FIELD OF THE INVENTION 
       [0001]    This invention pertains generally to the field of charge pumps and more particularly to clock generation circuit for use with a charge pump. 
       BACKGROUND 
       [0002]    Charge pumps use a switching process to provide a DC output voltage larger than its DC input voltage. In general, a charge pump will have a capacitor coupled to switches between an input and an output. During one clock half cycle, the charging half cycle, the capacitor couples in parallel to the input so as to charge up to the input voltage. During a second clock half cycle, the transfer half cycle, the charged capacitor couples in series with the input voltage so as to provide an output voltage twice the level of the input voltage. This process is illustrated in  FIGS. 1   a  and  1   b.  In  FIG. 1   a,  the capacitor  5  is arranged in parallel with the input voltage V IN  to illustrate the charging half cycle. In  FIG. 1   b,  the charged capacitor  5  is arranged in series with the input voltage to illustrate the transfer half cycle. As seen in  FIG. 1   b,  the positive terminal of the charged capacitor  5  will thus be 2*V IN  with respect to ground. 
         [0003]    Charge pumps are used in many contexts. For example, they are used as peripheral circuits on flash and other non-volatile memories to generate many of the needed operating voltages, such as programming or erase voltages, from a lower power supply voltage. A number of charge pump designs, such as conventional Dickson-type pumps, are know in the art. But given the common reliance upon charge pumps, there is an on going need for improvements in pump design, particularly with respect to trying to reduce the amount of layout area and the current consumption requirements of pumps. 
         [0004]      FIG. 2  is a top-level block diagram of a typical charge pump arrangement. The designs described here differ from the prior art in details of how the pump section  201 . As shown in  FIG. 2 , the pump  201  has as inputs a clock signal and a voltage Vreg and provides an output Vout. The high (typically Vext from the external power supply) and low (ground) connections are not explicitly shown. The voltage Vreg is provided by the regulator  203 , which has as inputs a reference voltage Vref from an external voltage source and the output voltage Vout. The regulator block  203  regulates the value of Vreg such that the desired value of Vout can be obtained. The pump section  201  will typically have cross-coupled elements, such at described below for the exemplary embodiments. (A charge pump is typically taken to refer to both the pump portion  201  and the regulator  203 , when a regulator is included, although in some usages “charge pump” refers to just the pump section  201 .) 
         [0005]    In a typical charge pump arrangement, a Dickson type pump for example, the pump  201  will have a capacitor (such as  5  of  FIG. 1 ) for each stage, where one plate is driven by input voltage to the pump or a previous stage and the other plate receives a clock signal. In path providing this clock signal there will generally be some parasitic capacitance, C par , driven at the clock frequency f clock , which leads to the generation of current and a corresponding power consumption. The amount of current consumption will also be dependent of the value of the external voltage supply, Vext, since the clock drivers typically use Vext to increase pump efficiency, and be of the form f clock C par Vext. Consequently, as supply the supply voltage increases, the pump will consume more power due to these clock driver parasitics. (The value Vext is from the power supply external to the device on which the charge pump is formed, as opposed to the high level on device, typically referred to as Vcc for example, which is external to pump, but not to the system.) 
       SUMMARY OF THE INVENTION 
       [0006]    In a first aspect, a charge pump system is formed on an integrated circuit that can be connected to an external power supply. The system includes a charge pump and a clock generator circuit. The clock circuit is coupled to provide a clock output, at whose frequency the charge pump operates and generates an output voltage from an input voltage. The clock frequency is a decreasing function of the voltage level of the external power supply. 
         [0007]    In another aspect, a method is described for reducing power consumption in a charge pump system formed on a circuit connectable to an external power supply. This includes receiving a voltage level from the external power supply at a clock circuit and generating in the clock circuit a clock signal having a frequency that is a decreasing function of the voltage level of the external power supply. The clock signal is provided to a charge pump, which operates at the frequency of this clock signal to generate an output voltage from an input voltage. 
         [0008]    Various aspects, advantages, features and embodiments of the present invention are included in the following description of exemplary examples thereof, which description should be taken in conjunction with the accompanying drawings. All patents, patent applications, articles, other publications, documents and things referenced herein are hereby incorporated herein by this reference in their entirety for all purposes. To the extent of any inconsistency or conflict in the definition or use of terms between any of the incorporated publications, documents or things and the present application, those of the present application shall prevail. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The various aspects and features of the present invention may be better understood by examining the following figures, in which: 
           [0010]      FIG. 1   a  is a simplified circuit diagram of the charging half cycle in a generic charge pump. 
           [0011]      FIG. 1   b  is a simplified circuit diagram of the transfer half cycle in a generic charge pump. 
           [0012]      FIG. 2  is a top-level block diagram for a regulated charge pump. 
           [0013]      FIG. 3  is a general clock generator. 
           [0014]      FIGS. 4A and 4B  illustrate a clock generator circuit with a period not directly dependent on the external power supply level. 
           [0015]      FIGS. 5A and 5B  show an exemplary embodiment of a clock generator circuit whose frequency is a decreasing function of the power supply voltage. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The typical pump design uses a constant clock frequency across supply voltage levels. As noted in the Background, as the supply voltage increases, because of clock driver parasitics, the pump consumes more power. To ameliorate this, the following presents a clock generator design that tracks the clock driver period with the external pump supply voltage. More specifically, the clock generator will have a frequency that is a decreasing function of the supply voltage, so that as the supply voltage increases, the frequency will decrease and vice versa. Consequently, the design will save on pump power consumption while maintaining the pump&#39;s I-V curve. 
         [0017]    More information on charge pumps, such Dickson type pumps and charge pumps generally, can be found, for example, in “Charge Pump Circuit Design” by Pan and Samaddar, McGraw-Hill, 2006, or “Charge Pumps: An Overview”, Pylarinos and Rogers, Department of Electrical and Computer Engineering University of Toronto, available on the webpage “www.eecg.toronto.edu/˜kphang/ece1371/chargepumps.pdf”. Further information on various other charge pump aspects and designs can be found in U.S. Pat. Nos. 5,436,587; 6,370,075; 6,556,465; 6,760,262; 6,922,096; and 7,135,910; and applications Ser. Nos. 10/842,910 filed on May 10, 2004; 11/295,906 filed on Dec. 6, 2005; 11/303,387 filed on Dec. 16, 2005; 11/497,465 filed on Jul. 31, 2006; 11/523,875 filed on Sep. 19, 2006; 11/845,903 and 11/845,939, both filed Aug. 28, 2007; 11/955,221 and 11/995,237, both filed on Dec. 12, 2007; and 12/135,945 filed on Jun. 9, 2008. 
         [0018]      FIG. 3  is a block diagram of an exemplary clock generator circuit. This shows a latch formed of the gates  311 ,  313 ,  315 , and  317  that has as its output the clock signal (CLK) used for the charge pump. The NAND gate  315  has as an input a reset signal RSTn to enable the latch. The output clock signal CLK from gate  317  is fed back into the delay element  301 , which in turn supplies an input to the gate  311 . The output of gate  313  is fed back to a similar delay element  303 , which in turn supplies the gate  315 . The output CLK begins clocking when RSTn is switched to logic high. If the period of the delay elements are each Δt delay , then the period for the clock signal CLK will be Δt period =2Δt delay .  FIG. 3  is just one example of a clock generator circuit and other arranges for latch shown here can be used to generator a clock signal from a delay element or elements. 
         [0019]      FIG. 4A  shows an example of a delay element that exhibits the sort of behavior typical in the prior art, where the amount of delay does not vary with the level of the external power supply. The output (OUT) of the delay element, which would be fed into  311  and  315  in  FIG. 3  is the output from DiffAmp  401 , whose − input is connected up to a reference voltage Vref. A pair of switches SW 1   411  and SW 2   413  are connected in series between a supply level V 1  and ground, with a current source  405  with a value of Iref connected in between. In the example here, V 1  would be the high voltage level on the chip, Vcc, with a value that of, say, Vcc=2.5V here. The voltage on the node above the current source  405 , Vcom, is fed into the + input of amp  401 . A capacitor  403  is also connected between the line at Vcom and ground. The switches  411  and  413  are controlled by the input from either  317  (CLK) or  313  as shown on  FIG. 3  to alternately open and close them. 
         [0020]    Initially, with switch SW 1   411  closed and SW 2   413  open, Vcom will precharge up to V 1 . At t 0 =0, SW 1   411  is opened and SW 2   413  is closed, so that Vcom is discharged by Iref through the current source  405 . The time, Δt delayO , it takes to switch OUT from High to Low is then: 
         [0000]    
       
         
           
             
               Δ 
                
               
                   
               
                
               
                 t 
                 delayO 
               
             
             = 
             
               
                 C 
                 Iref 
               
                
               
                 
                   ( 
                   
                     
                       V 
                       1 
                     
                     - 
                     Vref 
                   
                   ) 
                 
                 . 
               
             
           
         
       
     
         [0000]    Since each of the quantities does not have any real dependence on the value of the external power supply voltage Vext, the delay—and consequently the clock period—will not depend on the value of the external power supply either. (Again, V 1  would here be the on chip Vcc value, not the external supply level.) 
         [0021]      FIG. 4B  shows the clock signal from clock generator using a delay element such as in  FIG. 4A . In the top waveform, the external voltage Vext is about the same as the V 1  value, say the Vcc value of 2.5V. The lower waveform is for a higher external supply voltage, say Vext=3.5V. The frequency is the same. Consequently, there is no compensation from the pump&#39;s clock frequency to offset the increase in power consumption due to the increase in Vext. 
         [0022]      FIG. 5A  is an exemplary embodiment of a delay element whose delay increases with the external poser supply voltage level. Consequently, the frequency of the clock circuit using such a delay element will be a decreasing function of the power supply voltage. The arrangement of  FIG. 5A  is just an exemplary embodiment and other arrangements can also be used that provide a delay that is a decreasing function of the external power supply. 
         [0023]    In  FIG. 5A , the output (OUT) of the delay element, which would be fed into  311  and  315  in  FIG. 3  is again the output from DiffAmp  501 , whose − input is connected up to a reference voltage Vref. And as before a pair of switches SW 1   511  and SW 2   513  are connected in series between a supply level V 1  and ground, with a current source  505  with a value of Iref connected in between. In the example here, V 1  would be the high voltage level on the chip, Vcc, with a value that of, say, Vcc=2.5V here, and Vref would typically be a bandgap reference value less than V 1 , with a value of, say, Vref=1.2V here. The voltage on the node above the current source  505 , Vcom, is fed into the + input of amp  501 . The level on Vcom is now arranged differently. 
         [0024]    A capacitor C 1   507  is connected between the line at Vcom and V 1 . A second capacitor C 2   509  is also connected on the one side to the line at Vcom and at the other side through a switch SW 3   515  to, when position A, V 1  or, when in position B to V 2 , where V 2  is here the external supply voltage, V 2 =Vext. The switches SW 1   511 , SW 2   513 , and SW 3   515  are controlled but the input from either  317  (CLK) or  313  as shown on  FIG. 3  to alternately open and close them. 
         [0025]    Starting initially with SW 1   511  closed, SW 2   513  open, and SW 3   515  at position A, the top plates of the both C 1   507  and C 2   509  are at V 1  and Vcom is precharged to V 1 . Then, at t 0 =0, SW 1   511  is opened, SW 2   513  is closed, and SW 3  is at position B and connected to V 2 , so that Vcom is discharged by Iref through the current source  505 . The time, Δt delayNew , it takes to switch OUT from High to Low is then: 
         [0000]    
       
         
           
             
               Δ 
                
               
                   
               
                
               
                 t 
                 delayNew 
               
             
             = 
             
               
                 
                   
                     C 
                     1 
                   
                   + 
                   
                     C 
                     2 
                   
                 
                 Iref 
               
                
               
                 
                   ( 
                   
                     Vcom 
                     - 
                     Vref 
                   
                   ) 
                 
                 . 
               
             
           
         
       
     
         [0000]    Right after fed back input changes the switches at t 0 =0, Vcom is given by: 
         [0000]    
       
         
           
             
               At 
                
               
                   
               
                
               
                 
                   t 
                   
                     0 
                     + 
                   
                 
                 : 
                 Vcom 
               
             
             = 
             
               
                 
                   
                     
                       V 
                       1 
                     
                      
                     
                       C 
                       1 
                     
                   
                   + 
                   
                     
                       V 
                       2 
                     
                      
                     
                       C 
                       2 
                     
                   
                 
                 
                   
                     C 
                     1 
                   
                   + 
                   
                     C 
                     2 
                   
                 
               
               . 
             
           
         
       
     
         [0000]    Consequently, this gives 
         [0000]    
       
         
           
             
               Δ 
                
               
                   
               
                
               
                 t 
                 delayNew 
               
             
             = 
             
               
                 
                   
                     
                       C 
                       1 
                     
                      
                     
                       ( 
                       
                         
                           V 
                           1 
                         
                         - 
                         Vref 
                       
                       ) 
                     
                   
                   - 
                   
                     
                       C 
                       2 
                     
                     * 
                     Vref 
                   
                 
                 Iref 
               
               + 
               
                 
                   
                     C 
                     2 
                   
                   * 
                   
                     V 
                     2 
                   
                 
                 Iref 
               
             
           
         
       
     
         [0026]    As before in  FIG. 4A , V 1  would be the high voltage level on the chip, Vcc, with a value that of, say, Vcc=2.5V here. The Vref and Iref values will also be fixed, so that Δt delayNew  will increase/decrease as V 2  increases/decreases. Consequently, by taking V 2 =Vext, the frequency (being inversely related to Δt delayNew ) will be a decreasing function of Vext. 
         [0027]    This behavior is illustrated in  FIG. 5B . The upper waveform corresponds to a lower Vext value, say 2.5V, and the lower waveform corresponds to a higher value of, say 3.9V. As shown, the clock frequency decreases for the higher Vext supply value. 
         [0028]    As discussed in the Background, power consumption of the charge pump system has a contribution of the product of the external voltage and the clock frequency. By having the frequency as a decreasing function of Vext, the dependency of their product on the external supply level can be reduced, with the parameters (Vref, C 1 , . . . ) chosen accordingly. For example, if C 2  is taken so that: 
         [0000]    
       
         
           
             
               
                 C 
                 2 
               
               = 
               
                 
                   
                     C 
                     1 
                   
                    
                   
                     ( 
                     
                       
                         V 
                         1 
                       
                       - 
                       Vref 
                     
                     ) 
                   
                 
                 Vref 
               
             
             , 
           
         
       
     
         [0000]    then, putting this into the relation for Δt delayNew  gives f clock ˜1/Vext. Consequently, the dependence on Vext in the current consumption due to the parasitic capacitance will cancel out. 
         [0029]    Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. Consequently, various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as encompassed by the following claims.
A charge pump system is formed on an integrated circuit that can be connected to an external power supply. The system includes a charge pump and a clock generator circuit. The clock circuit is coupled to provide a clock output, at whose frequency the charge pump operates and generates an output voltage from an input voltage. The clock frequency is a decreasing function of the voltage level of the external power supply. This allows for reducing power consumption in the charge pump system formed on a circuit connectable to an external power supply.