Abstract:
Provided is an electronic device including a charge pump circuit whose circuit structure is simple and boosting efficiency is high. The charge pump circuit uses MOSFETs as charge transfer elements and has a structure in which a voltage of a gate of a charge transfer MOSFET is controlled to a predetermined level based on a dividing voltage caused by a first resistor connected between a source and the gate thereof and a second resistor connected between a drain and the gate thereof and a clock pulse for on/off control of the charge transfer MOSFET is supplied to the gate through a capacitor.

Description:
[0001]     This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No 2005-131003 filed Apr. 28, 2005, the entire content of which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a charge pump circuit employed for an electronic device and the like.  
         [0004]     2. Description of the Related Art  
         [0005]     An existing electronic device includes a plurality of ICs in order to realize a function thereof. The ICs are driven at different voltages, so that a plurality of voltages different from a power source voltage are required. Up to now, the plurality of voltages are generated by a switching regulator or a charge pump circuit.  
         [0006]     The switching regulator has high power efficiency. However, there is a disadvantage in that a harmonic noise is caused at the time of current switching, so that it is necessary to use a shielded power source circuit. In addition, a coil is required as an external part, with the result that the switching regulator is not suitable for a small electronic device.  
         [0007]     On the other hand, the charge pump circuit can generate a high voltage with a low noise. However, there is a disadvantage in that power efficiency is low, so that the charge pump circuit is not suitable as a power source circuit for a mobile device in which the power efficiency is set at the highest priority. Therefore, if a charge pump circuit having high power efficiency can be realized, the charge pump circuit becomes a power source most suitable for a small mobile device.  
         [0008]     In a fundamental charge pump circuit, diodes are used as charge transfer elements and a charge is successively transferred to a next stage to increase a voltage. In contrast to this, in a charge pump circuit mounted on a MOS integrated circuit, MOSFETs instead of the diodes are used as the charge transfer elements because of adaptability to a process. However, in the charge pump circuit using the MOSFETs as the charge transfer elements, a threshold voltage Vth of each of the MOSFETs is increased by the effect of a substrate used for the charge pump circuit, with the result that the power efficiency reduces as the number of stages increases. Therefore, there has been proposed a charge pump circuit in which a voltage loss caused by the threshold voltage Vth of each of charge transfer MOSFETs is reduced to improve the power efficiency (for example, see JP 2002-233134 A).  
         [0009]      FIG. 4  is a circuit diagram showing a conventional charge pump circuit using charge transfer MOSFETs.  
         [0010]     The conventional charge pump circuit using charge transfer MOSFETs includes N-type MOSFETs  700  to  703  in each of which a source thereof is connected with a substrate thereof, coupling capacitors  710  to  712  each of which is connected with a drain of corresponding one of the N-type MOSFETs  700  to  703 , a clock generating circuit  730 , and reverse level shifting circuits  720  to  723  for converting clock signals outputted from the clock generating circuit  730  into voltages and transferring the voltages to corresponding gates of the N-type MOSFETs  700  to  703 . A connection point between the N-type MOSFETs  702  and  703  is connected with a Dickson charge pump circuit including two N-type MOSFETs  704  and  705  and two coupling capacitors  713  and  714  (hereinafter referred to as a “branch charge pump circuit  733 ”). A power source terminal, which is located on a low-potential side, of each of the reverse level shifting circuits  720  to  723  is connected with the source of one of the N-type MOSFETs  700  to  703  which is provided in a corresponding stage. A power source terminal, which is located on a high-potential side, of each of the reverse level shifting circuits  720  and  721  is connected with the source of one of the N-type MOSFETs  702  and  703  which is provided in a second next stage.  
         [0011]     The reverse level shifting circuit  720  outputs a voltage V 2  to the gate of the N-type MOSFET  700  when a clock pulse CLK′ is an L-level, so that the N-type MOSFET  700  becomes an on state. When the clock pulse CLK′ is an H-level, the reverse level shifting circuit  720  outputs a voltage Vdd to the gate of the N-type MOSFET  700 , so that the N-type MOSFET  700  becomes an off state. Similarly, each of the reverse level shifting circuits shifts levels of the clock pulse CLK′ and a clock pulse CLKB′ and supplies a corresponding voltage to the gate of each of the N-type MOSFETs.  
         [0012]     Next, a boosting operation of the charge pump circuit which is in a steady state will be described. When each of the N-type MOSFETs  700  and  702  is in an on state (CLK′=L-level), V 1 =Vdd, V 2 =3 Vdd, and V 3 =3 Vdd. In the branch charge pump circuit  733 , (V 4 =5 Vdd−Vth) and (V 5 =5 Vdd−2 Vth). Here, Vth denotes a threshold voltage of each of the N-type MOSFETs  704  and  705 .  
         [0013]     On the other hand, when each of the N-type MOSFETs  701  and  703  is turned on (CLKB′=L), V 1 =2 Vdd, V 2 =2 Vdd, and V 3 =4 Vdd. In the branch charge pump circuit  733 , (V 4 =4 Vdd−Vth) and (V 5 =6 Vdd−2 Vth).  
         [0014]     As described above, an absolute value of Vgs at the time when each of the N-type MOSFETs is turned on becomes substantially the same value (2 Vdd) and the absolute value of Vgs at the time when each of the N-type MOSFETs is turned off becomes 0 V. Therefore, Vgs is a high voltage, so that an on-resistance of each of the N-type MOSFETS reduces. Thus, a high-efficiency charge pump circuit having a large output current capacity can be realized.  
         [0015]     However, the level shifting circuit is used for the conventional charge pump circuit using the charge transfer MOSFETs, so that the current consumption of the charge pump circuit is increased by the current consumption of the level shifting circuit and a through current flows at the time when an output of the level shifting circuit is reversed. Therefore, the level shifting circuit hinders the improvement of boosting efficiency.  
         [0016]     A level shifting circuit generates a voltage to be applied to the gate of a charge transfer MOSFET based on a potential caused in a second next stage. Therefore, there is a problem in that it takes a long time to obtain a stable state after a power source voltage is applied.  
       SUMMARY OF THE INVENTION  
       [0017]     The present invention has been made to solve the above-mentioned problems. According to the present invention, it is possible to provide a charge pump circuit which is a simple circuit and has high boosting efficiency and a short start time.  
         [0018]     The charge pump circuit according to the present invention employs a structure including the following means, in which a voltage is supplied to a gate of a charge transfer MOSFET. That is, the charge pump circuit employs a structure including:  
         [0019]     a plurality of charge transfer MOSFETs connected in series;  
         [0020]     first coupling capacitors in which first ends thereof are connected with respective connection points of the charge transfer MOSFETs and in which first clock pulses whose phases are reversed to each other are supplied to second ends of the first coupling capacitors;  
         [0021]     second coupling capacitors in which first ends thereof are connected with gates of the charge transfer MOSFETs and in which second clock pulses whose phases are reversed to each other and which have potentials different from those of the first clock pulses are supplied to second ends of the second coupling capacitors;  
         [0022]     first resistors whose first ends are connected with source of the charge transfer MOSFETs and whose second ends are connected with the gates of the charge transfer MOSFETs; and  
         [0023]     second resistors whose first ends are connected with drains of the charge transfer MOSFETs and whose second ends are connected with the gates of the charge transfer MOSFETs.  
         [0024]     According to the charge pump circuit in the present invention, a high voltage can be applied as a gate-source voltage Vgs of a charge transfer MOSFET by a simple circuit. Therefore, it is possible to provide a charge pump circuit having high boosting efficiency.  
         [0025]     A voltage to be supplied to the gate of a charge transfer MOSFET is based on voltages caused in a previous stage and a next stage. Thus, a time necessary to obtain a stable state after a power source voltage is applied can be shortened. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]     In the accompanying drawings:  
         [0027]      FIG. 1  is a circuit diagram showing a charge pump circuit according to a second embodiment of the present invention;  
         [0028]      FIG. 2  is a circuit diagram showing a charge pump circuit according to a first embodiment of the present invention;  
         [0029]      FIG. 3  is an explanatory timing chart showing an operation of the charge pump circuit according to the first embodiment of the present invention; and  
         [0030]      FIG. 4  is a circuit diagram showing a conventional charge pump circuit using charge transfer MOSFETs. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       [0031]      FIG. 2  is a circuit diagram showing a charge pump circuit according to a first embodiment of the present invention.  
         [0032]     In  FIG. 2 , n N-type MOSFETs for charge transfer  100  to  105  in each of which a source thereof is connected with a substrate thereof are connected in series. Nodes at which respective adjacent N-type MOSFETs for charge transfer are connected with each other are connected with first ends of first coupling capacitors  110  to  115 . A gate of each of the N-type MOSFETs for charge transfer  100  to  105  is connected with a source thereof through corresponding one of first resistors  130  to  135  and connected with a drain thereof through corresponding one of second resistors  140  to  145 . In addition, the gates of the N-type MOSFETs are connected with first ends of second coupling capacitors  120  to  125 . It is assumed that a resistance value of each of the first resistors is equal to that of each of the second resistors. A power source voltage Vdd is supplied as an input voltage Vin to the source of the N-type MOSFET for charge transfer  100  which is located in a first stage. A boosting voltage Vout is outputted from the drain of the N-type MOSFET for charge transfer  105  which is located in a final stage. The drain of the N-type MOSFET for charge transfer  105  is connected with an output capacitor  150  and a load  151 .  
         [0033]     First clock pulses CLK and CLKB whose phases are reversed to each other are alternately inputted to second ends of the first coupling capacitors  110  to  115 . Second clock pulses CLKG and CLKGB whose phases are reversed to each other are alternately inputted to second ends of the second coupling capacitors  120  to  125 . A peak value of each of the first clock pulses CLK and CLKB is Vdd. The second clock pulses CLKG and CLKGB are generated based on the first clock pulses CLK and CLKB. In order to prevent the reverse flow of currents flowing into the N-type MOSFETs for charge transfer  100  to  105 , a period for a Lo level is shortened and the peak value is set to a suitable voltage (for example, 2 Vdd) equal to or larger than Vdd for turning on/off the N-type MOSFETs for charge transfer  100  to  105 .  
         [0034]     A combination of the N-type MOSFET for charge transfer  100 , the first coupling capacitor  110 , the second coupling capacitor  120 , the first resistor  130 , and the second resistor  140  is set as a first-stage boosting unit. A voltage at a connection point between the N-type MOSFET for charge transfer  100  and the first coupling capacitor  110  is expressed by VI. The number of boosting unit in a boosting circuit is set to the number of stages of the boosting circuit. A combination of the N-type MOSFET for charge transfer  105 , the first coupling capacitor  115 , the second coupling capacitor  125 , the first resistor  135 , and the second resistor  145  is set as an n-th-stage boosting unit.  
         [0035]     On/off control of the N-type MOSFETs for charge transfer  100  to  105  will be described with reference to  FIG. 3 . It is assumed that a peak value of each of the second clock pulses CLKG and CLKGB is 2 Vdd.  
         [0036]     As shown in  FIG. 3 , when the first clock pulse CLK is an H-level in a second-stage boosting unit which is in a steady state, the voltage V 1  of the source of the N-type MOSFET for charge transfer  101  is equal to Vdd. The voltage V 2  of the drain of the N-type MOSFET for charge transfer  101  becomes  3  Vdd by the pumping operation of the first clock pulse CLK. As shown in  FIG. 3 , when the first clock pulse CLK is an L-level, the voltage V 1  of the source of the N-type MOSFET for charge transfer  101  becomes 2 Vdd by the pumping operation of the first clock pulse CLK. The voltage V 2  of the drain of the N-type MOSFET for charge transfer  101  becomes 2 Vdd. As shown by a broken line of  FIG. 3 , when the second coupling capacitor  121  is omitted, a voltage Vga of the gate (node A) of the N-type MOSFET for charge transfer  101  is a constant voltage, 2 Vdd, which is obtained by the voltages V 1  and V 2  by the first resistor and the second resistor. According to the pumping operation of the second clock pulse CLKGB supplied to a second end of the second coupling capacitor  121 , the voltage Vga changes with respect to 2 Vdd described above. That is, when the second clock pulse CLKGB is an L-level (first clock pulse CLK is an H-level), the voltage Vga becomes Vdd. On the other hand, when the second clock pulse CLKGB is an H-level (first clock pulse CLK is an L-level), the voltage Vga becomes 3 Vdd.  
         [0037]     In other words, when the first clock pulse CLK is the L-level, V 1 =2 Vdd and Vga=3 Vdd. Then, a gate-source voltage Vgs 2  of the N-type MOSFET for charge transfer  101  which is located in a second stage is obtained as follows. 
 
Vgs2=Vga−V1=3 Vdd−2 Vdd=Vdd 
 
         [0038]     Therefore, the N-type MOSFET for charge transfer  101  which is located in the second stage becomes an on state. On the other hand, when the first clock pulse CLK is an H-level, V 1 =Vdd and Vga=Vdd. Then, the gate-source voltage Vgs 2  of the N-type MOSFET for charge transfer  101  which is located in the second stage is obtained as follows. 
 
Vgs2=Vga−V1=Vdd−Vdd=0 V 
 
 Therefore, the N-type MOSFET for charge transfer  101  which is located in the second stage becomes an off state. 
 
         [0039]     The N-type MOSFET for charge transfer  102  which is a third-stage boosting unit operates in the same manner as described above. When the first clock pulse CLK is the L-level, V 2 =2 Vdd. At this time, according to the pumping operation of the first clock pulse CLKB, V 3 =4 Vdd. On the other hand, when the first clock pulse CLK is the H-level, V 2 =3 Vdd and V 3 =3 Vdd. According to the pumping operation of the second clock pulse CLKG, a voltage Vgb of a node B changes with respect to 3 Vdd. That is, when the second clock pulse CLKG is an H-level, the voltage Vgb becomes 2 Vdd. When the second clock pulse CLKG is an L-level, the voltage Vgb becomes 4 Vdd.  
         [0040]     In other words, when the first clock pulse CLK is the L-level, a gate-source voltage Vgs 3  of the N-type MOSFET for charge transfer  102  which is located in a third stage is obtained as follows. 
 
Vgs3=Vgb−V2=2 Vdd−2 Vdd=0 V 
 
 Therefore, the N-type MOSFET for charge transfer  102  which is located in the third stage becomes an off state. On the other hand, when the first clock pulse CLK is an H-level, the gate-source voltage Vgs 3  of the N-type MOSFET for charge transfer  102  which is located in the third stage is obtained as follows. 
 
Vgs3=Vgb−V2=4 Vdd−3 Vdd=Vdd 
 
 Therefore, the N-type MOSFET for charge transfer  102  which is located in the third stage becomes an on state. 
 
         [0041]     As described above, the charge pump circuit according to the present invention can obtain the same boosting efficiency as that of the conventional charge pump circuit shown in  FIG. 4  without using the level shifting circuit and the branch charge pump circuit for supplying the power source voltage to the level shifting circuit. In addition, according to the charge pump circuit of the present invention, the gate voltage of the N-type MOSFET for charge transfer which is provided in a stage is generated based on voltages caused in a previous stage and a next stage, so that a time necessary to obtain a stable state of a boosting operation after the power source voltage is applied can be shortened as compared with the case of the conventional charge pump circuit.  
       Second Embodiment  
       [0042]      FIG. 1  is a circuit diagram showing a charge pump circuit according to a second embodiment of the present invention. This is a charge pump circuit having an optimum structure in which voltages are applied to the gates of the N-type MOSFETs for charge transfer  100  and  105  which are the boosting unit located in the first stage and the boosting unit located in the final stage in the charge pump circuit according to the first embodiment of the present invention.  
         [0043]     A voltage applied to the gate of the N-type MOSFET for charge transfer  100  which is the boosting unit located in the first stage is generated based only on a voltage caused in a subsequent stage. A voltage applied to the gate of the N-type MOSFET for charge transfer  105  which is the boosting unit located in the final stage is generated based on a voltage of an output terminal.  
         [0044]     It is assumed that a power source voltage Vdd is supplied as the input voltage Vin for the first stage, a peak value of the first clock pulse is Vdd, and a peak value of the second clock pule is 2 Vdd. When the above-mentioned structure is used, the boosting unit located in the first stage and the boosting unit located in the final stage operate as described below.  
         [0045]     First, an operation of the boosting unit located in the first stage will be described. When the first clock pulse CLK is the H-level, charges are stored in the first coupling capacitor  110 , so that V 1  becomes Vdd. On the other hand, when the first clock pulse CLK is the L-level, V 1  becomes 2 Vdd by the pumping operation of the first coupling capacitor  110 . When the pumping operation of the second coupling capacitor is omitted, a gate voltage Vg 1  of the N-type MOSFET for charge transfer  100  becomes 1.5 Vdd which is an average level between Vdd and 2 Vdd by the CR effect caused by the second resistor  140  and the second coupling capacitor  120 . Therefore, the gate voltage Vg 1  is changed with respect to 1.5 Vdd by the pumping operation of the second clock pulse CLKG supplied to the second end of the second coupling capacitor.  
         [0046]     In other words, when the first clock pulse CLK is the L-level, a gate-source voltage Vgs 1  of the N-type MOSFET for charge transfer  100  which is located in a first stage is obtained as follows. 
 
Vgs1=Vg1−Vdd=0.5 Vdd−Vdd=−0.5 Vdd 
 
 Therefore, the N-type MOSFET for charge transfer  100  which is located in the first stage becomes an off state. On the other hand, when the first clock pulse CLK is an H-level, the gate-source voltage Vgs 1  of the N-type MOSFET for charge transfer  100  which is located in the first stage is obtained as follows. 
 
Vgs1=Vg1−Vdd=2.5 Vdd−Vdd=1.5 Vdd 
 
 Therefore, the N-type MOSFET for charge transfer  100  which is located in the first stage becomes an on state. 
 
         [0047]     In this case, a gate-source voltage Vgs 1  of the N-type MOSFET for charge transfer  100  which is located in the first stage becomes 1.5 Vdd at the time when it is in an on state. Therefore, a voltage applied to the gate becomes higher than that in each of the boosting units located in other stages. Thus, the N-type MOSFET for charge transfer  100  which is located in the first stage can be designed with a size smaller than that of each of the N-type MOSFETs for charge transfer which are located in the other stages.  
         [0048]     Next, an operation of the boosting unit located in the final stage will be described. In the boosting unit located in the final stage, a voltage of the drain of the N-type MOSFET for charge transfer  105  is an output terminal voltage Vout and continuously becomes a constant voltage of “n”×Vdd. That is, when the pumping operation of the second coupling capacitor  125  is omitted, a gate voltage Vgn of the N-type MOSFET for charge transfer  105  becomes “n”×Vdd. The gate voltage Vgn is changed with respect to “n”×Vdd by the pumping operation of the second clock pulse CLKGB.  
         [0049]     In other words, when the second clock pulse CLKGB is an L-level, the gate voltage Vgn becomes (n−1)×Vdd. When the second clock pulse CLKGB is the H-level, the gate voltage Vgn becomes (n+1)×Vdd.  
         [0050]     Therefore, when the first clock pulse CLK is the L-level, a gate-source voltage Vgsn of the N-type MOSFET for charge transfer  105  which is located in the final stage is obtained as follows. 
 
Vgsn=Vgn−V( n− 1)=( n+ 1)×Vdd− n ×Vdd=Vdd 
 
 Therefore, the N-type MOSFET for charge transfer  105  becomes an on state. 
 
         [0051]     Further, when the first clock pulse CLK is the H-level, a gate-source voltage Vgsn of the N-type MOSFET for charge transfer  105  which is located in the final stage is obtained as follows. 
 
Vgsn=Vgn−V( n− 1)=( n− 1)×Vdd−( n −1)×Vdd=0 V 
 
 Therefore, the N-type MOSFET for charge transfer  105  becomes an off state. 
 
         [0052]     As described above, the resistor which is provided in the boosting units located in the first stage and the resistor which is provided in the boosting unit located in the final stage are removed from the charge pump circuit according to the first embodiment. Therefore, a size of the charge pump circuit can be further reduced.