Abstract:
A refresh mechanism refreshes a supplied capacitor of a capacitor divider circuit at an interval that keeps an amount of charge degradation at a coupled up capacitor to less than a predetermined threshold. A node between the supplied capacitor and the coupled up capacitor provides a voltage sampling node having a divided voltage. Timing for the refresh operations may be established via internal clocks or internal oscillators running at multiples of other circuits already internal to the device utilizing the divided voltage. The divided voltage is then utilized for comparison, feedback (voltage regulation, for example), or other purposes. The invention is applicable to all types of circuits where degradation occurs due any type of leakage or other permutations affecting circuit operations.

Description:
COPYRIGHT NOTICE 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to voltage regulation. The invention is also related to dividing voltage sources. The invention is more particularly related to the division and provision of regulated voltage in microelectronic circuits. The invention is still more particularly related to the provision of capacitor divided voltage provided for power requirements of microelectronic devices, particulary flash memory devices. 
     2. Discussion of Background 
     Most electronic systems being designed today use some form of voltage regulation to ensure circuit stability in the face of changes in load current or supply voltage. In addition to being a requisite for proper or stable circuit operation, many times the circuit design can be simplified with resultant cost reductions if it is based on using a regulated supply. Regulated voltage supplies are also useful to provide a voltage level for comparison of other voltages, such as testing charge levels of other devices in an electronic device. The degree of accuracy that the voltage regulator must provide varies considerably with the function performed by the electronics. As a result, there are many different methods for providing the regulation available to the designer. 
     For many years gas tubes have been used to provide voltage regulation. This is because the characteristic maintaining voltage of these gas discharge tubes remains fairly constant over a relatively wide range of operating currents. Thus, they can absorb any reasonable voltage or current fluctuations occurring in normal operation, keeping the load voltage fairly constant. However, such devices have limited applicability and are not suitable for microelectronic devices. 
     Solid state voltage regulator diodes or Zeners have been popular since the late 1950&#39;s when they replaced bulky vacuum tubes. Zener diodes primarily serve as voltage regulators with variable operating currents when placed in parallel across a load to be regulated. 
     Generally speaking, voltage regulation within modern microelectronic devices utilize a voltage source which is regulated based on it&#39;s output. The process is illustrated in FIG.  1 . At step  100 , an output voltage is sampled. The output voltage is provided from a power supply. At step  110 , the sampled voltage is compared to a know value or reference (band gap reference, for example). The reference can be any stable value which provides an indication as to whether the power supply is providing the required or designed power output. Finally at step  120 , feedback from the step  110  comparison is fed back into the power supply to adjust the power level output (output voltage). In this manner a regulated power supply is provided. 
     FIG. 2 illustrates a typical arrangement of electronics for implementing the above described process. A voltage source  200  provides a supply voltage V supply . The V supply  is divided by resistor pair  210 , 220  to provide a divided voltage  230 . The divided voltage  230  is typically divided from the V supply  to provide a proper voltage level for any connected electronics and a design of the remaining circuit. In this case, the divided voltage provides a voltage for a comparison circuit  250 . 
     The divided voltage  230  is input to the comparison circuit  250  and compared to a reference voltage  240 . The comparison circuit  250  is a standard comparator or amplifier configured to produce some difference or adjustment between the divided voltage  230  and the reference voltage  240 , producing a feedback voltage  260 . 
     The feedback voltage  260  is then fed back into the voltage supply  200  where it has the effect of either lowering or increasing a voltage level produced, resulting in a regulated power supply. As thus illustrated, the circuit in FIG. 2 performs the method described above for power regulation and also illustrates a method of producing additional specific voltages (divided voltage  230 , for example) from a single voltage source using a resistor divider (resistor pair  210 , 220 ). 
     Resistor dividers are common methods for providing specific voltages from a single power source. However, such arrangements require that power is constantly drawn from the voltage source to provide the voltage, and are thus inefficient. One method that does not continuously draw power to provide a divided voltage is illustrated in FIG. 3. A capacitor pair  310 , 320  is shown that divides a voltage to produce a capacitor divided voltage  330 . The capacitor divided voltage is provided to comparison circuit  250  to produce the feedback voltage  260 . 
     The capacitor pair  310 , 320  are charged up by a voltage source similar to voltage source  200 . FIGS. 4A-B illustrates the charge of the capacitors  310 , 329  charged up from t 1  to t 2 , to provide a voltage level for comparison purposes. Unfortunately, capacitors leak charge (see t 2  to t 3 , for example) or will have charge drawn from them for circuit operation and therefore are unsuitable for stable continuous operations. 
     SUMMARY OF THE INVENTION 
     The present inventor has realized that continuous current voltage dividers, such as resistor pairs, are unsuitable for modern electronic devices, particularly for on chip devices or pumps that have limited power resources. The present inventor has realized that capacitor dividers are an ideal way to maintain a voltage level for operating purposes within modern microelectronic devices, and that a method of maintaining a charge level in the capacitors without continuous power application would overcome the problems in the prior art. 
     Roughly described, the present invention provides a nominal power application at predetermined intervals to refresh and maintain a charge level in a capacitor voltage divider circuit. The refresh is performed by calculating an amount of time that a capacitor in the capacitor divider circuit maintains a sufficient charge, and, before that time period has expired, turning the circuit off and reinitializing the capacitors in the circuit, and turning the circuit back on. The timing is generally based on a period of the pump (or voltage supply) and utilizes detailed set of electronics to implement the selected timing regiment. 
     The present invention is a device that provides a selected voltage level with a minimum of power drain from a supply voltage. The present invention includes a refresh mechanism for initializing capacitor nodes of a capacitor voltage divider circuit to maintain a predetermined voltage division of a voltage source between a capacitor pair of the capacitor voltage divider. 
     An embodiment of the present invention includes a timing circuit capable of refreshing a capacitor voltage diver circuit at a frequency tied to a frequency of a pump supplying the voltage divider by the capacitor voltage divider circuit. 
     The present invention can be constructed from parts already present on a microelectronic circuit to implement the timing and functions of the present invention. 
     The invention may be embodied in either of: a sampling circuit having, a sampling node, and a refresh circuit configured to refresh the sampling circuit at an interval rate of less than an amount of time that charge leakage of the sampling circuit degrades a voltage measurement made at the sampling node beyond a threshold amount; and a regulated voltage supply, having a voltage source, a sampling circuit connected to an output of said voltage source and configured to sample the output without drawing current from said voltage source, and a feedback circuit configured to provide a signal that adjusts an amount of voltage supplied at the output based on the sampled output. 
     The invention may also be embodied in a method of sampling voltage, comprising the steps of: retrieving a voltage from a divided node of a capacitor divider circuit having a supplied capacitor and a coupled up capacitor, and refreshing the capacitor divider circuit at an interval rate of less time than an amount of time for charge leakage of the capacitor circuit to degrade the voltage retrieved from the divided node. In addition, the invention may be embodied as a set of instructions, stored on a computer readable media, when loaded into a computer, cause the computer to perform the above identified steps; and as a means for performing reinitializing a capacitor voltage division circuit at a refresh rate of less than a predetermined amount of charge leakage occurs in a coupled up capacitor of the capacitor voltage division circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 is flow chart illustrating a conventional voltage regulation scheme; 
     FIG. 2 is a circuit diagram illustrating a conventional voltage regulation and voltage division mechanism; 
     FIG. 3 is a circuit diagram illustrating a conventional capacitor divider utilized for voltage regulation; 
     FIGS. 4A-B is a graph of charge leak or decay of a conventional capacitor voltage divider circuit; 
     FIGS. 5A-B is a graph illustrating a close up of a charge of a coupled up capacitor in a capacitor divider circuit; 
     FIG. 6 is a circuit diagram illustrating one embodiment of separation circuitry for isolating a supplied capacitor in a capacitor divider circuit according to the present invention; 
     FIG. 7 is a block diagram of a circuit for performing the present invention; 
     FIG. 8 is a timing diagram illustrating a relationship between a p-boost line and the reset/initialize signal; 
     FIG. 9 illustrates an example where a ratio between a frequency of an oscillator and initialize/reset signals; 
     FIG. 10 is a block circuit diagram of one possible implementation of an initialize and reset device; 
     FIGS. 11,  12 , and  13  each show alternate embodiments for each of the circuits described above; 
     FIG. 14 is a voltage and timing diagram illustrating responses for each of the PBOOST, OS 1 , and OS 2  signals described in FIG. 10; and 
     FIGS. 15-16 illustrates a timing and voltage diagram illustrating levels for the above illustrated circuits. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A capacitor divider circuit is ideal for providing a divided voltage under low current/power situations where a current drain would materially degrade circuit operations. However, a voltage of the divided node of the capacitor divider circuit leaks over time causing the circuit to not perform correctly. The present invention allows for continuous use of a capacitor divider circuit for any purpose such as the provision of a divided voltage, or a sample voltage for power regulation. 
     Referring again to the drawings, wherein like reference numerals designate identical or corresponding parts, and more particularly to FIGS. 4A-B thereof, it can be seen that the traditional capacitor divider, at node B, experiences significant charge decay within a short time period of being charged. Node A remains fully charged due to connection to a voltage supply. 
     The decay at node B results from the fact that the coupling up effect that charges capacitor  320  only occurs during a charge time of capacitor  310  (until time t 2 ), at which point no more coupling up effect occurs and capacitor  320  begins to decay via a finite leakage that is known to occur in capacitor devices. 
     Referring now to FIGS. 5A-B, Inset C, a charge decay is shown for the short time period after being charged. An acceptable level of charge for many operations is maintained until approximately time t 3 . For example, for a programming pulse that last 10 micro seconds, such a circuit would be completely acceptable. However, if a charge were to be maintained for seconds, or larger periods of a day, such an arrangement is unacceptable because the charge leaks away. 
     For voltage required for longer time intervals, the present invention proposes to disconnect the capacitor divider circuit to re-initialize the supplied and coupled up capacitors, before the decay of the coupled up capacitor has reached an unacceptable voltage level, and then reconnect it to allow the changing voltage of the now reinitialized supplied capacitor to refresh the coupled up capacitor. 
     FIG. 6 illustrates some basic electronics for disconnecting the capacitor voltage divider circuit having capacitors  510  and  520  arranged as a supplied capacitor and a coupled up capacitor respectively. Transistor  530  is connected between the supplied capacitor  510  and a voltage to be divided. The transistor  530  is configured to either connect of disconnect the supplied capacitor from a voltage supply (not shown) that provides the voltage to be divided. 
     Transistor  540  is connected at a divided voltage node D and to ground, and is configured to be normally off, not altering the circuit, and to ground a negative terminal of capacitor  510 , and a terminal of capacitor  520  when activated. Other methods or circuit arrangements for isolating the capacitor divider circuit or the supplied capacitor may also be utilized, as will now be apparent to those skilled in the art. 
     Once disconnected, the supplied capacitor  510  and coupled up capacitor  520  may then be re-initialized, i.e., the built up charge is zero-out or discharged. The timing is set such that the discharging of the supplied capacitor occurs after the circuit is disconnected, otherwise the discharge process will interfere with the operation of the pump, or power supply. The basic process is performed in three steps: 
     1) disconnect or isolate the supply capacitor; 
     2) Initialize (discharge/zero-out) the supplied capacitor and coupled up capacitor; and 
     3) reconnect the initialized supplied and coupled up capacitors. 
     The main principle of the invention is to refresh the capacitor divider circuit at an interval of time less than a leakage tolerance of the coupled up capacitor of the divider circuit. 
     In one embodiment the present invention is applied to a regulated power supply to sample a voltage from a power supply for comparison and calculation of feedback properties to the voltage supply. In another embodiment, the present invention is applied to a comparison circuit to compare against another on chip voltage level that is being tested. 
     FIG. 7 is a block diagram contain two circuits,  700  and  770 , according to the present invention each having  2  stage pumps,  701  and  771  driven by a single low frequency oscillator LFOSC. Each circuit provides a regulated power supply to a line (Word Line WL in  700 , and Select Gate line SG in  771 ). The pumps,  701  and  771 , if left unregulated would drive each of the Word line WL and SG too high for proper circuit operations. 
     Referring to circuit  700 , a capacitor divider circuit is connected to the Word line WL, including capacitors  710 ,  720 . A divided node  750  of the capacitor divider circuit is divided by a supplied capacitor  710  (0.4 pF, for example) connected to the divided node  750  and the word line WL, and a coupled up capacitor  720  (0.7 pF, for example) connected to ground and the divided node  750 . As will be appreciated by those of ordinary skill in the art, many other configurations of capacitors and connections are possible, for example, the couple up capacitor could be connected to another reference level other than ground. 
     The capacitor voltage divider circuit includes an init &amp; reset (ir) device  725  that controls an initialization process of the supplied capacitor  710  and coupled up capacitor  720 . The ir device  725  includes a control line (p-boost line  730 ) connected to a gate of a p-channel (p-boost) transistor  735 . The ir device controls application of a gate voltage to p-channel (p-boost) transistor  735  that is configured to disconnect the supplied capacitor  710  from the word line WL. 
     The ir device  725  also includes a reset and initialize signal  740  to initialize the supplied capacitor  710 , and a similar signal  745  for initializing the coupled up capacitor  720 . 
     In one embodiment, initialization of the supplied and coupled up capacitors comprises discharging the capacitors (connecting each of nodes  750  and  755  to ground via reset and initialize signals  740  and  745 . The ir device  725  operates as discussed above with regard to the steps of disconnecting the supplied capacitor from the voltage source (word line WL, in this example), initializing both capacitors,  710  and  720 ), and reconnecting the supplied capacitor to the voltage source. As discussed herein, the reinitialization can either be a complete grounding or zero out of the capacitors, or a predetermined amount of discharge sufficient to allow the proper voltage sample ratio to be maintained at node  750 . 
     Also provided for operation of the sampling circuit is a device, procedure, or architecture of the sampling circuit such that the initialization process has either minimized or no effect on the voltage line or source being regulated. In the embodiment illustrated in FIG. 7, the circuit architecture, specifically the comparator  705 , and VREF, are arranged such that when the initialization process is being performed the comparator senses that the voltage is not being regulated and the grounding transistor  760  is not activated (in other words, no shunting of word line WL is performed during the initialization process). 
     Other ways to maintain a status quo, or no operation, during the initialization process are also possible, disconnecting node  750  from the comparator along with an additional mechanism designed (connecting another small capacitor, for example) to maintain the voltage at the comparator just prior to initialization for the duration of the initialization process, for example. 
     In addition, the initialization time of capacitors  710  and  720 , in the current embodiment, relative to a frequency of the pump, in very low (on the order of {fraction (1/20)}th, {fraction (1/100)}th, or more that of the pump). Therefore, any spikes, dips, or other irregularities in regulation that might occur in other embodiments, as long as the pump frequencies are maintained at a higher order than required initialization time, are unlikely to substantially alter operation of the circuit. 
     In one embodiment, the ir device  725  operates in accordance with the timing diagram of FIG. 8, which shows activity of the p-boost line  730 , which is active between t 10  and t 13 , which activates the p-channel (p-boost) transistor  735 , disconnecting the capacitor divider circuit. At time t 11 -t 12 , the reset and initialize signal goes low which indicates the ir device  725  is initializing the capacitors  710  and  720 . 
     Alternatively, timing of the ir device  725  may be handled by a clocking mechanism entirely separate from the oscillator driving the voltage pump, so long as the capacitors are refreshed before being diminished beyond an acceptable threshold, due to charge leakage, for example. However, utilizing an existing oscillator has the advantage of re-using existing parts already present on an electronic device and is therefore an efficient implementation. 
     In addition, other advantages can be realized by timing the ir device from an oscillator driving the pump. For example, in the case where the oscillator frequency corresponds to actual pumps of voltage, or moments of pump inactivity, it may be advantageous to have the timing of the ir  725  in synch at a certain interval with the voltage pump such that the discharge of the capacitors occurs during a time of pump inactivity, rather than voltage production, for example. Other timing scenarios will be apparent based on the application and configuration of the capacitor divider circuit. 
     A comparator  705  is connected, by comparator inputs, to a divided node  750  of the capacitor divider circuit and to a reference voltage VREF. The VREF is any stable voltage source, or a voltage to be compared. An output line of the comparator  705  is connected to a gate of a grounding transistor  760  that is connected between ground and the Word line WL. 
     In operation, voltage on the word line is sampled and divided by the capacitors  710  and  720 . The divided voltage sample (at node  750 ) is input to the comparator  705  where it is compared to the reference voltage VREF. While the divided voltage sample is less than VREF, the output of comparator  705  is in a lower region, leaving the grounding transistor in an off condition not affecting the word line WL. However, once the divided voltage sample reaches more than VREF, the comparator then output reaches a higher region where the grounding transistor is active, or turned on, which causes a shunt current to flow between the word line WL and ground. In this example, VREF would be supplied such that the grounding transistor would be inactive (open, not affecting the word line WL voltage), until the pump began charging the word line WL above a 3.5 v level. Thus the VREF would be provided based on a voltage present at the divided node when 3.5 v is present on the word line WL. 
     However, the comparator provides a continuous signal to the grounding transistor  760 , and, because the transistor does not operate as an absolute switch, a small amount of shunting occurs below a WL level of 3.5 v, up to full shunting at some point above a 3.5 v WL level. 
     As stated above many other configurations are possible. For example, the comparator could be configured to provide only an absolute low of high signal to the grounding transistor  760 , forcing the transistor to operate in a more switch like fashion, however, such a configuration might cause irregularities in the WL line voltage level or pump operation. Also, the output from the comparator, or another comparison circuit could be fed back to the pump  701 , shutting it down for one or two cycles when the voltage reaches a predetermined threshold (the above stated fraction of 3.5 v, for example). Regardless of the feedback mechanism for controlling the pump output, the present invention provides the divided voltage without current drain from the line being tested. Or, in yet another alternative, the grounding transistor could be replaced with another device for regulating a shunt path between the word line WL and ground (a variable resistor, for example). 
     Circuit  771 , has many components similar to those described in circuit  700  for regulating a select gate voltage. Circuit  771  utilizes a slightly different line configuration and demonstrates how plural divided sampled voltage schemes may be reinitialized from a single reinit signal and having pumps driven by a single oscillator (LFOSC). 
     The present invention may also be configured to sample voltages for comparison in determining a logic level represented by the sampled voltage, such as inside a memory cell or other device. The present invention may be applied to any device that samples a voltage, and is particularly advantageous in situations where continuous low power drain sampling is needed. 
     As discussed above, the present inventor has also determined that coupling a timing of the ir device  725  to a frequency generator LFOSC 2 . The LFOSC 2  provides a frequency to the ir device  725  that controls a timing of the p-boost  730  and reset  740  signals. The provided frequency is sufficient to provide timing consistent with the requirements of the capacitor divider circuit. FIG. 9 illustrates an example where a ratio between a frequency of an oscillator and the initialize and reset signals provide timing for the reset and initialize signal. 
     FIG. 10 illustrates another embodiment and one possible implementation of the ir device  725 . A REINIT signal (from LFOSC 2 , for example) is provided to input circuitry  1000  that accepts the REINIT signal which is then produces a PBOOST signal and OS 1  signal via split/boost circuitry  1005 . Split/boost circuitry  1005  includes an upper path connecting the input circuitry  1000  and to a first terminal of transistor  1006 . A gate of transistor  1006  is connected to VCC, and a second terminal of transistor  1006  is connected to a PBOOST line for activating the p-channel transistor  1035 . Split/boost circuitry  1005 , also includes a lower path that includes a series of inverters  1007  that act as a delay circuit, and a capacitor  1008  (0.25PF, for example) connected to the series inverters and the PBOOST line. 
     In operation, the capacitor  1008  is charged up to a level of approximately VCC (VCC−V Threshold  of transistor  1006 , see FIG. 10, for example). Then, the delay signal kicks in (in about 1.05 usecs, for example), and because the voltage across the capacitor does not change, the PBOOST line signal is boosted to a higher voltage level. The PBOOST signal is applied to a p-channel transistor  1035  which then disconnects a supplied capacitor  1035  from a word line supply WLSUPPLY. 
     In this embodiment, because the word line supply WLSUPPLY is being boosted above a level of VCC (an external power supply of the device), in order to achieve a disconnection of the supplied capacitor  1035  from the word line WLSUPPLY, a voltage applied to the gate of p-channel transistor  1035  is boosted higher than a voltage of the word line WLSUPPLY (a boost up to at least a voltage of WLSUPPLY may also suffice, however, a boost above the WLSUPPLY level provides better assurance of a good disconnect). 
     The OS 1  signal is delayed by delay circuitry  1020  and then provided as a control signal activating both grounding transistor  1025  (coupled between p-channel transistor  1035  and supplied capacitor  1010 ) and grounding transistor  1030  (coupled between a divided node  140  and ground). The delay of circuit  1020  is calculated so the PBOOST signal and operation of transistor  1035 , disconnecting the supplied capacitor  1010 , is complete before grounding transistors  1025  and  1030  re-initialize the supplied capacitor  1010  and coupled up capacitors  1015  by discharging (a full discharge is primarily envisioned, however a partial discharge may also be sufficient to maintain a proper charge in the coupled up capacitor, which would then be fully consistent with the present invention). Delay circuit  1020  also provides an appropriate voltage level to assure solid ground contacts for each of grounding transistors  1025  and  1030 . 
     In FIG. 10, the divided voltage (at divided node  1045 ) is provided to an lpreg (low power regulator, which is a differential comparator), along with a reference voltage VREF, and a power down control signal PWRDWN. The lpreg utilizes these inputs to provide a regulator output signal REGOUT that controls a transistor MNREG, coupled between WLSUPPLY and ground, that, when activated, pulls the WLSUPPLY line low, thus providing an additional example of how the present invention is applicable within another device. As will be appreciated by those of ordinary skill in the art, the circuits of FIG. 10 can take on many different configurations, still consistent with the teachings herein. Generally speaking the present invention will be highly sought for use in any device that requires a sampled voltage where limited current drain is desired. 
     FIG. 11 is an example of a low frequency oscillator  1100  (LFOSC of FIG. 7, for example). The oscillator  1100  is similar to a traditional ring oscillator with some additional circuitry making it low power (tri-stating p-channel and n-channel transistors, on the top of each inverter, controlled by the previous 2 inverter stages—note that node  1110  between stages A 2  and A 3  feeds each of transistors  1120  and  1130  at stages A 0  and A 5 , respectively). In a traditional ring oscillator, power is wasted from VCC to ground because as each stage changes that occurs during a time period that is also driving the next stage, resulting in non-sharp transitions between each stage in the ring oscillator. Here, in FIG. 11, power is allowed to flow to the inverter only once the input to the inverter reaches a full level (full VCC or full VSS). 
     FIG. 12 illustrates a two stage pump  1200  that could be utilized as pump  701  or pump  771  as illustrated in FIG.  7 . The two stage pump  1200  takes a standard VCC input voltage and pumps it up to a higher voltage. 
     FIG. 13 is an example of a differential comparator (lpreg of FIG. 10 for example) having an active load and a current source. The differential comparator compares an IN+ signal to an IN− signal, which if approximately equal will maintain a mid-level signal on an OUT line. If IN+ is above IN−, the OUT signal goes high, and if N+ is below N−, the OUT signal goes low. Other comparison circuits may be utilized. 
     FIG. 14 illustrates responses for each of the PBOOST, OS 1 , and OS 2  signals described above for the circuit as illustrated in FIG.  10 . As will be appreciated by those of ordinary skill in the art, any circuit implementing a similar signal or timing arrange may be substituted for the circuit shown in FIG.  10 . 
     In FIG. 14, the PBOOST signal is first activated (disconnecting the supplied capacitor  1010 ), and remains active for about 0.2-0.25 useconds (usecs). During this time period OS 1  and then OS 2  are subsequently activated (OS 1  in approx 0.025 usecs, and OS 2  approx 0.025 usecs after OS 1 ). Two important aspects of the timing are: 
     1) the length pboost remains active (which needs to encompass the period that the capacitors ( 1010  and  1015 ) are being discharged to prevent the discharging circuitry from affecting the WLSUPPLY); and 
     2) the length of time OS 2  remains active (which needs to remain active long enough to incur sufficient discharge of the capacitors ( 1010  and  1015 )). 
     FIG. 15 illustrates a timing and voltage level PUMP_OUT for the pump  701  attached to the word line WL of FIG.  7 . The waveform of PUMP_OUT illustrates continuous pumping of the pump  701  even during a regulation period (when REGOUT, comparator  700  output, is high). FIG. 16 illustrates a timing and voltage diagram according to an aspect of the present invention. 
     In addition to an embodiment consisting of specifically designed integrated circuits or other electronics, the present invention may be conveniently implemented using a conventional general purpose or a specialized digital computer or microprocessor programmed according to the teachings of the present disclosure, as will be apparent to those skilled in the computer art. 
     Appropriate software coding can readily be prepared by skilled programmers based on the teachings of the present disclosure, as will be apparent to those skilled in the software art. The invention may also be implemented by the preparation of application specific integrated circuits or by interconnecting an appropriate network of conventional component circuits, as will be readily apparent to those skilled in the art. 
     The present invention includes a computer program product which is a storage medium (media) having instructions stored thereon/in which can be used to program a computer to perform any of the processes of the present invention. The storage medium can include, but is not limited to, any type of disk including floppy disks, optical discs, DVD, CD-ROMs, microdrive, and magneto-optical disks, ROMs, RAMS, EPROMs, EEPROMS, DRAMs, VRAMs, flash memory devices, magnetic or optical cards, nanosystems (including molecular memory ICs), or any type of media or device suitable for storing instructions and/or data. 
     Stored on any one of the computer readable medium (media), the present invention includes software for controlling both the hardware of the general purpose/specialized computer or microprocessor, and for enabling the computer or microprocessor to interact with a human user or other mechanism utilizing the results of the present invention. Such software may include, but is not limited to, device drivers, operating systems, and user applications. Ultimately, such computer readable media further includes software for performing at least one of additive model representation and reconstruction. 
     Included in the programming (software) of the general/specialized computer or microprocessor are software modules for implementing the teachings of the present invention, including, but not limited to, separating planes of a source image, averaging at least one of foreground and background colors, replacing colors, and compensating for error introduced by color replacement in one plane by feeding error into a second plane, storage, and communication of results according to the processes of the present invention. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.