abstract:
Charge pumps and methods for regulating charge pumps. The charge pump includes a voltage booster circuit and a voltage regulator circuit. The voltage booster circuit includes first and second input terminals that respectively receive a regulation voltage and an input voltage. The voltage booster circuit generates an output voltage having a polarity that is different from the input voltage. The output voltage is adjusted by the regulation voltage and provided to an output terminal. The voltage regulator circuit is coupled between the first input terminal and the output terminal of the voltage booster circuit. The voltage regulator circuit shifts the output voltage to a level shifted voltage and generates the regulation voltage responsive to the level shifted voltage.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of charge pumps and, more particularly, to methods and circuits for regulating charge pumps. 
     BACKGROUND OF THE INVENTION 
     In many electronic devices, it is desirable to generate a voltage having a magnitude that is greater than a magnitude of a supply voltage providing power to the device. In other applications, it is desirable to generate a polarity that is different from the polarity of the supply voltage providing power to a device. Charge pumps may be used for both of these purposes. Although a wide variety of charge pumps have been developed, many charge pumps use capacitors to obtain a boosted voltage or a voltage having a different polarity. 
     Typically, a supply voltage is sampled on a first terminal of a capacitor (by charging the capacitor to the supply voltage) during a first phase of a cycle. During a second phase of the cycle, one of the terminals is coupled to a load. If the first terminal of the capacitor is coupled to the load and the second terminal is held at ground, a boosted voltage may be generated. Because the capacitor was charged to the supply voltage during the first phase when the second terminal was connected to ground, the voltage on the first terminal is approximately twice the supply voltage during the second phase. If, during the second phase, the second terminal of the capacitor is coupled to the load and the first terminal is held at ground, a voltage with a reverse polarity may be generated. Because the capacitor was charged to the supply voltage during the first phase when the second terminal was connected to ground, the voltage on the second terminal is approximately a negative supply voltage during the second phase. The charge pump repeatedly alternates between the first and second phases, each cycle generating an output voltage that is approximately twice the supply voltage V AA  or of a reversed polarity. 
     Charge pumps are presently used in a wide variety of applications. For example, charge pumps are typically used in memory devices to provide a negative substrate voltage or to provide a boosted voltage that may be applied to the gate of an NMOS transistor to allow the transistor to couple the supply voltage to an output node. Charge pumps are also used in CMOS imagers to generate voltages of different polarities and magnitudes during various operations carried out by the imagers. For example, charge pumps are commonly used to supply power having a polarity that is different from that of the supply voltage to the imaging array of CMOS imagers. 
     The time required for a charge pump to output a target voltage is sometimes referred to as a time constant of the charge pump. In general, the time constant of a charge pump driving a resistance load is very short as long as the current demands of the load do not exceed the current that may be supplied by the charge pump. The time constant, however, of a charge pump driving a capacitive load may be very long because the voltage applied to a load incrementally increases through a charge sharing process each cycle. The time constant of the charge pump may affect the magnitude of capacitance relative to the load capacitance, as well as the difference between the supply voltage and the load voltage to which the capacitive load has been charged. The charge pump, thus, may be slow to reach the target voltage because the charge pump may not produce more charge than the combination of the pump capacitance, supply voltage and the load voltage. In addition, charge pumps typically do not compensate for charge lost when the charge pump is inactive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be understood from the following detailed description when read in connection with the accompanied drawing. Included in the drawing are the following figures: 
         FIG. 1  is a schematic diagram of a charge pump according to one example of the invention; 
         FIG. 2  is a schematic diagram of a charge pump according to another example of the invention including two voltage boosters arranged in counter-phase; 
         FIG. 3  is a timing diagram of control signals and generated output voltage for the circuit shown in  FIG. 1 ; 
         FIG. 4  is a schematic diagram of a charge pump according to a further example of the invention; 
         FIG. 5  is a timing diagram of generated output voltage and clock signals for the circuit shown in  FIG. 4 ; 
         FIG. 6  is a timing diagram of a voltage step signal generated by the charge pump shown in  FIG. 4  when pumping charge from a load; and 
         FIG. 7  is a block diagram of a CMOS imager using one or more of the charge pumps shown in  FIG. 1 ,  2  or  4  or a charge pump according to another example of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a charge pump, designated generally as  100 , according one example of the invention. Charge pump  100  includes voltage booster circuit  102 , voltage regulator circuit  104  and clock generator  106 . Clock generator circuit  106  provides a set of nonoverlapping clock signals (φ 1 , φ 2 ) to control voltage booster circuit  102  and voltage regulator circuit  104 . Clock signals (φ 1 , φ 2 ) represent a first phase (φ 1 ) and second phase (φ 2 ) of a cycle. 
     Voltage booster circuit  102  includes capacitance C p , two switches  110 ,  112  that are closed during the first phase of each cycle and two switches  114 ,  116  that are closed during the second phase of cycle. Voltage booster circuit  102  may be used to supply a negative voltage when powered by a positive supply voltage  118  of voltage V AA . A load L is connected to output node  126  of charge pump  100 . Switches  110 ,  112  that are closed during the first phase of each cycle are open during the second phase, and switches  114 ,  116  that are closed during the second phase are open during the first phase. The load L is assumed to be the array of a CMOS imager, which may be highly capacitance, with a capacitance of C L . The voltage across the capacitance load C L  is designated as V L . Voltage booster circuit  102  is coupled to voltage regulator circuit  104  via node  128  and switch  114 . Voltage booster circuit  102  provides voltage V IN  to voltage regulator circuit  104  via node  128  and receives regulator voltage V REG  from voltage regulator circuit  104  via switch  114 . 
     Although a capacitive load C L  is illustrated in  FIG. 1 , load L may not be entirely capacitive. According to one embodiment, load L may include both capacitor and diode components, where the diode may introduce some charge leakage. For example, load L may correspond to transfer (TX) gates of active pixels (not shown) of a CMOS imager. In this example, voltage booster circuit  102  may be coupled to the TX gates via respective source diffusion regions of a number of driver transistors (not shown), for example, about 480-2500 driver transistors. Accordingly, there may be a significant load from respective forward-biased diffusion diodes of the corresponding pixels. For example, the area of the diodes may be large enough to produce a detectable amount of charge leakage, even for driving voltages (i.e. V L ) for example, of about 250 mV below a threshold voltage of the diode. 
     Voltage regulator circuit  104  includes supply voltage  120  of voltage V AA , capacitor C FB , switches  122 ,  124  and differential amplifier  108 . Switch  122  is closed during the first phase of each cycle and switch  124  is closed during the second phase of each cycle. Switch  122  that is closed during first phase of each cycle is open during the second phase, and switch  124  that is closed during the second phase is open during the first phase. Switch  122  operates together with switch  110 ,  112  and switch  124  operates in together with switches  114 ,  116 . 
     Capacitor C FB , switches  122 ,  124  and supply voltage  120  form level shift circuit  130  that receives voltage V IN  from voltage booster circuit  102  generates level shifted voltage V SHIFT . Differential amplifier  108  receives a reference voltage V REF  at the non-inverting input terminal and level shifted voltage V SHIFT  at the inverting input terminal and produces regulation voltage V REG . Reference voltage V REF  represents a target voltage corresponding to a desired negative pumping voltage. Accordingly, to reach an output level of −m volts (where m is an integer), reference voltage V REF  can be set at (V AA −m). Capacitor C FB  is a feedback capacitor which is used by differential amplifier  108  to detect load voltage V L . In general, capacitor C L  is large, for example, 100 times larger, compared to capacitor C P . 
     As discussed above, in one embodiment, load L may include a diode component, which may introduce a charge leakage to load L. It may be appreciated that the charge leakage may increase exponentially with increasing voltage V L . Accordingly, a threshold voltage for the target voltage (and thus a suitable maxiumum reference voltage V REF ) may be determined such that charge pump  100  may compensate for the charge leakage. Charge pump  100 , thus, may produce a regulated load voltage V L  that may substantially reduce noise due to charge leakage. 
     During the first phase of each cycle, supply voltage  122  is connected to an upper terminal of capacitor C P  by switch  110  while switch  112  connects the lower terminal of capacitor C P  to ground. Capacitor C P  is therefore charged to −V AA  during the first phase. In addition, supply voltage source  120  of voltage regulator circuit  104  is connected to one terminal of capacitor C FB , while switch  112  connects the other terminal of capacitor C FB  to ground (i.e., such that V IN  is at ground). During the first phase, differential amplifier  108  is disconnected from voltage booster circuit  102  and level shift circuit  102 , and capacitors C P  and C FB  are each charged to V AA . 
     During the second phase of each cycle, switch  114  is closed to connect the upper terminal of capacitor C P  to receive regulation voltage V REG  and the other switch  116  is closed to connect the other terminal of capacitor C P  to load L. In addition, switch  124  is closed to connect one terminal of capacitor C FB  to the inverting input terminal of differential amplifier  108  and the other terminal of C FB  is connected to load L and thus to load voltage V L . Thus, a voltage difference of (V AA −V L ) is generated across level shift circuit  130 . 
     It may be appreciated that, during the second phase, a feedback circuit is provided by differential amplifier  108  and capacitances C P , C FB . It may also be appreciated that voltage V SHIFT  at the inverting input of differential amplifier  108  is at a voltage V AA  higher than V L  (i.e., it is level shifted). Differential amplifier  108  provides unity gain feedback from V REG  to V L . Because of the feedback configuration, differential amplifier  108  adjusts V REG  to compensate for any lost charge and to maintain output node  126  at a voltage of V REF −V AA . 
     In operation, when voltage regulator circuit  104  determines that load voltage V L  is outside of a target voltage range, differential amplifier  108  slews to ground (for example, acting as a current sink), such that all charge across C P  is pushed into load C L . Accordingly, both output node  126  and level shifted voltage V SHIFT  are reduced by ±(C P /C L )·V AA . Thus, the entire supply voltage V AA  range is used. When the load voltage V L  is within the target voltage range, differential amplifier  108 , acting as a voltage buffer, generates regulation voltage V REG  to provide sufficient charge into capacitor C P  such that the inverting and non-inverting input terminals of differential amplifier  108  are maintained at a substantially same voltage. 
     By repeating the sequence of first and second phases using a clock, for example, of a few tens of MHz, a large amount of charge may be efficiently moved into capacitor C P  while maintaining a smooth settling for the boosted voltage, when the load voltage V L  is within the target voltage range. Namely, voltage regulator circuit  104  may 1) rapidly pump load L to within a target voltage range (where differential amplifer  108  acts as a current sink) and 2) apply differential amplifier  108 , acting as a voltage buffer, to reach the target voltage. As described further below, a size of capacitor C P , used in charge pump  100 , may be reduced. Because the size of capacitor C P  may be reduced, a size of an output stage of differential amplifier  108  may also be reduced, thus generating a smaller output current. Accordingly, it may be appreciated that, even with the smaller output current differential amplifier  108  may still be capable of slewing from V AA  to ground within a clock phase. 
     In addition, if differential amplifier  108  has a gain that is fairly high, for example, a gain of greater than 100, voltage booster  102  may keep pumping within the full range (i.e., V AA ) until the load voltage V L  is within the target voltage range. Because capacitor C P  may be charged to supply voltage V AA , a size of capacitor C P  may be reduced. Because the output voltage V IN  of voltage booster circuit  102  is level shifted by a higher predetermined value (e.g., V AA ), voltage booster circuit  102  may be operated using a regulation voltage V REG , which generally produces for a larger voltage (compared to a target voltage) that may be used across capacitor C P . Because a larger voltage may be used in voltage booster circuit  102 , a capacitor size needed to reach a target voltage within the time constant may be reduced. 
     The devices for implementing switches  110 ,  112 ,  114 ,  116 ,  122 ,  124  are conventional as are circuitry for controlling them during the first and second phases of each cycle. Therefore, a more detailed explanation of these devices and control circuits have been omitted. 
       FIG. 2  illustrates a charge pump, designated generally as  200 , according to another example of the invention. Charge pump  200  is the same as charge pump  100  ( FIG. 1 ) with an exception. In addition to containing voltage booster circuit  102 - 1  (including switches  110 - 1 ,  112 - 1 ,  114 - 1 ,  116 - 1 , capacitor C P  and supply voltage force  118 - 1 ), charge pump  200  also includes a second voltage booster circuit  102 - 2 . Voltage booster circuits  102 - 1 ,  102 - 2  are each connected to voltage regulator circuit  104 . Voltage booster circuit  102 - 2  includes switches  110 - 2 ,  112 - 2 ,  114 - 2 ,  116 - 2 , capacitance C P  and voltage source  118 - 2 . Switches  110 - 2 ,  112 - 2 ,  114 - 2 ,  116 - 2  are operated out of phase with correspondingly numbered switches  110 - 1 ,  112 - 1 ,  114 - 1 ,  116 - 1 . As a result, capacitor C P  of voltage booster circuit  102 - 1  applies a voltage to load L during the second phase of each cycle and capacitor C P  of voltage booster circuit  102 - 2  applies a voltage to load L during the first phase of each cycle. 
     According to another embodiment, charge pumps  100 ,  200  ( FIGS. 1 and 2 ) may include a gate (not shown) as part of clock generator  106  or separate from clock generator  106 . The gate may be used to inactivate voltage booster circuit  102  and/or voltage regulator circuit  104  at particular times. In this manner charge pump  100 ,  200  ( FIGS. 1 and 2 ) may stop pumping, for example, during sampling of a pixel output of imager array to minimize the introduction of switch noise into sampled pixels. 
     Referring to  FIG. 3 , a timing diagram of generated output voltage and control signals as a function of time are shown. In particular,  FIG. 3  shows input clock signal  302 , pump clock signal  304  (i.e. φ 1 , φ 2 ), stop clock signal  306 , capacitive voltage signal  308 , load voltage signal  310  (i.e. V L ) and output voltage signal  312 . Input clock signal  302  is provided to clock generator  106  ( FIG. 1 ). Pump clock signal  304  is used to control charge pump  100  ( FIG. 1 ). Stop clock signal  306  is used to inactivate charge pump  100  ( FIG. 1 ), as described above. Capacitive voltage signal  308  represents a voltage capacitively coupled to load L that cycles during sampling. Load voltage signal  310  is the load voltage V L  provided by charge pump  100  ( FIG. 1 ). Output voltage signal  312  represents a voltage of a single metal line across a CMOS imager array. 
     As shown in  FIG. 3 , load voltage signal  310  is pulled down rapidly after initialization. Load voltage signal  310  is shown to recover quickly after a hold period when a substantial amount of charge is pulled from the imager array. It may be seen that some of the coupling of the load to load voltage  310  during sampling (when stop clock signal  306  is asserted) may be accounted for by capacitive voltage signal  308 . When capacitive voltage signal  308  is pulled down, it also pulls down load voltage signal  310 , such that an amount of charge is leaked from output voltage signal  310 . When capacitive voltage signal  308  is released, load voltage signal  310  is initially less negative than prior to stop clock signal  306  being asserted. It may be appreciated that load voltage signal  310  is then reduced, thus, compensated by charge pump  100  ( FIG. 1 ). It may also be appreciated that charge lost to output voltage signal  312 , during the assertion of stop clock signal  306 , is also quickly compensated by charge pump  100  ( FIG. 1 ). 
       FIG. 4  is a schematic diagram of a charge pump, designated generally as  400 , according to another example of the invention. Charge pump  400  includes voltage booster circuit  402  and regulator circuit  404 . Voltage booster circuit  402  supplies a negative voltage to output node  434 . Regulator circuit  404  generates a set of pump clock signals (φ 1 ′, φ 2 ′) to control voltage booster circuit  402 . A capacitive load C L  is connected to output node  434  of charge pump  400  and the voltage across capacitance load C L  is designated as V L . 
     As described further below, pump clock signals (φ 1 ′, φ 2 ′) are generated to activate and control operation of voltage booster circuit  402  when the load voltage V L  at node  434  is less than a reference voltage V REF . When load voltage V L  is greater than or equal to reference voltage V REF , pump clock signals (φ 1 ′, φ 2 ′) are set to a low value (i.e. 0) and voltage booster circuit  402  is inactive. Pump clock signals (φ 1 ′, φ 2 ′) represent a first phase (φ 1 ′) and second phase (φ 2 ′) of an active pump cycle. 
     Voltage booster circuit  402  includes supply voltage  432  of voltage V AA , capacitor C P , two switches  424 ,  426  that are closed during the first phase (φ 1 ′) of the pump cycle and two switches  428 ,  430  that are closed during the second phase (® 2 ′) of the pump cycle. Switches  424 ,  426  that are closed during the first phase of each cycle are open during the second phase. Switches  428 ,  430  that are closed during the second phase are open during the first phase. 
     Regulator circuit  404  includes voltage detector circuit  406  and clock generator circuit  408 . Voltage detector circuit  406  receives and samples load voltage V L  from node  434  and provides a detected voltage V SENSE  to clock generator circuit  408 . Clock generator circuit  408  generates a set of clock signals (φ 1 , φ 2 ) to control voltage detector circuit  408  and the set of pump clock signals (φ 1 ′, φ 2 ′) to control voltage booster circuit  402 . Clock signals (φ 1 , φ 2 ) represent a first phase (φ 1 ) and second phase (φ 2 ) of a clock cycle. As described further below, clock signals (φ 1 , φ 2 ) are generated each clock cycle. Clock signals (φ 1 ′, φ 2 ′), however, are activated when V L  is less than reference voltage V REF . Accordingly, regulator circuit  404  detects the load voltage V L  and determines whether to activate or deactivate voltage booster circuit  402 . 
     Voltage detector circuit  406  includes capacitor C s , first set of switches  416 ,  418  and second set of switches  420 ,  422 . Voltage detector circuit  406  receives and samples load voltage V L  on capacitor C s  according to the set of clock signals (φ 1 , φ 2 ). Switches  416 ,  418  that are closed during the first phase of each cycle are open during the second phase. Switches  420 ,  422  that are closed during the second phase of each cycle are open during the first phase. During the second phase, capacitor C s  samples load voltage V L  when switches  420 ,  422  are closed. During the first phase, switches  416 ,  418  are closed and the detected voltage sampled by capacitor C s  is inverted and provided to clock generator circuit  408  as detected voltage V SENSE . 
     Clock generator circuit  408  includes comparator  410 , clock generator  412  and AND gates  414 - 1 ,  414 - 2 . Comparator  410  compares detected voltage V SENSE  with reference voltage V REF  and generates a pump signal (pump). Comparator  410  generates a high pump signal (i.e.,  1 ) when V SENSE  is less than V REF . Comparator  410  generates a low pump signal (i.e.,  0 ) when V SENSE  is greater than or equal to V REF . Clock generator  412  generates the set of clock signals φ 1 , φ 2  which is provided to voltage detector circuit  406 , regardless of the state of the pump signal. Clock signals φ 1 , φ 2  are gated with the pump signal by AND  414 - 1 ,  414 - 2 , respectively, to produce the set of pump clock signals φ 1 ′, φ 2 ′ used to control operation of voltage booster circuit  402 . Clock generator circuit  408  sets the set of pump clock signals φ 1 ′, φ 2 ′ to zero when the pump signal is low, thus causing pumping of voltage booster circuit  402  to cease. 
     Reference voltage V REF  represents a target voltage for the load voltage V L  at output node  434 . Although in one embodiment, V REF  is a positive value of 400 mV, it is understood that any suitable reference voltage may be used, based on the load voltage. As described above, load L may also include a diode component that may generate a charge leakage. Accordingly, a suitable V REF  may also be based on the charge leakage from the diode component. 
     In operation, the set of clock signals φ 1 , φ 2  for clock generation circuit  406  continues for each cycle such that voltage detector circuit  406  continually detects load voltage V L . Voltage booster circuit  402 , however, is activated when the pump signal is high. 
     When the pump signal is high and during the second phase of the pump cycle, supply voltage  432  is connected to capacitor C P  by switch  430 , while switch  428  connects the other terminal of capacitor C P  to ground. Capacitor C P  is therefore charged to −V AA  during the second phase of the pump cycle. During the first phase of the pump cycle, switch  426  is closed to connect the lower terminal of capacitor C P  to ground and switch  424  is closed to connect the other terminal of capacitor C P  to load L. 
     In another embodiment, charge pump  400  may include first and second voltage booster circuits  402  (not shown), each connected to regulator circuit  404 . The first and second voltage booster circuits  402  are similar to each other except that they are operated out of phase. Accordingly, first and second voltage booster circuits  402  may apply a voltage to load L, as described above, during the first and second phases of each cycle, respectively. 
     According to one embodiment, when charge pump  400  is used with an imager array, clock generator circuit  408  may be configured with a gate (not shown) to inactivate voltage booster circuit  402  at particular times. In this manner, charge pump  400  may stop pumping, for example, during sampling of a pixel output of imager array to minimize the introduction of switch noise into sampled pixels. 
     Referring to  FIG. 5 , a timing diagram of a charge pump output voltage and clock sequences are shown for charge pump  400 . In particular,  FIG. 5  shows input clock signal  502 , sense clock signal  504  (i.e., φ 1 , φ 2 ), pump clock signal  506  (i.e. φ 1 ′, φ 2 ), stop clock signal  508 , pump signal  510 , load voltage signal  512  (i.e. V L ) and output voltage signal  514 . Input clock signal  502  is provided to clock generator  412  ( FIG. 4 ). Sense clock signal  504  is used to control voltage detector circuit  406  ( FIG. 4 ). Pump clock signal  506  is used to control voltage booster circuit  402  ( FIG. 5 ). Stop clock signal  508  is used to inactivate charge pump  400  ( FIG. 4 ), as described above. Pump signal  510  is used in clock generator cirucit  408  ( FIG. 4 ) that is used to produce pump clock signal  506 . Load voltage signal  512  is the load voltage V L  provided by charge pump  400  ( FIG. 4 ). Output voltage signal  514  represents a voltage of a single metal line across a CMOS imager array. 
     To produce the timing diagram shown in  FIG. 5 , voltage booster circuit  402  has a capacitance C P  of 20 pF and a capacitive load C L  of 5 nF. As shown in  FIG. 5 , voltage booster circuit  402  is capable of pulling down capacitive load C L  to −0.5 V in less than 1.75 microseconds (load voltage signal  512 ). After load voltage signal  512  has reached the reference voltage, pump signal  510  is inactivated. When switching a load of 2 pF, voltage booster  402  ( FIG. 4 ) may compensate for lost charge within about 2 cycles. In contrast, conventional charge pumps typically have a start up time of about 30 microsecond with a 140 pF capacitor C P . 
     As described above, although a capacitive load C L  is shown in  FIG. 4 , load L may include a diode component that may contribute charge leakage. As shown in  FIG. 5 , pump signal  510  may be activated at some interval to compensate for the charge leakage.  FIG. 5  also illustrates that pump signal  510  may be triggered after release of stop clock signal  508 , in order to compensate for any charge leakage during inactivation of charge pump  400  ( FIG. 4 ). Furthermore, cycling of output voltage signal  514  may cause some charge leakage by the metal lines across the CMOS array. Accordingly, pump signal  510  is activated in order to compensate for the charge leakage during cycling of output voltage signal  514 . 
     Referring to  FIG. 6 , a timing diagram illustrating a voltage step when pumping charge from a load is shown, for charge pump  400  ( FIG. 4 ). In particular,  FIG. 6  shows input clock signal  602 , sense clock signal  604  (i.e., φ 1 , φ 2 ), pump clock signal  606  (i.e. φ 1 ′, φ 2 ′), stop clock signal  608 , pump signal  610 , load voltage signal  612  (i.e. V L ) and output voltage signal  614 . Signals  602 - 614  are similar to signals  502 - 514 , except that a different capacitive load is used. To produce the timing diagram shown in  FIG. 6 , voltage booster  402  has a capacitance C P  of 20 pF and a capacitive load C L  of 4 nF. 
     As shown in  FIG. 6 , load voltage signal  612  slowly drifts upwards due to charge leakage (due to a diode component of load L) before it is pumped down (by activating pump signal  610 ) at about 3.08 microseconds. A small difference is illustrated between the transient responses (for example, between about 3.08 microseconds and about 3.1 microseconds) of load voltage signal  612  and output voltage signal  614 . The difference in the transient responses may be due to a resistance, capacitance (RC) delay between charge pump  400  ( FIG. 4 ) and output voltage  614 . The RC delay may reduce an amount of overshoot in output voltage  614  as compared with load voltage signal  612 . 
     Although not specifically shown in the drawings, it will be understood that charge pumps  100 ,  200 ,  400  or a charge pump according to another example of the invention may be adapted to provide a positive rather than negative load voltage V L . Further, by adding additional switches and a capacitor, charge pumps  100 ,  200 ,  400  or a charge pump according to the other example of the invention may generate both positive and negative voltages. 
     Charge pumps  100 ,  200 ,  400  or a charge pump according to some other example of the invention can be used in a wide variety of applications. They are particularly suitable for use in a CMOS imager because the imaging arrays of such devices are highly capacitive (as well as typically including a diode component that may generate a charge leakage). For example, CMOS imager  700  shown in  FIG. 7  include CMOS imaging array  706  that responds to a received image to generate corresponding signals. Array  706  is coupled to control and addressing circuit  702 , which interrogates imaging array  706  to output signal S i  corresponding to the image received by imaging array  706 . CMOS imager  700  also includes charge pump  704  connected to imaging array  706  to supply imaging array  706  with a negative voltage. Charge pump  704  may be one of charge pumps  100 ,  200 ,  400  shown in respective  FIG. 1 ,  2  or  4  or a charge pump according to some other example of the invention. 
     Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.