Abstract:
An n-channel MOS transistor negative-voltage charge pump is disclosed in which the bulks of the n-channel MOS transistors are biased in such a manner as to prevent turning on the parasitic bipolar transistor inherent in the CMOS environment of the charge pump structure.

Description:
PRIORITY CLAIM  
         [0001]    This application claims priority to Italian Application Serial Number 2002A000821, filed Sep. 20, 2002.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to charge pump circuits. More particularly, the present invention relates to a negative charge pump that switch the bulk of each transistor stage to the lowest potential node to minimize body effect.  
           [0004]    2. The State of the Art  
           [0005]    In integrated circuit applications such as flash memory, EEPROMs and the like, generation of a negative voltage is required. In the case of non-volatile memories that operate with only one level of supply voltage, the internal high voltages are generated with charge pumps. The charge pumps are used to generate both positive and negative voltages. Charge pumps for generating negative voltages are usually formed using triple-well processes. Such negative charge pumps use n-channel MOS transistors pump a voltage line to a negative value.  
           [0006]    Referring to FIG. 1, a schematic diagram depicts a commonly employed prior-art implementation of a negative charge pump formed from n-channel MOS transistors. Charge pump  10  includes three stages,  12 ,  14 , and  16 , driven by a four-phase clock. Each stage includes two n-channel MOS transistors and two capacitors.  
           [0007]    Stage  12  includes n-channel MOS transistors  18  and  20 . N-channel MOS transistor  18  has its drain coupled to ground, its source coupled to the source of n-channel MOS transistor  20  and its gate coupled to the drain of n-channel MOS transistor  20  and to the phase-D signal of the clock through capacitor  22 . The gate of n-channel transistor  20  is coupled to the drain of n-channel MOS transistor  18  and to the phase-A signal of the clock through capacitor  24 .  
           [0008]    Stage  14  includes n-channel MOS transistors  26  and  28 . N-channel MOS transistor  26  has its drain coupled to the sources of n-channel MOS transistors  18  and  20  from stage  12 , its source coupled to the source of n-channel MOS transistor  28  and its gate coupled to the drain of n-channel MOS transistor  28  and to the phase-B signal of the clock through capacitor  30 . The gate of n-channel transistor  28  is coupled to the drain of n-channel MOS transistor  26  and to the phase-C signal of the clock through capacitor  32 .  
           [0009]    Stage  16  includes n-channel MOS transistors  34  and  36 . N-channel MOS transistor  34  has its drain coupled to the sources of n-channel MOS transistors  26  and  28  from stage  14 , its source coupled to the source of n-channel MOS transistor  36  and its gate coupled to the drain of n-channel MOS transistor  36  and to the phase-D signal of the clock through capacitor  38 . The gate of n-channel transistor  36  is coupled to the drain of n-channel MOS transistor  34  and to the phase-A signal of the clock through capacitor  40 .  
           [0010]    As may be seen from an examination of FIG. 1, each of the n-channel MOS transistors  18 ,  20 ,  26 ,  28 ,  34 , and  36  has its bulk connected to the most negative node (VNEG at reference numeral  42 ) that serves as the output of the charge pump. The reason for this is to avoid turning on the parasitic bipolar transistor formed in each stage by the buried n-well, the p-well and the n+ source and drain regions of the n-channel MOS transistors.  
           [0011]    In the charge-pump circuit of FIG. 1, the parasitic bipolar transistor in the last stage  16  can be turned on during the transition toward the steady state (from 0 to VNEG) when the phase-A signal of the clock goes low to sink current from the load. If the bipolar transistor turns on, the efficiency of the charge pump is compromised because the current is no longer sunk by the load but from the grounded buried-n-well collector of the bipolar transistor.  
           [0012]    Moreover another drawback of the implementation of FIG. 1 is that body effect of the n-channel MOS transistors of the charge pump increases moving from right to left of the pump. This can severely limit the performance of the charge pump in terms of maximum negative voltage in those applications where very low power supply voltages are employed.  
           [0013]    Referring now to FIG. 2, a schematic diagram shows a prior-art solution that can be adopted to reduce but does not eliminate the body effect inside each stage of the charge pump. The circuit of FIG. 2 is substantially similar to the circuit of FIG. 1, except that the bulks of the two n-channel MOS transistors in each stage are coupled to the output node of the stage. Thus, the bulks of n-channel MOS transistors  18  and  20  are coupled to their common sources; the bulks of n-channel MOS transistors  26  and  28  are coupled to their common sources; and the bulks of n-channel MOS transistors  34  and  36  are coupled to their common sources. This configuration does not solve the parasitic bipolar turn-on problem in the last stage  16 .  
           [0014]    Another technique to reduce the body effect is disclosed in U.S. Pat. No. 6,130,572. This circuit has the same drawback of FIG. 2. In particular, for low-voltage applications, the problem of threshold-voltage increase due to body effect is very important because the difference between the V DD  and V th  of the MOS transistors is reduced and degrades performance.  
           [0015]    Another prior-art technique used to reduce the influence of the body effect is to use a level shifter to boost the phase of the charge pumps but in this way the efficiency (I load /V DD ) of the charge pump is reduced. Another drawback of this method is that the silicon area is undesirably increased.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0016]    The present invention provides a n-channel MOS transistor charge pump in which the bulks of the n-channel MOS transistors are biased in such a manner as to prevent turning on the parasitic bipolar transistor inherent in the CMOS environment of the charge pump structure.  
           [0017]    A negative-voltage charge pump has a plurality of operating phases and comprises a plurality of stages, each stage comprising at least two n-channel MOS transistors each including bulk regions. Each of said stages also includes a parasitic bipolar transistor. The bulk regions are switchably coupled during each of the operating phases to a circuit node having a potential such that the parasitic bipolar transistor will not turn on. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES  
       [0018]    [0018]FIG. 1 is a schematic diagram depicting a common implementation of a negative charge pump employing n-channel MOS transistors.  
         [0019]    [0019]FIG. 2 is a schematic diagram depicting another prior-art implementation of a negative charge pump employing n-channel MOS transistors.  
         [0020]    [0020]FIG. 3 is a schematic diagram depicting a single stage of a negative charge pump employing n-channel MOS transistors according to then present invention.  
         [0021]    [0021]FIG. 4 is a schematic diagram depicting multiple stages of a negative charge pump employing n-channel MOS transistors according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    Persons of ordinary skill in the art will realize that the following description of the present invention is only illustrative and not in any way limiting. Other embodiments of this invention will be readily apparent to those skilled in the art having benefit of this disclosure.  
         [0023]    The purpose of the present invention is to overcome some of the aforementioned drawbacks by biasing the bulk of each stage of the charge pump in such a way that body effect is eliminated (Vbs=0 when transistors are on) and the parasitic bipolar transistors never turn on even in the output stage.  
         [0024]    Referring now to FIG. 3, a schematic diagram shows an illustrative embodiment of a single stage  50  of an illustrative charge pump that operates in accordance with the principles of the present invention. For an ease of understanding the present invention, FIG. 3 illustrates a stage corresponding to the second stage of the charge pumps of FIGS. 1 and 2 and the same reference numerals as used in those figures will be used in FIG. 3 to identify corresponding circuit elements.  
         [0025]    Stage  50  includes n-channel MOS transistors  26  and  28 . As in the prior-art charge pump circuits of FIGS. 1 and 2, n-channel MOS transistor  26  has its drain coupled to the common sources of the two n-channel MOS transistors from the preceding stage (which in this case would be the sources of n-channel MOS transistors corresponding to reference numerals  18  and  20  of FIGS. 1 and 2) (or to ground if stage  50  is the first stage), its source coupled to the source of n-channel MOS transistor  28  and its gate coupled to the drain of n-channel MOS transistor  28  and to the phase-B signal of the clock through capacitor  30 . The gate of n-channel transistor  28  is coupled to the drain of n-channel MOS transistor  26  and to the phase-C signal of the clock through capacitor  32 . The phase-A signal of the clock is shown coupled to the common sources of n-channel MOS transistors  26  and  28  through capacitor  40  as it is in the charge pumps depicted in FIGS. 1 and 2.  
         [0026]    The bulks of n-channel MOS transistors  26  and  28  are connected together to a node  50 . Node  50  is coupled to the drains of both n-channel MOS transistors  52  and  54  as well as to their bulk regions. The source of n-channel MOS transistor  52  is coupled to the common sources of the two n-channel MOS transistors of the previous stage, and the source of n-channel MOS transistor  54  is coupled to the common sources of the two n-channel MOS transistors  26  and  28 . The gate of n-channel MOS transistor  52  is coupled to the drain and gate of n-channel MOS transistor  28  and the gate of n-channel MOS transistor  54  is coupled to the common sources of n-channel MOS transistors  26  and  28 .  
         [0027]    This single stage  50  works as before: when the phase-A signal of the clock is high and the phase-C signal of the clock is low, the phase-B signal of the clock also goes high and turns on n-channel MOS transistor  26 , allowing current to flow from capacitor  40  to capacitor  30  thus discharging capacitor  40  and charging up capacitor  30 . Then the phase-A signal of the clock goes low and receives charge from the following stage while the phase-C signal of the clock goes high, transferring charge to the previous stage.  
         [0028]    Adding the two transistors  52  and  54  to each stage prevents the parasitic bipolar transistor from being turned on. When the phase-C signal of the clock is high and the phase-A signal of the clock is low, the phase-B signal of the clock is also low, n-channel MOS transistor  52  is turned off and n-channel MOS transistor  54  is turned on, thus biasing node  50  to the same potential of as the common sources of n-channel MOS transistors  26  and  28 , which is the lowest voltage seen by the transistors of this stage. In the other half period when the phase-A signal of the clock is high and the phase-C signal of the clock is low, the phase-B signal of the clock is also high and n-channel MOS transistor  54  is turned off but n-channel MOS transistor  52  is turned on, thus assuring that the bulk regions of n-channel MOS transistors  26  and  28  are at a potential that is more negative or the same as any n+ region of the stage.  
         [0029]    Referring now to FIG. 4, a schematic diagram shows an illustrative charge-pump circuit  60  in accordance with the present invention including multiple charge-pump stages. As with the circuit of FIG. 3, the same reference numerals as used in FIGS. 1 and 2 will be used in FIG. 4 to identify corresponding circuit elements.  
         [0030]    As shown in the charge-pump circuit  10  of FIG. 1, charge-pump circuit  60  of FIG. 4 includes three stages,  62 ,  64 , and  66 , driven by a four-phase clock. Each stage includes the same two n-channel MOS transistors and two capacitors.  
         [0031]    Stage  62  includes n-channel MOS transistors  18  and  20 . N-channel MOS transistor  18  has its drain coupled to ground, its source coupled to the source of n-channel MOS transistor  20  and its gate coupled to the drain of n-channel MOS transistor  20  and to the phase-D signal of the clock through capacitor  22 . The gate of n-channel transistor  20  is coupled to the drain of n-channel MOS transistor  18  and to the phase-A signal of the clock through capacitor  24 .  
         [0032]    In addition, stage  62  includes n-channel MOS transistors  68  and  70  having their drains coupled together to node  72  comprising the bulk regions of n-channel MOS transistors  18  and  20  as well as the bulk regions of n-channel MOS transistors  68  and  70 . The source of n-channel MOS transistor  68  is coupled to the drain of n-channel MOS transistor  18  and its gate is coupled to the drain of n-channel MOS transistor  20 . The source of n-channel MOS transistor  70  is coupled to the common sources of n-channel MOS transistors  18  and  20  and its gate is coupled to the drain of n-channel MOS transistor  18 .  
         [0033]    Stage  64  includes n-channel MOS transistors  26  and  28 . N-channel MOS transistor  18  has its drain coupled to the sources of n-channel MOS transistors  18  and  20  from stage  62 , its source coupled to the source of n-channel MOS transistor  28  and its gate coupled to the drain of n-channel MOS transistor  28  and to the phase-B signal of the clock through capacitor  30 . The gate of n-channel transistor  28  is coupled to the drain of n-channel MOS transistor  26  and to the phase-C signal of the clock through capacitor  32 .  
         [0034]    In addition, stage  64  includes n-channel MOS transistors  74  and  76  having their drains coupled together to node  78  comprising the bulk regions of n-channel MOS transistors  26  and  28  as well as the bulk regions of n-channel MOS transistors  74  and  76 . The source of n-channel MOS transistor  74  is coupled to the drain of n-channel MOS transistor  26  and its gate is coupled to the drain of n-channel MOS transistor  28 . The source of n-channel MOS transistor  76  is coupled to the common sources of n-channel MOS transistors  26  and  28  and its gate is coupled to the drain of n-channel MOS transistor  26 .  
         [0035]    Stage  66  includes n-channel MOS transistors  34  and  36 . N-channel MOS transistor  34  has its drain coupled to the sources of n-channel MOS transistors  26  and  28  from stage  64 , its source coupled to the source of n-channel MOS transistor  36  and its gate coupled to the drain of n-channel MOS transistor  36  and to the phase-D signal of the clock through capacitor  38 . The gate of n-channel transistor  36  is coupled to the drain of n-channel MOS transistor  34  and to the phase-A signal of the clock through capacitor  40 .  
         [0036]    In addition, stage  66  includes n-channel MOS transistors  80  and  82  having their drains coupled together to node  84  comprising the bulk regions of n-channel MOS transistors  34  and  36  as well as the bulk regions of n-channel MOS transistors  80  and  82 . The source of n-channel MOS transistor  80  is coupled to the drain of n-channel MOS transistor  34  and its gate is coupled to the drain of n-channel MOS transistor  36 . The source of n-channel MOS transistor  82  is coupled to the common sources of n-channel MOS transistors  34  and  36  and its gate is coupled to the drain of n-channel MOS transistor  34 . Stage  66  also includes capacitor  86  coupling the phase-C signal of the clock to the sources of n-channel MOS transistors  34  and  36 .  
         [0037]    The output of the charge pump of FIG. 4 is the VNEG node  88  at the source of n-channel MOS transistor  90 . The drain of n-channel MOS transistor  90  is coupled to the sources of n-channel MOS transistors  26  and  28 . The gate of n-channel MOS transistor is coupled to the drain of n-channel MOS transistor  36 . The bulk of n-channel MOS transistor  90  is coupled to node  78 .  
         [0038]    In the embodiment of FIG. 4, the last stage  66  is not used to transfer charge, but is present for the purpose of properly biasing the gate of n-channel MOS transistor  90 .  
         [0039]    The capacitors  22 ,  24 ,  30 ,  32 , 38 ,  40 , and  86  used in the circuits of FIGS. 3 and 4 may be formed as either poly-1 to poly-2 capacitors or as MOS capacitors. Typical values for these capacitors may be from about 500 fF to about 7 pF, although capacitors  38  and  86  in the last stage may have low values since they are not used to transfer charge to the load.  
         [0040]    Persons of ordinary skill in the art will appreciate that, from the disclosure of FIG. 4, charge-pump circuits according to the principles of the present invention may be realized using any number of stages.  
         [0041]    Using the circuit shown in FIG. 4, the problem of turning on the parasitic bipolar transistor in the output stage can be overcome. All transistors  34 ,  36 ,  80 , and  82  have their bulk regions biased more negatively or at the same potential of any n+ junction of the stage.  
         [0042]    While embodiments and applications of this invention have been shown and described, it would be apparent to those skilled in the art that many more modifications than mentioned above are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims.