Abstract:
A voltage conversion circuit incorporate many of what were previously discrete components into a single IC, whereby the number of externally disposed discrete components is reduced as much as possible to reduce the overall size while maintaining high power conversion efficiency. This is achieved by using multiple wells within wells, and coupling the wells to specific voltage potentials to protect the circuit from failure.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a voltage conversion circuit used, for example, in the power supply of a liquid crystal panel and configured to output two voltages, that is, a positive and negative voltage, based on an input dc voltage.  
           [0003]    2. Description of the Related Art  
           [0004]    A first example of a voltage conversion circuit of this type according to the prior art (referred to below as first prior art circuit) is shown in FIG. 29.  
           [0005]    As shown in FIG. 29 this first prior art circuit has a timing signal generator  1 , MOS transistors Q 1  to Q 6  that are controllably switched ON/OFF by the output from the timing signal generator  1 , capacitors C 1  and C 2  producing a positive voltage twice the dc input voltage Vin according to the ON/OFF operation of MOS transistors Q 1  to Q 4 , capacitors C 3  and C 4  producing a negative voltage −1 times the dc input voltage Vin according to the ON/OFF operation of MOS transistors Q 1 , Q 2 , Q 5 , and Q 6 , and a level shift circuit  2 .  
           [0006]    MOS transistors Q 1  and Q 5  of MOS transistors Q 1  to Q 6  are n-type, and MOS transistors Q 2  to Q 4  and Q 6  are p-type. As shown in the figure, level shift circuit  2  consists of resistors, diodes, and the like.  
           [0007]    Note also that the timing signal generator  1  and MOS transistors Q 1  to Q 4  enclosed in the dotted square in FIG. 29 in this first prior art circuit, , are integrated onto the same semiconductor substrate, forming an IC chip. The level shift circuit  2 , MOS transistors Q 5  and Q 6 , and capacitors C 1  to C 4  are separate components externally connected to the IC chip.  
           [0008]    The operation of this first prior art circuit is described next with reference to FIG. 29 to FIG. 31.  
           [0009]    The timing signal generator  1  generates and supplies timing signals (control signals) A, XB, XA 2 , XB 2  to the gate of MOS transistors Q 1  to Q 4 , respectively, to switch the MOS transistors Q 1  to Q 4  ON or OFF. Timing signals A and XB from timing signal generator  1  are level shifted by the level shift circuit  2 , which outputs the resulting signals AS and XBS to the gates of MOS transistors Q 5  and Q 6  to switch MOS transistors Q 5  and Q 6  ON or OFF.  
           [0010]    This operation causes MOS transistors Q 1 , Q 3 , Q 5  to be ON and MOS transistors Q 2 , Q 4 , Q 6  to be off in period T 1  shown in FIG. 30. The circuit equivalent to operation period T 1  is shown in FIG. 31A. Capacitor C 1  is charged by the dc source Vin. The positive output voltage VOUT 1  is the sum of the source voltage Vin and the stored charge voltage of capacitor C 2 . At the same time the charge of capacitor C 3  is shared with capacitor C 4 , and the end voltage of capacitor C 3  becomes the negative output voltage VOUT 2 .  
           [0011]    In period T 2  in FIG. 30 MOS transistors Q 2 , Q 4 , Q 6  switch on, and MOS transistors Q 1 , Q 3 , Q 5  switch off. The circuit equivalent to period T 2  is shown in FIG. 31B. Capacitor C 3  is charged by the dc source Vin, and the negative output voltage VOUT 2  is the voltage across capacitor C 4 . At the same time, the charge of capacitor C 1  is shared with capacitor C 2 , and the positive output voltage VOUT 1  becomes the sum of the source voltage Vin and the stored charge voltage of capacitor C 1 . The voltage drops VF are due to the forward voltage drops of the diodes within the level shift circuit  2 . This first prior art circuit thus operates as a charge-pump type dc-dc converter by simply repeating the operation of periods T 1  and T 2 .  
           [0012]    As a result of this operation, the values of positive output voltage VOUTI and negative output voltage VOUT 2  from this first prior art circuit can be determined from equations (1) and (2) where the ground GND potential is 0 V.  
           VOUT 1 = Vin ·2  (1)  
           VOUT 2   =Vin ·(−1)  (2)  
           [0013]    where Vin is the dc input voltage.  
           [0014]    [0014]FIG. 32 shows the configuration of the MOS transistors Q 1  to Q 4  inside the dotted line in FIG. 29 integrated to a semiconductor substrate in this first prior art circuit.  
           [0015]    As shown in FIG. 32, reference numeral  11  is a p-type semiconductor substrate. An NMOS transistor Q 1  with a source S, gate G, and drain D is formed in this p-type semiconductor substrate  12 . Three n-wells  11  to  14  are also formed in p-type semiconductor substrate  11 , and PMOS transistors Q 2  to Q 4  each having a source S, gate G, and drain D are formed in each of these n-wells  12  to  14 .  
           [0016]    Connections between parts of MOS transistors Q 1  to Q 4  and the p-type semiconductor substrate  11  are indicated by the bold lines in FIG. 32.  
           [0017]    A second example of a voltage conversion circuit of this type according to the prior art (referred to below as second prior art circuit) is described next with reference to FIG. 33.  
           [0018]    As shown in FIG. 33, this second prior art circuit replaces MOS transistors Q 5  and Q 6  of the first prior art circuit with diodes D 1 , D 2 , and eliminates the level shift circuit  2 . The configuration of other parts is identical to the configuration of the first prior art circuit, and further description thereof is thus omitted.  
           [0019]    Operation of this second prior art circuit is basically the same as that of the first prior art circuit except that since the switching devices are diodes, and not complementary MOS transistors, a voltage drop, VFa, equivalent to the forward voltage drop of the diodes is introduced at the output. Therefore, VOUT 2  of this embodiment differs from that of the embodiment of FIG. 29 in that the negative output voltage VOUT 2  is as shown in equation (3).  
           VOUT 2 =[ Vin ·(−1)]+[ VFa· 2]  (3)  
           [0020]    where VFa is the forward voltage drop of diodes D 1 , D 2 .  
           [0021]    A problem with the first prior art circuit as shown in FIG. 29 is that it is not possible to reduce the overall size of the circuit because of the many externally connected components, including parts of the level shift circuit  2  and MOS transistors Q 5  and Q 6 .  
           [0022]    Furthermore, because a level shift circuit  2  is needed, the level of timing signal A from timing signal generator  1  is gradually lowered by level shift circuit  2 , resulting in signal AS (see FIG. 30) being applied to the gate of MOS transistor Q 5 . This drop increases if the frequency of signal AS is low, and potentially adversely affects the operation of MOS transistor Q 5 .  
           [0023]    The second prior art circuit has an advantage over the first prior art circuit in that there are fewer external parts. However, the negative output voltage VOUT 2  is decreased by the forward voltage VFa component of the diode as shown by equation (3), and power conversion efficiency thus drops.  
         OBJECTS OF THE INVENTION  
         [0024]    Therefore, with consideration for the above problems, an object of the present invention is to provide a voltage conversion circuit that reduces the number of external parts as much as possible and thus enables an overall reduction in size while maintaining high power conversion efficiency.  
         SUMMARY OF THE INVENTION  
         [0025]    To resolve the above problems, a voltage conversion circuit includes a plurality of MOS transistors that are switched ON/OFF to charge a capacitance with an input dc voltage, and this charging voltage is used to convert the input dc voltage to a specific positive and negative output voltage, wherein: the plural MOS transistors include PMOS and NMOS transistors for positive voltage conversion, and a NMOS transistor for negative voltage conversion; the NMOS transistors for positive voltage conversion are formed in a p-type semiconductor substrate; the PMOS transistors for positive voltage conversion are formed in an n-type first well formed in the p-type semiconductor substrate; and the NMOS transistor for negative voltage conversion is formed in a p-type third well, which is formed in an n-type second well formed in the p-type semiconductor substrate.  
           [0026]    The present invention can thus form the MOS transistors used for voltage conversion in the same p-type semiconductor substrate, and can therefore reduce the external components to capacitors only. It is therefore possible to reduce the overall size while maintaining a high power conversion efficiency.  
           [0027]    The present invention can alternatively be implemented as a voltage conversion circuit having a plurality of MOS transistors that are switched ON/OFF to charge a capacitance with an input dc voltage, and using this charging voltage to convert the input dc voltage to a specific positive and negative output voltage, wherein: the plural MOS transistors include PMOS and NMOS transistors for positive voltage conversion, and an NMOS transistor for negative voltage conversion; a specific NMOS transistor for positive voltage conversion is formed in a p-type semiconductor substrate; the PMOS transistors for positive voltage conversion are formed in an n-type first well formed in the p-type semiconductor substrate; and a NMOS transistors for positive voltage conversion other than said specific NMOS transistor, and the NMOS transistor for negative voltage conversion, are formed in a p-type third well, which is formed in an n-type second well formed in the p-type semiconductor substrate.  
           [0028]    This embodiment of the invention can thus form the MOS transistors used for voltage conversion in the same p-type semiconductor substrate, and can therefore reduce the external components to capacitors only. It is therefore possible to reduce the overall size while maintaining high power conversion efficiency.  
           [0029]    Furthermore, the NMOS transistor for positive voltage conversion is also isolated from a substrate bias effect with the invention described in claim  2 , and problems such as an increased threshold value therefore do not occur.  
           [0030]    In a third embodiment of the present invention includes a voltage conversion circuit having a plurality of MOS transistors that are switched ON/OFF to charge a capacitance with the input dc voltage, and using this charging voltage to convert the input dc voltage to a specific positive and negative output voltage, wherein: the plural MOS transistors include PMOS and NMOS transistors for positive voltage conversion, and PMOS and NMOS transistors for negative voltage conversion; the NMOS transistors for negative voltage conversion are formed in a p-type semiconductor substrate; the PMOS transistors for positive voltage conversion and PMOS transistors for negative voltage conversion are formed in an n-type first well formed in the p-type semiconductor substrate; and the NMOS transistors for positive voltage conversion are formed in a p-type third well, which is formed in an n-type second well formed in the p-type semiconductor substrate.  
           [0031]    This third embodiment of the invention can thus form the MOS transistors used for voltage conversion in the same p-type semiconductor substrate, and can therefore reduce the external components to capacitors only. It is therefore possible to reduce the overall size while maintaining high power conversion efficiency.  
           [0032]    Additionally, with the third embodiment of the present invention, substrate biasing to a potential lower than the ground potential is prevented, and PMOS transistors can therefore be used as the MOS transistors producing a negative potential.  
           [0033]    Alternatively, a fourth embodiment of the present invention is a voltage conversion circuit having a plurality of MOS transistors that are switched ON/OFF to charge a capacitance with the input dc voltage, and using this charging voltage to convert the input dc voltage to a specific positive and negative output voltage, wherein: the plural MOS transistors include PMOS and NMOS transistors for negative voltage conversion, and PMOS transistors for positive voltage conversion; the PMOS transistors for negative voltage conversion are formed in an n-type semiconductor substrate; the NMOS transistors for negative voltage conversion are formed in a p-type first well in the n-type semiconductor substrate; and the PMOS transistors for positive voltage conversion are formed in an n-type third well, which is formed in a p-type second well formed in the n-type semiconductor substrate.  
           [0034]    The fourth embodiment of the present invention can thus form the MOS transistors in the same n-type semiconductor substrate, and can therefore reduce the external components to capacitors only. It is therefore possible to reduce the overall size while maintaining high power conversion efficiency.  
           [0035]    Alternatively, the first through fourth embodiments can further include an ON/OFF control means for switching the plural MOS transistors ON/OFF, the ON/OFF control means can be formed in the p-type or n-type semiconductor substrate.  
           [0036]    In any of the embodiments, it is also preferably that at least one of the MOS transistors has an offset area in the semiconductor substrate around the gate insulation layer, the offset area being a low concentration impurity layer disposed below a LOCOS layer in the semiconductor substrate.  
           [0037]    By providing a low concentration impurity layer as an offset region below a LOCOS (local oxidation of silicon) layer in the invention, the offset region can be made deep relative to the channel area when compared with a configuration in which a LOCOS layer is not formed. As a result, the field around the drain is effectively saturated, the withstand voltage of the drain is increased, and a high withstand voltage can be achieved.  
           [0038]    Alternatively, in any of the embodiments, a voltage producing a potential is applied between the semiconductor substrate and second well to assure that they remain in reverse bias condition, or at the same potential during operation. Additionally, and a voltage producing a reverse bias during operation is applied between the second well and third well.  
           [0039]    Alternatively, the invention may be implemented with a booster circuit for boosting an input dc voltage n-fold and plurality of MOS transistors, outputting the boosted voltage of the booster circuit as a positive voltage, switching the plural MOS transistors ON/OFF to charge a capacitance with at least the boosted voltage of the booster circuit, and using this charging voltage to produce a negative voltage, wherein: the plural MOS transistors include PMOS and NMOS transistors used for generating the negative voltage; specific NMOS transistors are formed in a p-type semiconductor substrate; the PMOS transistors are formed in an n-type first well formed in the p-type semiconductor substrate; and specific NMOS transistors are formed in a p-type third well, which is formed in an n-type second well formed in the p-type semiconductor substrate.  
           [0040]    The invention in this embodiment can thus form the MOS transistors used for voltage conversion in the same p-type semiconductor substrate, and can therefore reduce the external components to capacitors only. It is therefore possible to reduce the overall size while maintaining high power conversion efficiency.  
           [0041]    This same embodiment can further include an ON/OFF control means for switching the plural MOS transistors ON/OFF. The ON/OFF control means and the booster circuit can be formed on the p-type semiconductor substrate.  
           [0042]    Additionally, at least one of the MOS transistors can have an offset area in the semiconductor substrate around the gate insulation layer, the offset area being a low concentration impurity layer disposed below a LOCOS layer in the semiconductor substrate. This permits the invention to achieve a high withstand voltage.  
           [0043]    In this same embodiment, it is preferably that the semiconductor substrate and second well received a voltage to assure that they are maintained reversed biased, or at the same potential, during operation. Also, a voltage producing a reverse bias condition between the second well and third well during operation, is preferably applied.  
           [0044]    Lastly, the an n-type fourth well may further be formed in the third well, and a PMOS transistor used for a logic circuit or a PMOS transistor used for generating a negative voltage is formed inside the fourth well.  
           [0045]    Means of Resolution  
           [0046]    This circuit has the timing signal generator and MOS transistors integrated into the same p-type semiconductor substrate, thus forming an IC chip, with capacitors externally connected to the IC chip. At least one NMOS transistor is formed in the p-type semiconductor substrate, while PMOS transistors are formed in a first well of n-type conductivity formed in the p-type semiconductor substrate. Other NMOS transistors are formed in a third well of p-type conductivity, which is formed in a second well of n-type conductivity formed in the p-type semiconductor substrate.  
           [0047]    Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0048]    In the drawings wherein like reference symbols refer to like parts.  
         [0049]    [0049]FIG. 1 is a circuit diagram showing the circuit design of a voltage conversion circuit according to a first embodiment of the present invention;  
         [0050]    [0050]FIG. 2 is a waveform diagram for the timing signal generator;  
         [0051]    [0051]FIG. 3 is a section view showing the physical configuration of the MOS transistors;  
         [0052]    [0052]FIG. 4 is a section view showing the configuration achieving a high withstand voltage in the MOS transistors in this first embodiment;  
         [0053]    [0053]FIG. 5 is a section view showing another configuration achieving a high withstand voltage;  
         [0054]    [0054]FIG. 6 is a section view showing yet another configuration achieving a high withstand voltage;  
         [0055]    [0055]FIG. 7 is a circuit diagram showing the circuit design of a voltage conversion circuit according to a second embodiment of the present invention;  
         [0056]    [0056]FIG. 8 is a waveform diagram for the timing signal generator;  
         [0057]    [0057]FIG. 9 is a section view showing the physical configuration of the MOS transistors;  
         [0058]    [0058]FIG. 10 is a circuit diagram showing the circuit design of a voltage conversion circuit according to a third embodiment of the present invention;  
         [0059]    [0059]FIG. 11 is a waveform diagram for the timing signal generator;  
         [0060]    [0060]FIGS. 12A and 12B show an operationally equivalent circuit;  
         [0061]    [0061]FIG. 13 is a circuit diagram showing the circuit design of a voltage conversion circuit according to a fourth embodiment of the present invention;  
         [0062]    [0062]FIG. 14 is a section view showing the physical configuration of the MOS transistors;  
         [0063]    [0063]FIG. 15 is a circuit diagram showing the circuit design of a voltage conversion circuit according to a fifth embodiment of the present invention;  
         [0064]    [0064]FIG. 16 is a section view showing the physical configuration of the MOS transistors;  
         [0065]    [0065]FIG. 17 is a circuit diagram showing the circuit design of a voltage conversion circuit according to a sixth embodiment of the present invention;  
         [0066]    [0066]FIG. 18 is a waveform diagram for the timing signal generator;  
         [0067]    [0067]FIGS. 19A and 19B show an operationally equivalent circuit;  
         [0068]    [0068]FIGS. 20A and 20B are a section view showing the physical configuration of the MOS transistors;  
         [0069]    [0069]FIG. 21 is a circuit diagram showing the circuit design of a voltage conversion circuit according to a seventh embodiment of the present invention;  
         [0070]    [0070]FIG. 22 is a section view showing the physical configuration of the MOS transistors;  
         [0071]    [0071]FIG. 23 is a circuit diagram showing the circuit design of a dc-dc converter used in a voltage conversion circuit according to an eighth embodiment of the present invention;  
         [0072]    [0072]FIG. 24 is a circuit diagram showing the circuit design of a voltage conversion circuit according to a ninth embodiment of the present invention;  
         [0073]    [0073]FIG. 25 is a circuit diagram showing the circuit design of a voltage conversion circuit according to a tenth embodiment of the present invention;  
         [0074]    [0074]FIG. 26 is a circuit diagram showing the internal configuration of the operational amplifier;  
         [0075]    [0075]FIG. 27 is a circuit diagram showing another internal configuration for the operational amplifier;  
         [0076]    [0076]FIG. 28 is a circuit diagram showing the circuit configuration of a voltage conversion circuit according to an eleventh embodiment of the present invention;  
         [0077]    [0077]FIG. 29 is a circuit diagram showing the configuration of a first conventional circuit;  
         [0078]    [0078]FIGS. 30A and 30B show the output waveform of the timing signal generator;  
         [0079]    [0079]FIG. 31 shows an operationally equivalent circuit;  
         [0080]    [0080]FIG. 32 is a section view showing the physical configuration of the MOS transistors; and  
         [0081]    [0081]FIG. 33 is a circuit diagram showing the configuration of a second conventional circuit. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0082]    A first embodiment of a voltage conversion circuit according to the present invention is described below with reference to FIG. 1 to FIG. 3.  
         [0083]    With reference to FIG. 1, a voltage conversion circuit according to the first embodiment of the present invention has a timing signal generator  25 , MOS transistors Q 11  to Q 16 ON/OFF, and capacitors C 1  to C 4  configured to generate a positive output voltage VOUT 1  and a negative output voltage VOUT 2  based on dc input voltage Vin. Transistors Q 11  to Q 16  are switched ON/OFF in accordance with their appropriate timing signals XB 2 , XA 2 , XB, A, B 3  and A 3  from timing signal generator  15 .  
         [0084]    As further described below, the area enclosed in the dotted line in FIG. 1, that is, timing signal generator  25  and MOS transistors Q 11  to Q 16 , are integrated onto the same p-type semiconductor substrate, and are thus configured as IC chip  20 . Capacitors C 1  to C 4  are discrete components externally connected to IC chip  20 .  
         [0085]    The timing signal generator  25  generates timing signals XB 2 , XA 2 , XB, A, B 3 , and A 3  as shown in FIG. 2 to control the ON/OFF state of their corresponding MOS transistors Q 11  to Q 16 .  
         [0086]    MOS transistors Q 11  to Q 14  are used to convert dc input voltage Vin to positive output voltage VOUT 1 , and MOS transistors Q 11 , Q 12 , Q 15 , and Q 16  are used to convert dc input voltage Vin to negative output voltage VOUT 2 .  
         [0087]    MOS transistors Q 11  and Q 12  are used for conversion to both positive and negative output voltages in this example, but it will be noted that another pair of MOS transistors could be provided so that MOS transistors Q 11  and Q 12  are used to convert the input voltage to only one of a positive or negative output voltage.  
         [0088]    Yet more specifically, NMOS transistor Q 11  and PMOS transistor Q 12  are connected between ground line  21  and input line  22  with their respective drain nodes coupled together, as shown in FIG. 1. PMOS transistor Q 13  and PMOS transistor Q 14  are connected between input line  22  and output line  23 , with the drain node of transistor Q 14  coupled to the source node of transistor Q 13 . NMOS transistor Q 15  and NMOS transistor Q 16  are connected between ground line  21  and output line  24 , with the drain node of transistor Q 15  coupled to the source node of Q 16 . The timing signals XB 2 , XA 2 , XB, A, B 3 , and A 3  from timing signal generator  25  are applied to the gates of MOS transistors Q 11  to Q 16 . Capacitor C 1  is connected to the node between NMOS transistor Q 11  and PMOS transistor Q 12  and the node between PMOS transistor Q 13  and PMOS transistor Q 14 . Capacitor C 2  is connected between input line  22  and output line  23 . Capacitor C 3  is connected to the node between NMOS transistor Q 11  and PMOS transistor Q 12  and the node between NMOS transistor Q 15  and NMOS transistor Q 16 . Capacitor C 4  is connected between ground line  21  and output line  24 .  
         [0089]    As noted above, MOS transistors Q 11  to Q 16  are integrated to the same p-type semiconductor substrate in the voltage conversion circuit according to this first embodiment, and the configuration thereof is described next below.  
         [0090]    In FIG. 3 reference numeral  31  is the p-type semiconductor substrate. Two n-type wells  32  and  33  are formed in the p-type semiconductor substrate  31 , and two p-type wells  34  and  35  are formed in n-type well  33 .  
         [0091]    NMOS transistor Q 11  having a source S, gate G, and drain D is formed in p-type semiconductor substrate  31 . PMOS transistor Q 12  having a source S, gate G, and drain D is formed in n-type well  32 . NMOS transistors Q 15  and Q 16  each having a source S, gate G, and drain D are formed respectively in p-type wells  34  and  35 . MOS transistors Q 13  and Q 14  shown in FIG. 1 are configured identically to MOS transistor Q 12  shown in FIG. 3, and are therefore not shown in FIG. 3.  
         [0092]    Wells  33 ,  34 ,  35  are designed with consideration for the withstand voltage and threshold value of the MOS transistors formed in wells  33 ,  34 , and the pass-through voltage and junction voltage between the wells. The following description assumes that the MOS transistor withstand voltage is at least 10 V, and is preferably 20 V to 30 V.  
         [0093]    The depth of the wells  33 ,  34 , and  35  is determined with consideration for the MOS transistor withstand voltage and the pass-through voltage between the p-type semiconductor substrate  31  and wells  34 / 35 . In this example the depth of well  33  is 15 μm to 18 μm and the depth of wells  34 / 35  is 6 μm to 8 μm.  
         [0094]    The impurity concentration of wells  33 ,  34 , and  35  is determined according to the threshold value and withstand voltage of the MOS transistors. The surface concentration of impurities in well  33  is, for example, 1×10 16  to 3×10 16  atms/cm 3 , and the surface concentration of impurities in wells  34 / 35  is 1×10 16  to 3×10 16  atms/cm 3 .  
         [0095]    Wells  34  and  35  are shallower than well  33 , and are preferably ½ to ⅓ the depth of well  33  considering the MOS transistor withstand voltage and the passthrough voltage between p-type semiconductor substrate  31  and wells  34 / 35 .  
         [0096]    It should be noted that the configuration of these wells  33  to  35  also applies to the wells equivalent to wells  33  to  35  formed in the semiconductor substrate in each of the embodiments described below.  
         [0097]    The parts of MOS transistors Q 15  and Q 16  in FIG. 3 are electrically connected as shown in FIG. 1, and the parts of MOS transistors Q 11  and Q 12  are electrically connected as shown in FIG. 1.  
         [0098]    Negative output voltage VOUT 2  is applied to p-type well  34  and dc input voltage Vin is applied to n-type well  33  so that the pn junction of p-type well  34  and n-type well  33  is reversed biased during use as shown in FIG. 3.  
         [0099]    In addition, ground potential GND is applied to p-type well  35  and dc input voltage Vin is applied to n-type well  33  to that the pn junction of p-type well  35  and n-type well  33  is reverse biased during use.  
         [0100]    Furthermore, ground potential GND is applied to p-type semiconductor substrate  31  and dc input voltage Vin is applied to n-type well  33  so that the pn junction of p-type semiconductor substrate  31  and n-type well  33  is reverse biased when used.  
         [0101]    Yet further, ground potential GND is applied to p-type semiconductor substrate  31  and dc input voltage Vin is applied to n-type well  32  so that the pn junction of p-type semiconductor substrate  31  and n-type well  32  is reverse biased during use.  
         [0102]    Note that the relationship between dc input voltage Vin, ground potential GND, and negative output voltage VOUT 2 , is Vin&gt;GND&gt;VOUT 2 .  
         [0103]    Potential is applied to each well in the configuration shown in FIG. 3 so that there is a reverse bias between n-type wells, but it is also possible to apply the ground potential GND to n-type well  33  so that n-type well  33  and p-type semiconductor substrate  31  are set to the same potential.  
         [0104]    The operation of the first embodiment thus comprised is described next with reference to the figures.  
         [0105]    The timing signal generator  25  generates timing signals XB 2 , XA 2 , XB, A, B 3 , and A 3  as shown in FIG. 2, supplies the timing signals to the gate of corresponding MOS transistors Q 11  to Q 16 , and thereby controls the ON/OFF state of MOS transistors Q 11  to Q 16 .  
         [0106]    As a result of this control, MOS transistors Q 11 , Q 13 , Q 15  are ON and MOS transistors Q 12 , Q 14 , Q 16  are OFF in period T 1  in FIG. 2, and the equivalent circuit is as shown in FIG. 31A. As shown in FIG. 2, in period T 2 , MOS transistors Q 12 , Q 14 , Q 16  are ON and MOS transistors Q 11 , Q 13 , Q 15  are OFF, and the equivalent circuit is as shown in FIG. 31B.  
         [0107]    By repeating the operation of periods T 1  and T 2 , this first embodiment operates as a charge pump type dc-dc converter.  
         [0108]    As a result of this operation, positive output voltage VOUT 1  and negative output voltage VOUT 2  in this first embodiment are as shown in equations (1) and (2) above if the ground potential GND is 0 V.  
         [0109]    As described above, MOS transistors Q 15  and Q 16  used to produce negative output voltage VOUT 2  in this first embodiment are n-type, and these NMOS transistors Q 15  and Q 16  are formed by forming an n-type well  33  inside a p-type semiconductor substrate  31 , forming p-type wells  34  and  35  inside the n-type well  33 , and then forming the NMOS transistors Q 15  and Q 16  inside the p-type wells  34  and  35 . In addition, the junctions between p-type wells  34  and  35  and the n-type well  33  are reverse biased, the junction between p-type semiconductor substrate  31  and n-type well  33  is reverse biased, and the junctions are used electrically separated. Note that n-type well  33  and p-type semiconductor substrate  31  can have the same potential.  
         [0110]    It is therefore possible with this first embodiment to form the timing signal generator  25  and MOS transistors Q 11  to Q 16  in a p-type semiconductor substrate  31  in a single IC chip, and the conventionally needed level shift circuit and MOS transistors of the prior art can be omitted. The only required external components are capacitors C 1  to C 4 . It is therefore possible to reduce the overall size while maintaining high power conversion efficiency.  
         [0111]    Alternate configurations as shown in FIGS.  4  to  6  are preferable as a means of achieving a higher withstand voltage in MOS transistors Q 11  to Q 16  in this first embodiment. This configuration is described as applied to MOS transistor Q 15 , as a typical example of transistors Q 11 -Q 16 .  
         [0112]    [0112]FIG. 4 shows an example in which a high withstand voltage is achieved by using a LOCOS offset structure for MOS transistor Q 15 .  
         [0113]    More specifically, in this embodiment, MOS transistor Q 15  has a gate insulation layer  37  formed at a specific location on p-type well  34  with gate G formed on this gate insulation layer  37 . An offset LOCOS layer  38  is formed around gate insulation layer  37 , and an offset impurity layer  39  is formed below this offset LOCOS layer  38  from a low concentration n-type impurity layer. The source S and drain D are formed from a high concentration n-type impurity layer around the outside of this offset LOCOS layer  38 .  
         [0114]    Also shown in FIG. 4 are element-isolating LOCOS layer  40  for isolating MOS transistor Q 15  from other MOS transistors, and a low concentration n-type impurity layer  41  formed below this element-isolating LOCOS layer  40 .  
         [0115]    This gate insulation layer  37  preferably has a film thickness of 60 nm to 80 nm when a voltage of at least 10 V, or more specifically a voltage of 10 V to 30 V, is applied, although the specific thickness will depend upon the required MOS transistor withstand voltage.  
         [0116]    [0116]FIG. 5 shows a MOS transistor Q 15  in which a high withstand voltage is achieved by using a drain over structure. That is, the bottom side of source S and drain D in this MOS transistor Q 15  is surrounded by a low concentration n-type impurity  42  and  42 , respectively.  
         [0117]    [0117]FIG. 6 shows a MOS transistor Q 15  in which an even higher withstand voltage is achieved by using both a LOCOS offset structure, as shown in FIG. 4, and a drain over structure, as shown in FIG. 5.  
         [0118]    More specifically, the MOS transistor Q 15  of FIG. 6 has offset LOCOS layer  38  formed around the gate insulation layer  37 , and offset impurity layer  39 , which is a low concentration n-type impurity layer, formed below this offset LOCOS layer  38 . A low concentration n-type impurity layer  43  is also formed below the element-isolating LOCOS layer  40 , source S, and offset impurity layer  39 , and a low concentration n-type impurity layer  43  is formed below the element-isolating LOCOS layer  40 , drain D, and offset impurity layer  39 .  
         [0119]    The structures shown in FIG. 4 to FIG. 6 can also be applied to the MOS transistors in the embodiments described below, and a high withstand voltage can be achieved by such applications.  
         [0120]    A second embodiment of a voltage conversion circuit according to the present invention is described next with reference to FIG. 7 to FIG. 9.  
         [0121]    The voltage conversion circuit according to this second embodiment replaces PMOS transistor Q 12  of the first embodiment shown in FIG. 1 with an NMOS transistor Q 17  as shown in FIG. 7, and the timing signal generator  25 A produces the timing signals shown in FIG. 8 and applies timing signal B 2  to the gate of MOS transistor Q 17  (see FIG. 8).  
         [0122]    The NMOS transistors Q 11  and  17  in the voltage conversion circuit according to the second embodiment are configured as shown in FIG. 9.  
         [0123]    As shown in FIG. 9 reference numeral  31  identifies a p-type semiconductor substrate, an n-type well  45  is formed in this p-type semiconductor substrate  31 , and a p-type well  46  is formed in this n-type well  45 . NMOS transistor Q 17  having a source S, gate G, and drain D is then formed in p-type well  46 . In addition, NMOS transistor Q 11  having source S, gate G, and drain D is formed in p-type semiconductor substrate  31 .  
         [0124]    The drain D voltage of MOS transistor Q 11  and the source S voltage of MOS transistor Q 17  are applied to p-type well  46 , and ground potential GND is applied to n-type well  45  to reverse bias the pn junction of p-type well  46  and n-type well  45 .  
         [0125]    The internal configuration of other parts of the IC chip  20 A of this second embodiment is the same as in IC chip  20  according to the first embodiment, like parts are identified by like reference numerals, and further description is therefore omitted. The operation of this second embodiment is also the same as the operation of the first embodiment, and description thereof is therefore also omitted.  
         [0126]    It will thus be apparent that MOS transistors Q 11  and Q 17  are n-type in this second embodiment, NMOS transistor Q 11  is formed in the p-type semiconductor substrate  31  as shown in FIG. 9, and NMOS transistor Q 17  is formed inside p-type well  46 , which is formed in n-type well  45 , which is formed in the p-type semiconductor substrate  31  as shown in FIG. 9. Moreover, the junction between p-type well  46  and n-type well  45  is used reverse biased so that the elements are used electrically separated.  
         [0127]    As a result, there is no substrate bias effect on MOS transistor Q 17 , there is no concern about an increase in the threshold value, and the overall size of this second embodiment can be reduced as compared to the first embodiment.  
         [0128]    A voltage conversion circuit according to a third embodiment of the present invention is described next with reference to FIG. 10 to FIG. 12.  
         [0129]    The voltage conversion circuit according to this third embodiment has a circuit design as shown in FIG. 10, and based on the dc input voltage Vin outputs a positive output voltage VOUT 1  at a voltage level three times dc input voltage Vin and outputs a negative output voltage VOUT 2  at a voltage level −2 times the dc input voltage Vin.  
         [0130]    As shown in FIG. 10, the voltage conversion circuit of this third embodiment is therefore based on the structure of the first embodiment shown in FIG. 1, adding to this basic circuit MOS transistors Q 21  to Q 24  and capacitors C 5  and C 6 , and replacing timing signal generator  25  with timing signal generator  25 C.  
         [0131]    The physical structure of MOS transistors Q 11  to Q 16  is therefore identical to the MOS transistors Q 11  to Q 16  of the first embodiment.  
         [0132]    To describe the physical structure of MOS transistors Q 21  to Q 24 , MOS transistor Q 21  is identical in structure to MOS transistor Q 14 , and MOS transistor Q 22  is identical in structure to MOS transistor Q 15 . In addition, MOS transistors Q 23  and Q 24  are identical in structure to MOS transistors Q 11  and Q 12 .  
         [0133]    The timing signal generator  25 C generates timing signals XB 2 , XA 2 , XB, A, XA, B, B 3 , and A 3 , applies the timing signals to the gates of MOS transistors Q 11  to Q 16  and Q 21  to Q 24  to ON/OFF controls the MOS transistors.  
         [0134]    The configuration of other parts of the IC chip  20 B of this third embodiment is basically the same as in IC chip  20  according to the first embodiment, like parts are identified by like reference numerals, and further description is therefore omitted.  
         [0135]    An example of the operation of this third embodiment is described next with reference to FIG. 10 to FIG. 12.  
         [0136]    The timing signal generator  25 C generates timing signals XB 2 , XA 2 , XB, A, XA, B, B 3 , and A 3  as shown in FIG. 11, applies the timing signals to the gates of corresponding MOS transistors Q 11  to Q 16  and Q 21  to Q 24 , and thus switches the MOS transistors Q 11  to Q 16  and Q 21  to Q 24  ON and OFF.  
         [0137]    As a result of this control, MOS transistors Q 11 , Q 13 , Q 21 , Q 15 , Q 24  are ON in period T 1  shown in FIG. 11, and MOS transistors Q 12 , Q 14 , Q 16 , Q 22 , Q 23  are OFF. The period T 1  equivalent circuit is shown in FIG. 12 (A).  
         [0138]    As a result, capacitor C 1  is charged by dc source voltage Vin in period T 1  and the charge of capacitor C 5  in the previous period T 2  moves to capacitor C 2 . The positive output voltage VOUT 1  at this time is the sum of dc input voltage Vin and both end voltages of capacitor C 5 .  
         [0139]    Also in period T 1  capacitor C 6  is charged by the voltage sum of the dc source voltage Vin and the stored charge voltage of capacitor C 3  from the previous period T 2 . The negative output voltage VOUT 2  at this time is the voltage across capacitor C 4 .  
         [0140]    In period T 2  in FIG. 11 MOS transistors Q 12 , Q 14 , Q 16 , Q 22 , Q 23  are ON and MOS transistors Q 11 , Q 13 , Q 21 , Q 15 , Q 24  are OFF. The period T 2  equivalent circuit is as shown in FIG. 12(B).  
         [0141]    Capacitor C 5  is therefore charged in period T 2  by the voltage sum of the dc source voltage Vin and the stored charge voltage of capacitor C 1  from period T 1 . The positive output voltage VOUT 1  at this time is therefore the sum of dc input voltage Vin and the voltage across capacitor C 2 .  
         [0142]    Also in period T 2 , capacitor C 3  is charged by dc input voltage Vin, and the charge of capacitor C 6  stored in period T 1  is shared with capacitor C 4 . The negative output voltage VOUT 2  at this time is the voltage of both ends of capacitor C 6 .  
         [0143]    As a result of this operation, the positive output voltage VOUT 1  and negative output voltage VOUT 2  in this third embodiment are as shown in equations (4) and (5).  
         VOUT 1 =(3)· Vin   (4)  
         VOUT 2 =(−2)· Vin   (5)  
         [0144]    As described above, timing signal generator  25 C and MOS transistors Q 11  to Q 16  and Q 21  to Q 24  can be integrated to a p-type semiconductor substrate in this third embodiment, thus forming an IC chip, and the only required external parts are capacitors C 1  to C 6 . As a result, the overall size can be reduced.  
         [0145]    A voltage conversion circuit according to a fourth embodiment of the present invention is described next with reference to FIG. 13 and FIG. 14.  
         [0146]    The voltage conversion circuit according to this fourth embodiment replaces the NMOS transistors Q 15  and Q 16  of the third embodiment shown in FIG. 10 with PMOS transistors Q 25  and Q 26  as shown in FIG. 13, and replaces timing signal generator  25 C with timing signal generator  25 D.  
         [0147]    Timing signal generator  25 D produces the timing signals similar to the timing signals of generator  25 C with the exception that the timing signal B 3  that is coupled to transistor Q 16  in FIG. 10 is replaced with new timing signal XB 3  coupled to transistor Q 26  in FIG. 13, and timing signal A 3  that is coupled to transistor Q 15  in FIG. 10 is replaced with new timing signal XA 3  coupled to transistor Q 25  in FIG. 13. In other words, timing signals XA 3  and XB 3  are applied to the gates of PMOS transistors Q 25  and Q 26  to control their ON/OFF operation. Note that these timing signals XA 3  and XB 3  are the inverse of timing signals A 3  and B 3  generated by timing signal generator  25 C (see FIG. 11).  
         [0148]    The physical structure of the embodiment of FIG. 13 is described next with particular reference to MOS transistors Q 25  and Q 26 , as shown in FIG. 14.  
         [0149]    In FIG. 14 reference numeral  31  is a p-type semiconductor substrate. Also, n-type wells  61  to  63  are formed in this p-type semiconductor substrate  31 , and p-type well  64  is formed in n-type well  63 . An NMOS transistor Q 22  with source S, gate G, and drain D is formed in p-type semiconductor substrate  31 . PMOS transistors Q 25 , Q 26 , Q 12  each having a source S, gate G, and drain D are formed in corresponding n-type wells  61 ,  62 ,  63 . NMOS transistor Q 11  with source S, gate G, and drain D is formed in p-type well  64 .  
         [0150]    This fourth embodiment applies the negative output potential VOUT 2  of output line  24  to p-type semiconductor substrate  31 , applies ground potential GND of ground line  21  to n-type well  62  and p-type well  64 , and applies dc input voltage Vin of input line  22  to n-type well  63 . The pn junctions between wells, and the pn junction between well and p-type semiconductor substrate are thus used with a reverse bias.  
         [0151]    MOS transistors Q 25  and Q 26  in this fourth embodiment are p-type transistors formed in n-type wells  61  and  62 , as shown in FIG. 14. Furthermore, NMOS transistor Q 11  is formed in p-type well  64 , as shown in FIG. 14. In addition, the ground potential GND (0 V) of ground line  21  is applied to p-type well  64  and n-type well  62 .  
         [0152]    It is therefore possible with this fourth embodiment to form a PMOS transistor without causing a reverse bias to a potential below the ground line  21 , and the overall size can be reduced as in the third embodiment.  
         [0153]    A voltage conversion circuit according to a fifth embodiment of the present invention is described next with reference to FIG. 15 and FIG. 16.  
         [0154]    The voltage conversion circuit according to this fifth embodiment has a timing signal generator  25 , MOS transistors Q 31  to Q 36  that have their ON/OFF state controlled by the timing signals from timing signal generator  25 , and capacitors C 1  to C 4  as shown in FIG. 15. Timing signal generator  25  and MOS transistors Q 31  to Q 36  enclosed in the dotted line in FIG. 15 are integrated to the same n-type semiconductor substrate as further described below, and thus form IC chip  20 D.  
         [0155]    This fifth embodiment generates a lx positive output voltage VOUT 1  and a −2× negative output voltage VOUT 2  based on negative dc input voltage Vin.  
         [0156]    The physical structure of MOS transistors Q 31  to Q 36  is described next with reference to FIG. 16.  
         [0157]    In FIG. 16 reference numeral  51  is an n-type semiconductor substrate  51 . Two p-type wells  52  and  53  are formed in this n-type semiconductor substrate  51  and two n-type wells  54  and  55  are formed in p-type well  53 .  
         [0158]    PMOS transistor Q 31  with source S, gate G, and drain D is formed in n-type semiconductor substrate  51 . NMOS transistor Q 32  with source S, gate G, and drain D is formed in p-type well  52 . PMOS transistors Q 35  and Q 36  each having a source S, gate G, and drain D are formed in n-type wells  54  and  55 , respectively. NMOS transistors Q 33  and Q 34  shown in FIG. 15 are constructed identically to MOS transistor Q 32  shown in FIG. 16, and are therefore omitted in FIG. 16.  
         [0159]    As shown in FIG. 16, specific voltages are applied to reverse bias the well-well pn junction and the well to n-type semiconductor substrate pn junction. Note that the potential of p-type well  53  can be set to GND so that p-type well  53  and n-type semiconductor substrate  51  are the same potential.  
         [0160]    With this fifth embodiment of the invention configured as described above, the timing signal generator  25  and MOS transistors Q 31  to Q 36  can be formed on the n-type semiconductor substrate  51  to create an IC chip, and the only necessary external parts are therefore capacitors C 1  to C 4 . As a result, the overall size can be reduced.  
         [0161]    A sixth embodiment of a voltage conversion circuit according to the present invention is described next with reference to FIG. 17 to FIG. 20.  
         [0162]    As shown in FIG. 17, the voltage conversion circuit according to this sixth embodiment has a charge pump-type n-fold booster circuit  71  for boosting the dc input voltage VC N-times, timing signal generator  25 E, MOS transistors Q 41  to Q 48  whose ON/OFF state is determined by the timing signals output from timing signal generator  25 E, and capacitors CP 1 , CB 0 , CB 1 , CP 2 , CB 2 . The structure is configured to output a positive output voltage V 3  at output line  75 , negative output voltages VDDy at output line  76 , and negative output voltage MV 3  at output line  77 .  
         [0163]    In the voltage conversion circuit according to this sixth embodiment, the parts enclosed in the dotted line in FIG. 17, that is, n-fold booster circuit  71 , timing signal generator  25 E, and MOS transistors Q 41  to Q 48  are integrated on the same p-type semiconductor substrate, thus forming IC chip  20 E. Capacitors CP 1 , CB 0 , CB 1 , CP 2 , and CB 2  are discrete components externally connected to IC chip  20 E.  
         [0164]    If the n-fold booster circuit  71  has a gain of three times, for example, the n-fold booster circuit  71  comprises the parts of the voltage conversion circuit shown in FIG. 10 for generating a positive voltage.  
         [0165]    The timing signal generator  25 E generates timing signals XB 2 , A 2 , XB 1 , A 1 , B 3 , and A 3 , and applies these timing signals to switch the MOS transistors Q 41  to Q 48  ON and OFF.  
         [0166]    MOS transistors Q 41  to Q 44  are used to generate negative output voltage MV 3 , and MOS transistors Q 45  to Q 48  are used to generate negative output voltage VDDy.  
         [0167]    More specifically, NMOS transistor Q 41  and PMOS transistor Q 42  are connected between input line  73  and output line  75  with the drain node of NMOS transistor Q 41  coupled to the drain node of PMOS transistor Q 42 , as shown in FIG. 17. NMOS transistor Q 43  and NMOS transistor Q 44  are connected between input line  73  and output line  77 , with the drain node of transistor Q 44  coupled to the source node of transistor Q 43 . Capacitor CP 1  has a first end connected to the junction node between NMOS transistor Q 41  and PMOS transistor Q 42 , and a second end connected to the junction node between NMOS transistor Q 43  and NMOS transistor Q 44 . Capacitor CB 0  is connected between ground line  72  and output line  75 .  
         [0168]    NMOS transistor Q 45  and PMOS transistor Q 46  are connected between ground line  72  and input line  74 , with the drain node of PMOS transistor Q 46  coupled to the drain node of NMOS transistor Q 45 . NMOS transistor Q 47  and NMOS transistor Q 48  are connected between output line  76  and output line  77  with the drain electrode of transistor Q 47  coupled to the source electrode of transistor Q 48 . Capacitor CP 2  has a first end connected to the junction node between NMOS transistor Q 45  and PMOS transistor Q 46 , and has a second end connected to the junction node between NMOS transistor Q 47  and NMOS transistor Q 48 . Capacitor CB 1  is connected between ground line  72  and output line  77 , and capacitor CB 2  is connected between output line  76  and output line  77 .  
         [0169]    As noted above, MOS transistors Q 41  to Q 48  are integrated onto the same p-type semiconductor substrate in the voltage conversion circuit according to this sixth embodiment, and the configuration thereof is therefore described next below with reference to FIG. 20.  
         [0170]    With reference to FIG. 20A, reference numeral  31  is a p-type semiconductor substrate, and n-type wells  72 / 73  are formed in this p-type semiconductor substrate  31 . Two p-type wells  74 / 75  are formed in n-type well  73 .  
         [0171]    NMOS transistor Q 45  having a source S, gate G, and drain D is formed in p-type semiconductor substrate  31 . PMOS transistor Q 46  having a source S, gate G, and drain D is formed in n-type well  72 . NMOS transistors Q 47 , Q 48  each having a source S, gate G, and drain D are formed in p-type wells  74 ,  75 .  
         [0172]    As shown in FIG. 20B, n-type well  73 A is formed in p-type semiconductor substrate  31  and p-type well  74 A is formed in n-type well  73 A. NMOS transistor Q 41  having a source S, gate G, and drain D is formed in p-type well  74 A.  
         [0173]    MOS transistor Q 42  shown in FIG. 17 is configured identically to MOS transistor Q 46  shown in FIG. 20, and MOS transistors Q 43 , Q 44  shown in FIG. 17 are configured identically to MOS transistors Q 47 , Q 48  shown in FIG. 20, and these are therefore not shown in FIG. 20.  
         [0174]    NMOS transistors Q 47  and Q 48  are equivalent to MOS transistors Q 15  and Q 16  of FIG. 3, and the conditions for the configuration of wells  33  to  35  can therefore be applied to the configuration of wells  73  to  75 .  
         [0175]    Negative output voltage MV 3  is applied to p-type well  74  and the dc input voltage VDD is applied to n-type well  73  so that the pn junction of p-type well  74  and n-type well  73  is reverse biased as shown in FIG. 20. In addition, negative output voltage VDDy is applied to p-type well  75  and dc input voltage VDD is applied to n-type well  73  so that the pn junction between p-type well  75  and n-type well  73  is reverse biased.  
         [0176]    Furthermore, ground potential VSS is applied to p-type semiconductor substrate  31  and dc input voltage VDD is applied to n-type well  73  so that the pn junction between p-type semiconductor substrate  31  and n-type well  73  is reverse biased. Note that n-type well  73  surrounding NMOS transistors Q 47  and Q 48  can be set to the ground potential. Potential VSS is also applied to p-type semiconductor substrate  31  and dc input voltage VDD is applied to n-type well  72  so that the pn junction between p-type semiconductor substrate  31  and n-type well  72  is reverse biased.  
         [0177]    Note that the relationship between dc input voltage VDD, potential VSS, negative output voltage VDDy, and negative output voltage MV 3  is VDD&gt;GND&gt;VDDy&gt;MV 3 .  
         [0178]    Operation of the sixth embodiment thus comprised is described next with reference to FIG. 17 to FIG. 19.  
         [0179]    Timing signal generator  25 E generates timing signals XB 2 , A 2 , XB 1 , A 1 , B 3 , and A 3  as shown in FIG. 18, and applies these timing signals to the gate of the corresponding MOS transistors Q 41  to Q 48  to thereby control switching between the ON and OFF states the MOS transistors Q 41  to Q 48 .  
         [0180]    By using this sort of control, MOS transistors Q 41 , Q 43 , Q 45  and Q 47  turn ON, and MOS transistors Q 42 , Q 44 , Q 46  and Q 48  turn OFF during an interval T 1  shown in FIG. 18. The resulting equivalent circuit during interval T 1  is as shown in FIG. 19A.  
         [0181]    In contrast, during an interval T 2  shown in FIG. 18, the MOS transistors Q 42 , Q 44 , Q 46  and Q 48  turn ON, and the MOS transistors Q 41 , Q 43 , Q 45  and Q 47  turn OFF. The resulting equivalent circuit during interval T 2  is as shown in FIG. 19B.  
         [0182]    In FIG. 18, it is understood that the states shown in FIGS. 19A and B alternate repeatedly.  
         [0183]    As shown in FIG. 17, N voltage multiplier circuit  71  receives inputs Vss and VC, and outputs a positive potential V 3  equivalent to N times direct current voltage vC.  
         [0184]    First, in a state that corresponds to the interval T 2 , in other words the state shown in FIG. 19B, capacitor CP 1  is coupled across the V 3  output from N voltage multiplier  71  and DC voltage VC. Capacitor CP 1  therefore receives a charge from the N voltage multiplier  71  and develops a potential difference across equal to V 3 -VC. Since V 3  is (N·VC), the potential across capacitor CP 1  can be written as [(N·VC)−VC], which is Vc·(N−1).  
         [0185]    Subsequently, in a state that corresponds to the interval T 1 , in other words the state shown in FIG. 19A, the charge in capacitor CP 1  that was charged during the previous interval T 2  is shared with capacitor CB 1 . At this stage, the potential difference across capacitor CP 1  is equal to |VC·(N−1)|. Therefore, if ground level VSS serves as the reference, then MV 3  has a potential as shown below in equation (7).  
         [0186]    Now, from a different perspective, during interval T 1 , capacitor CP 2  is charged by direct current power supply voltage VSS and by the charge that was stored in capacitor CP 1  during the previous interval and was moved to the capacitor CP 2 . At this stage, a voltage having a negative output voltage VDDy as shown below in equation (8) is outputted.  
         [0187]    By repeating the above operation, positive output voltage V 3 , negative output voltage MV 3 , and negative output voltage VDDy are output as shown by equations (6) to (8) below in this sixth embodiment of the invention.  
         V 3 = VC·N   (6)  
         MV 3 =(−1) VC ·( N− 2)  (7)  
           VDDy =MV 3 + VDD   (8)  
         [0188]    As described above, the only required external components in this sixth embodiment are the capacitors because the n-fold booster circuit  71 , timing signal generator  25 E, and MOS transistors Q 41  to Q 48  can be formed in a p-type semiconductor substrate and integrated as an IC chip. As a result, the overall device size can be reduced.  
         [0189]    A voltage conversion circuit according to a seventh embodiment of the present invention is described next with reference to FIG. 21 and FIG. 22.  
         [0190]    As shown in FIG. 21 the voltage conversion circuit according to this seventh embodiment is based on the sixth embodiment of FIG. 17, but adds a logic circuit  77  consisting of an NMOS transistor Q 51  and PMOS transistor Q 52 , and thus forming IC chip  20 F.  
         [0191]    [0191]FIG. 22 is a section view showing the physical structure of this voltage conversion circuit particularly as concerns the MOS transistors Q 51  and Q 52  of the output line  77 .  
         [0192]    Referring to FIG. 22, reference numeral  31  is a p-type semiconductor substrate, n-type wells  72  and  81  are formed in p-type semiconductor substrate  31 , p-type well  82  is formed in n-type well  81 , and n-type well  83  is formed in p-type well  82 . NMOS transistor Q 45  having a source S, gate G, and drain D is formed in p-type semiconductor substrate  31 . PMOS transistor Q 46  having a source S, gate G, and drain D is formed in n-type well  72 . NMOS transistor Q 51  having a source S, gate G, and drain D is formed in p-type well  82 . PMOS transistor Q 52  having a source S, gate G, and drain D is formed in n-type well  83 .  
         [0193]    Other parts of the IC chip  20 F according to this seventh embodiment are identical to the IC chip  20 E according to the sixth embodiment of FIG. 17, and further description thereof is thus omitted.  
         [0194]    As described above this seventh embodiment is based on the sixth embodiment, and therefore achieves the same operational effects as the sixth embodiment.  
         [0195]    In addition, because an n-type well  83  is further formed inside p-type well  82 , and PMOS transistor Q 52  is formed inside this n-type well  83 , PMOS transistor Q 52  can be used at a lower potential than the potential (VSS=0 V) of the p-type semiconductor substrate  31 , and it is therefore possible to provide a CMOS inverter or other logic circuit  77  between two power supplies with a potential lower than the substrate potential.  
         [0196]    A voltage conversion circuit according to an eighth embodiment of the present invention is described next with reference to FIG. 23.  
         [0197]    The voltage conversion circuit of this eighth embodiment replaces the charge-pump type n-fold booster circuit  71  of the sixth embodiment (see FIG. 17) with a synchronous commutating type switching regulator  86  as shown in FIG. 23. The switching regulator  86  of FIG. 23 along with the timing signal generator  25 E and MOS transistors Q 41  to Q 48  of FIG. 17 may be formed on the same p-type semiconductor substrate, thereby reducing the number of external components and thus reducing the overall size.  
         [0198]    As shown in FIG. 23, the switching regulator  86  includes oscillator circuit  87 , comparator  88 , switching control circuit  89 , MOS transistors Q 61  and Q 62 , voltage-dividing resistors RA and RB, coil L 1 , and capacitor C 11 . The elements of the part enclosed in the dotted line are formed on a p-type semiconductor substrate, and coil L 1  and capacitor C 11  are externally connected.  
         [0199]    The switching regulator  86  thus comprised controls the switching of the ON/OFF states of MOS transistors Q 61  and Q 62  by means of switching control circuit  89 . First, MOS transistor Q 61  is switched ON to store electromagnetic energy in coil L 1  by means of the dc supply. Then, MOS transistor Q 62  turns ON and the sum of the dc supply voltage and the voltage across coil L 1  is taken as output voltage VOUT 1 .  
         [0200]    Output voltage VOUT 1  is voltage divided by the voltage-divider circuit consisting of resistors RA, RB to produced a stepped-down voltage representation of VOUT 1 . Comparator  88  compares the stepped-down voltage with reference voltage Vrefl. The switching control circuit  89  adjusts the ON times of MOS transistors Q 61  and Q 62  according to the output from comparator  88 , and thereby holds output voltage VOUT 1  constant.  
         [0201]    A voltage conversion circuit according to a ninth embodiment of the invention is described next with reference to FIG. 24.  
         [0202]    As shown in FIG. 24, the voltage conversion circuit according to this ninth embodiment outputs positive and negative boosted voltages from a single dc source by using a combination of the synchronous commutating switching regulator  86  of FIG. 23 to output a positively boosted voltage and a synchronous commutating switching regulator  90  to output a negatively boosted voltage.  
         [0203]    The switching regulator  86  in FIG. 24 is configured identically to the switching regulator  86  shown in FIG. 23, like parts are therefore referenced by like reference numerals, and further description is omitted. As shown in FIG. 24, switching regulator  90  comprises a comparator  91 , switching control circuit  92 , MOS transistors Q 63  and Q 64 , voltage-dividing resistors RC and RD, coil L 2 , and capacitor C 12 . The functionality of switching regulator  90  is similar to that of switching regulator  86  with the exception that the polarity of the power supplies coupled to switching regulator  90  are reversed so that it may provide a negative voltage output.  
         [0204]    The elements of the part enclosed in the dotted line in FIG. 24 are formed in a p-type semiconductor substrate in this ninth embodiment using the same method as in the above first and sixth embodiments, thus forming an IC chip to which coils L 1  and L 2  and capacitors C 11  and C 12  are externally connected.  
         [0205]    With this ninth embodiment of the invention the NMOS transistor Q 64  for generating the negative voltage can be formed in the p-type semiconductor substrate, the only external components are the coils L 1  and L 2  and capacitors C 11  and C 12 , and the overall device size can therefore be reduced.  
         [0206]    A voltage conversion circuit according to a tenth embodiment of the present invention is described next with reference to FIG. 25 to FIG. 27.  
         [0207]    As shown in FIG. 25, the voltage conversion circuit according to this tenth embodiment comprises a charge pump type dc-dc converter  95  and a plurality of operational amplifiers  96  to  98 , forming a dc-dc converter for converting dc input voltage Vin to a specific dc voltage.  
         [0208]    The dc-dc converter  95  has a charge pump design and outputs voltages at 0.5, 2, 3, 4, and 5 times the dc input voltage Vin, and these output voltages are supplied as shown in the figure as the supply voltages to the operational amplifiers  96  to  98 , which are configured as voltage followers.  
         [0209]    Output voltage (VIN·5) of dc-dc converter  95  is voltage divided by voltage-dividing resistors R 1  to R 3 , and these voltage-divided potentials are observed at the output terminals of the operational amplifiers  96  to  98 . Note that the internal capacitors of dc-dc converter  95  and any output stabilizing capacitors are not shown in FIG. 25, but are considered to be within the general understanding in the art.  
         [0210]    It should be noted that the dc-dc converter  95  could be configured using a plurality of switching regulators, or a combination of charge pump type dc-dc converters and switching regulators.  
         [0211]    [0211]FIG. 26 shows a first exemplary internal configuration for operational amplifiers  96  to  98 . In the present configuration, identified herein as a push mode operational amplifier, the output current flow is modulated by transistor Q 79  functioning as a current source.  
         [0212]    As shown in FIG. 26, the push mode operational amplifiers  96 - 98  includes MOS transistors Q 71  to Q 79 , which are formed in a semiconductor substrate in the same way as MOS transistors Q 11 , Q 17 , Q 13 , and Q 14  of the second embodiment (see FIGS. 7 and 9) so that there is no substrate bias effect (i.e. no back gate, or back bias, effect) on NMOS transistors Q 72  to Q 76 .  
         [0213]    [0213]FIG. 27 shows as second exemplary internal configuration for operational amplifiers  96  to  98 . In the present configuration, identified herein as a pull mode operational amplifier, the output current is flow modulated by transistor Q 89  functioning as a current drain.  
         [0214]    As shown in FIG. 27, the pull mode operational amplifiers  96 - 98  includes MOS transistors Q 81  to Q 89 , which are formed in a semiconductor substrate in the same way as MOS transistors Q 11 , Q 17 , Q 13 , Q 14  in the second embodiment so that there is no substrate bias effect (i.e. no back gate, or back bias, effect) on NMOS transistors Q 81  and Q 87  to Q 89 . Note that a phase compensation capacitor is omitted in FIG. 26 and FIG. 27.  
         [0215]    Various types of amplifiers may be configured as amplifiers  96 - 98 . For example, a class B amplifier, also known as a push-pull amplifier, wherein two active devices alternately drive the output node for alternate half cycles and neither drive the output for a transition region of the operating range between the two cycles, may be use. Alternatively, a class AB amplifier wherein two active devices alternatively drive the output for alternate half cycles throughout the entire operating range, may also be used.  
         [0216]    As described above, this tenth embodiment of the invention provides a circuit configuration whereby the NMOS transistors of the operational amplifiers  96  to  98  are free of the substrate bias, or back bias, effect, and operation is therefore normal even if an intermediate potential output from dc-dc converter  95  is applied as the source voltage of the operational amplifiers  96  to  98 .  
         [0217]    It will also be noted that by applying an intermediate potential from the dc-dc converter  95  as the supply voltage VDD, VSS to the operational amplifiers  96  to  98 , power consumption by the operational amplifiers can be suppressed and power conversion efficiency is significantly improved when a negative overcurrent is supplied. For example, when negative overcurrent Io flows from operational amplifier  97  in the direction shown in FIG. 25, a current of five times Io would flow to the input side of a conventional circuit, but is reduced to only three times Io with the design shown in FIG. 25.  
         [0218]    A voltage conversion circuit according to an eleventh embodiment of the present invention is described next with reference to FIG. 28.  
         [0219]    The voltage conversion circuit according to this eleventh embodiment uses a process whereby the circuit of the seventh embodiment as shown in FIG. 21 is integrated as shown in FIG. 22, for example, and assembles p-channel and channel MOS transistors to a desired potential irrespective of the system supply voltage.  
         [0220]    This eleventh embodiment is therefore a dc-dc converter combining a voltage converter  101  comprising a charge-pump type dc-dc converter and switching regulator arrangement with plural operational amplifiers  102  to  104  so as to generate a specific positive and negative voltage based on the dc input voltage Vin.  
         [0221]    The voltage converter  101  outputs, for example, a positive voltage that is three or five times the dc input voltage Vin, and negative voltages at −1 and −3 or −5 times the dc input voltage Vin. These output voltages are supplied as the source voltages to the operational amplifiers  102  to  104  configured as voltage followers as shown in FIG. 28. Output voltages from the voltage converter  101  are also voltage divided by voltage dividing resistors R 11  to R 13 , and the voltage-divided voltages are taken from the output terminals of the operational amplifiers  102  to  104 .  
         [0222]    Advantages of the Invention  
         [0223]    As will be known from the preceding description, the MOS transistors used for voltage conversion can be formed in the same p-type semiconductor substrate with the present invention, thereby reducing the externally disposed parts to the capacitors only and making it possible to reduce the overall size while maintaining high power conversion efficiency.  
         [0224]    It is also possible according to the present invention to form the MOS transistors used for voltage conversion in the same n-type semiconductor substrate, thereby reducing the externally disposed parts to the capacitors only and making it possible to reduce the overall size while maintaining high power conversion efficiency.  
         [0225]    While the invention has been described in conjunction with several specific embodiments, it is evident to those skilled in the art that many further alternatives, modifications and variations will be apparent in light of the foregoing description. Thus, the invention described herein is intended to embrace all such alternatives, modifications, applications and variations as may fall within the spirit and scope of the appended claims.