Patent Publication Number: US-2007109035-A1

Title: Charge pump

Description:
TECHNICAL FIELD  
      The present invention relates to a charge pump, and more particularly, to a charge pump having capacitors split unevenly, allowing optimization for better power efficiency, speed and area.  
     BACKGROUND OF THE INVENTION  
      Charge pumps are well known in the art. Charge pumps are used to pump a voltage from a first level to a second level. Typically charge pumps are used in non-volatile memories to increase the voltage from a source so that the increased voltage can be used to program or to erase selected cells in the memory.  
      Referring to  FIG. 1  there is shown a schematic circuit diagram of a charge pump  10  of the prior art. The pump  10  comprises a plurality of serially connected like stages  20  ( a - f ). Each stage,  20   a , comprises a pair of cross coupled inverters. Further, the pair of cross coupled inverters has a first input  22   a , and a second input  24   a . A capacitor  26   a  has two ends, with one end connected to the first input  22   a , and another end  28   a  for receiving a clock signal CLK 2 . A capacitor  30   a  has two ends, with one end connected to the second input  24   a , and another end  32   a  for receiving a clock signal CLK 1 . Each of the capacitors  26   a  and  30   a  is of the same size. Further, the clock signal CLK 1 , is connected to the ends  32   a  of each capacitor  30   a , and the clock signal CLK 2  is connected to the ends  28   a  of each capacitor  26   a.    
      Each inverter of the pair of cross coupled inverters has a N channel transistor having a first end and a second end, and a P channel transistor having a first end and a second end. The N channel transistor is connected in series with the P channel transistor, with the first end of the N channel transistor connected to the first end of the P channel transistor. Each of the N channel transistor and the P channel transistor has a gate, with the gates connected together and to the first end of the N channel transistor of the other inverter. The second ends of the N channel transistors of the two inverters in a stage  20   a  are connected together. The second ends of the P channel transistors of the two inverters in a stage  20   a  are connected together and to the second end of the N channel transistors of an immediately adjacent stage  20   b . The second end of the N channel transistors of the first stage  20   a  is connected to a voltage source supplying Vdd. The second end of the P channel transistor of the final stage  20   f  is connected to another capacitor  40  which supplies the pumped voltage.  
     SUMMARY OF THE INVENTION  
      In the present invention, a charge pump comprises a plurality of like stages which are connected in series. Each stage comprises a pair of cross coupled inverters having a first input and a second input. Each inverter comprises a N channel transistor having a first end and a second end, and a P channel transistor having a first end and a second end with the first end of the N channel transistor connected to the first end of the P channel transistor. A first capacitor has a first end and a second end. The first end is connected to the first input with the second end receiving a first clock signal. A second capacitor, different in size from the first capacitor, has a first end and a second end. The first end is connected to the second input with the second end receiving a second clock signal. The second end of the P channel transistor of one of the inverters is connected to the second end of the N channel transistor of a corresponding inverter of an adjacent stage. The second end of the P channel transistor of another of the inverters is connected to the second end of the N channel transistor of a corresponding inverter of an adjacent stage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a circuit diagram of a charge pump of the prior art.  
       FIG. 2  is a circuit diagram of a charge pump of the present invention.  
       FIG. 3  is a graph showing the improvement in area and power consumption by reducing the connected capacitors by 12% with no impact on efficiency of 1, 2, and 3 stage charge pump of the present invention compared to the charge pump of the prior art.  
       FIG. 4  is a graph showing the improvement in time and final voltage with no impact on area and power consumption of 1, 2, and 3 stage pumps of the present invention compared to the charge pump of the prior art. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Referring to  FIG. 2 , there is shown a circuit diagram of a charge pump  110  of the present invention. The pump  110  comprises a plurality of serially connected like stages  120  ( a - f ). Each stage,  120   a , comprises a pair of cross coupled inverters. Further, the pair of cross coupled inverters has a first input  122   a , and a second input  124   a . A capacitor  126   a  has two ends, with one end connected to the first input  122   a , and another end  128   a  for receiving a clock signal CLK 2 . A capacitor  130   a  has two ends, with one end connected to the second input  124   a , and another end  132   a  for receiving a clock signal CLK 1 . Each of the capacitors  126   a  and  130   a  is of a different size. In the example shown, capacitor  130   a  is smaller than the capacitor  126   a . Further, the clock signal CLK 1 , is connected to the end  132   a  of capacitor  130   a  of stage  120   a , and to the end  128   b  of capacitor  126   b  of stage  120   b , and to the end  132   c  of capacitor  130   c  of stage  120   c  and so on. The clock signal CLK 2  is connected to the end  128   a  of capacitor  126   a  of stage  120   a , and to the end  132   b  of capacitor  130   b  of stage  120   b , and to the end  128   c  of capacitor  126   c  of stage  120   c , and so on. Thus the clock signals CLK 1  and CLK 2  are connected to alternate inputs of each stage  120 .  
      Each inverter of the pair of cross coupled inverters has a N channel transistor having a first end and a second end, and a P channel transistor having a first end and a second end. The N channel transistor is connected in series with the P channel transistor, with the first end of the N channel transistor connected to the first end of the P channel transistor. Each of the N channel transistor and the P channel transistor has a gate, with the gates connected together and to the first end of the N channel transistor of the other inverter. The second ends of the P channel transistors of the two inverters in a stage  20   a  are connected to the respective second ends of the N channel transistors of an immediately adjacent stage  20   b . Further, the N and P channel transistors whose first ends are connected together and to the end  124   a  of the capacitor  130   a  are smaller in size than the N and P channel transistors in the same stage.  
      The second ends of the N channel transistors of the first stage  20   a  are connected to a voltage source supplying Vdd.  
      The final stage  120   f  has one inverter with a N channel transistor and a P channel transistor, and a second inverter with only a single N channel transistor. The gates of the N and P channel transistors are connected to the first end of the N channel transistor of the other inverter. The gate of the N channel transistor of the other inverter is connected to the first end of the N channel transistor of the one inverter. The second end of the P channel transistor is connected to another capacitor  140  which supplies the pumped voltage.  
      Referring to  FIG. 3  there is shown a graph of time vs. voltage showing the improvement in area and power consumption by reducing the connected capacitors by 12% with no impact on efficiency of 1, 2, and 3 stage charge pump of the present invention compared to the charge pump of the prior art.  
      Referring to  FIG. 4  there is shown a graph of time vs. voltage showing the improvement in time and final voltage with no impact on area and power consumption of 1, 2, and 3 stage pumps of the present invention compared to the charge pump of the prior art.  
      As can be seen from  FIGS. 3 and 4 , the advantage of the charge pump  110  of the present invention compared to the pump  10  of the prior art is that there is improvement in area and power consumption, as well as efficiency.