Abstract:
Provided is a reference voltage circuit for generating a low constant voltage (1.25 V or lower) having less temperature dependence. The reference voltage circuit includes: a bandgap voltage generation circuit including two PN junctions, for outputting a voltage (Vk) which is based on any one of the two PN junctions and a current (Ik) which is based on a voltage difference between the two PN junctions; and a voltage divider circuit for dividing the voltage (Vk). The voltage divider circuit corrects a divided voltage based on the current (Ik) input thereto, and outputs the corrected divided voltage as a reference voltage.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2011-197357 filed on Sep. 9, 2011, 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 reference voltage circuit for generating a constant voltage having less temperature dependence. 
         [0004]    2. Description of the Related Art 
         [0005]    Conventionally, as a reference voltage circuit for generating a constant voltage having less temperature dependence, there is known a bandgap reference voltage circuit for generating a voltage which is substantially equal to a bandgap value of silicon (see, for example, Japanese Patent Application Laid-open No. 2008-305150). 
         [0006]      FIG. 4  is a configuration diagram illustrating a conventional bandgap reference voltage circuit. The conventional bandgap reference voltage circuit includes a PN junction  401 , a PN junction  402 , a resistor  403  having a resistance value R 1 , a transistor  404 , a transistor  405 , a transistor  406 , a resistor  407  having a resistance value R 2  which is of the same type as the resistor  403  (which has equal temperature characteristics), a PN junction  408 , and an amplifier  409 . The PN junction  401  and the PN junction  402  have a relationship in which an effective area ratio (for example, an anode-cathode junction area ratio) is 1:K 1 . 
         [0007]    The transistor  404  and the transistor  405  have the same gate-source voltage, and hence a current based on the size ratio flows therethrough. For example, when the size ratio is 1:1, substantially equal currents flow through the transistor  404  and the transistor  405 . It is herein assumed that the current of the transistor  404  and the current of the transistor  405  are substantially equal to each other. The amplifier  409  controls the currents flowing through the transistor  404  and the transistor  405  so that a voltage VA and a voltage VB may be equal to each other. In this case, a current Ib flowing through the transistor  405  is expressed by Expression (1). 
         [0000]        Ib=Vt× {ln( K 1)}/ R 1  (1)
 
         [0008]    In Expression (1), VT represents a thermal voltage and is expressed by kT/q, where q represents the unit electron charge, k represents the Boltzmann constant, and T represents the absolute temperature. 
         [0009]    A current based on the current Ib flows through the transistor  406 . When the size ratio between the transistor  405  and the transistor  406  is 1:1, and a voltage generated at the PN junction  408  is represented by a voltage Vpn 3 , a reference voltage Vref is expressed by Expression (2). 
         [0000]        V ref= Vpn 3+( R 2 /R 1)× VT ×{ln( K 1)}  (2)
 
         [0010]    The first term exhibits a negative temperature characteristic because the voltage Vpn 3  has a negative temperature characteristic of about −2.0 mV/° C. The second term exhibits a positive temperature characteristic because the thermal voltage VT has a positive temperature characteristic. 
         [0011]    Expression (2) is differentiated with respect to T, and the condition in which Vref becomes zero is obtained as expressed by Expression (3). 
         [0000]      ( R 2/ R 1)×( k/q )×{ln( K 1)}=0.002  (3)
 
         [0012]    Therefore, by setting (R 2 /R 1 ) so as to satisfy Expression (3), it is possible to realize a reference voltage Vref having no temperature dependence. 
         [0013]    In this manner, a reference voltage circuit for generating a voltage having less temperature dependence can be obtained. 
         [0014]    In the conventional bandgap reference voltage circuit, however, the reference voltage Vref is about 1.25 V based on Expressions (2) and (3). Therefore, there has been a problem in that an operating voltage cannot be set equal to or lower than a voltage which is limited by the reference voltage Vref. 
       SUMMARY OF THE INVENTION 
       [0015]    The present invention has been made in order to solve the above-mentioned problem, and realizes a reference voltage circuit for generating a lower voltage having less temperature dependence. 
         [0016]    A reference voltage circuit according to the present invention includes: a bandgap voltage generation circuit including two PN junctions, for outputting a voltage Vk which is based on the two PN junctions and a current Ik which is based on a voltage difference between the two PN junctions; and a voltage divider circuit for dividing the voltage Vk. The voltage divider circuit corrects a divided voltage based on the current Ik input thereto, and outputs the corrected divided voltage as a reference voltage. 
         [0017]    The present invention can provide the reference voltage circuit for generating a lower reference voltage having less temperature dependence. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    In the accompanying drawings: 
           [0019]      FIG. 1  is a configuration diagram illustrating a reference voltage circuit according to a first embodiment of the present invention; 
           [0020]      FIG. 2  is a configuration diagram illustrating a reference voltage circuit according to a second embodiment of the present invention; 
           [0021]      FIG. 3  is a configuration diagram illustrating a reference voltage circuit according to a third embodiment of the present invention; and 
           [0022]      FIG. 4  is a configuration diagram illustrating a conventional bandgap reference voltage circuit. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]      FIGS. 1 to 3  are configuration diagrams each illustrating a reference voltage circuit according to embodiments of the present invention. 
         [0024]    The reference voltage circuit according to the embodiments of the present invention includes a bandgap voltage generation circuit  100  and a voltage divider circuit  101 . The bandgap voltage generation circuit  100  generates and outputs a voltage Vk and a current Ik based on voltages of two PN junctions (having a relationship in which an effective area ratio, for example, an anode-cathode junction area ratio is 1:K 1 ). The voltage divider circuit  101  outputs a reference voltage Vref based on the voltage Vk and the current Ik which are input from the bandgap voltage generation circuit  100 . 
       First Embodiment 
       [0025]      FIG. 1  illustrates a configuration diagram of a reference voltage circuit according to a first embodiment of the present invention. 
         [0026]    A bandgap voltage generation circuit  100  includes PN junctions  401  and  402 , a resistor  403 , transistors  404  and  405 , an amplifier  409 , and a transistor  11 . A voltage divider circuit  101  includes an amplifier  12  and resistors  13  and  14 . 
         [0027]    The transistor  404  and the PN junction  401  are connected in series between a power source and the ground. The transistor  405 , the resistor  403 , and the PN junction  402  are connected in series between the power source and the ground. The amplifier  409  has an inverting input terminal connected to a node between the transistor  404  and the PN junction  401 . The amplifier  409  has a non-inverting input terminal connected to a node between the transistor  405  and the resistor  403 . The amplifier  409  has an output terminal connected to a gate terminal of each of the transistor  404 , the transistor  405 , and the transistor  11 . 
         [0028]    As a voltage Vk based on the PN junctions, a voltage VA generated at the PN junction  401  is used. As a current Ik based on the PN junctions, a current supplied by the transistor  11 , whose gate terminal is connected in common to the gate terminals of the transistor  404  and the transistor  405 , is used. 
         [0029]    The amplifier  12  has an inverting input terminal to which the voltage Vk is input. An output terminal and an inverting input terminal of the amplifier  12  are connected to each other. The resistors  13  and  14  are connected in series between the output terminal of the amplifier  12  and the ground. A node between the resistors  13  and  14  is connected to a drain terminal of the transistor  11 , and is connected to an output terminal of the reference voltage circuit. 
         [0030]    Now, the operation of the reference voltage circuit according to this embodiment is described. 
         [0031]    The amplifier  409  controls currents flowing through the transistor  404  and the transistor  405  so that the voltage VA and a voltage VB may be equal to each other. 
         [0032]    A current Ib flowing through the transistor  405  is a value obtained by dividing a voltage difference between a voltage Vpn 1  generated at the PN junction  401  and a voltage Vpn 2  generated at the PN junction  402  by a resistance value R 1  of the resistor  403 . In other words, the current Ib based on the voltage difference of the two PN junctions flows through the transistor  405 . 
         [0033]    In this case, the transistor  11  and the transistor  405  have the same gate-source voltage, and hence a current based on the size ratio flows therethrough. For example, when the size ratio is 1:1, substantially equal currents Ib flow through the transistor  11  and the transistor  405 . In other words, the current Ik, which is equal to the current Ib based on the voltage difference of the two PN junctions, flows through the transistor  11 . 
         [0034]    The current Ik flowing through the transistor  11  is expressed by Expression (4). 
         [0000]        Ik=VT× {ln( K 1)/} R 1  (4)
 
         [0035]    In Expression (4), VT represents a thermal voltage and is expressed by kT/q, where q represents the unit electron charge, k represents the Boltzmann constant, and T represents the absolute temperature. 
         [0036]    The voltage Vref is expressed by Expression (5). 
         [0000]        V ref= Ik ×( R 3× R 4)/( R 3+ R 4)+ Vk×R 3/( R 3 +R 4)={ R 3/( R 3 +R 4)}×( R 4/ R 1)× VT× {ln( K 1)}+ Vk}   (5)
 
         [0000]    where R 3  represents a resistance value of the resistor  13 , and R 4  represents a resistance value of the resistor  14 . 
         [0037]    In Expression (5), (R 4 /R 1 )×VT×{ln(K 1 )} exhibits a positive temperature characteristic because the thermal voltage VT has a positive temperature characteristic, and Vk exhibits a negative temperature characteristic because Vpn 1  has a negative temperature characteristic of about −2.0 mV/° C. Therefore, through appropriate setting of (R 4 /R 1 ), {(R 4 /R 1 )×VT×{ln(K 1 )}+Vk} in Expression (5) can have less temperature dependence. Then, merely through appropriate setting of {R 3 /(R 3 +R 4 )}, the reference voltage Vref can be obtained as a divided voltage of {(R 4 /R 1 )×VT×{ln(K 1 )}+Vk} in Expression (5), whose absolute value can be set freely. 
         [0038]    As described above, the reference voltage Vref of the reference voltage circuit according to the first embodiment can be obtained as a low voltage (1.25 V or lower) having less temperature dependence. Therefore, the operating voltage of the reference voltage circuit can also be reduced. 
         [0039]    Note that, the reference voltage circuit of the first embodiment has a configuration in which the voltage Vk is subjected to impedance conversion by the amplifier  12 , but, in the case where the impedance of the voltage Vk is low, the voltage Vk may be connected to the resistor  14  directly. 
         [0040]    Further, in the reference voltage circuit of the first embodiment, as the voltage Vk based on the PN junctions, the voltage VA generated at the PN junction  401  is used, but the voltage VB or another voltage may be used. 
         [0041]    Further, in the reference voltage circuit of the first embodiment, as a circuit for generating the voltage VB, the circuit configuration in which the PN junction  402  and the resistor  403  are connected in series in this order from the ground is used, but the same effect can be obtained even when the PN junction  402  and the resistor  403  are connected in the reverse order. 
       Second Embodiment 
       [0042]      FIG. 2  illustrates a configuration diagram of a reference voltage circuit according to a second embodiment of the present invention. 
         [0043]    A bandgap voltage generation circuit  100  includes PN junctions  401  and  402 , a resistor  403 , transistors  21 ,  22 ,  23 ,  24 ,  25 , and  27 , a PN junction  26 , and a transistor  11 . 
         [0044]    The PN junctions  401  and  402  and the resistor  403  are configured similarly to the reference voltage circuit of the first embodiment. The transistors  21  and  22  and the transistors  23 ,  24 , and  25  form current mirror circuits, respectively. The transistor  27 , the transistor  25 , and the PN junction  26  are connected in series between a power source and the ground. The transistor  27  and the transistor  11  form a current mirror circuit. 
         [0045]    The current mirror circuits allow equal currents to flow through the PN junctions  401  and  402  and the resistor  403 , and hence a voltage VA and a voltage VB become equal to each other. 
         [0046]    As a voltage Vk based on the PN junctions, the voltage VA generated at the PN junction  401  is used. As a current Ik based on the PN junctions, a current supplied by the PN junction  26  and the transistor  25 , whose gate terminal is connected in common to the gate terminals of the transistor  23  and the transistor  24 , is used. 
         [0047]    Even by the reference voltage circuit of the second embodiment having the configuration illustrated in  FIG. 2  described above, the same effect as in the reference voltage circuit of the first embodiment can be obtained. 
         [0048]    Note that, the reference voltage circuit of the second embodiment has a configuration in which the voltage Vk is subjected to impedance conversion by the amplifier  12 , but, in the case where the impedance of the voltage Vk is low, the voltage Vk may be connected to the resistor  14  directly. 
         [0049]    Further, in the reference voltage circuit of the second embodiment, as the voltage Vk based on the PN junctions, the voltage VA generated at the PN junction  401  is used, but the voltage VB or another voltage may be used. 
         [0050]    Further, in the reference voltage circuit of the second embodiment, as a circuit for generating the voltage VB, the circuit configuration in which the PN junction  402  and the resistor  403  are connected in series in this order from the ground is used, but the same effect can be obtained even when the PN junction  402  and the resistor  403  are connected in the reverse order. 
       Third Embodiment 
       [0051]      FIG. 3  illustrates a configuration diagram of a reference voltage circuit according to a third embodiment of the present invention. 
         [0052]    A bandgap voltage generation circuit  100  includes current sources  31   a  and  31   b,  PN junctions  401  and  402 , transistors  33   a  and  33   b,  resistors  34   a  and  34   b , amplifiers  39   a  and  39   b,  and transistors  35  and  11 . 
         [0053]    The current source  31   a  and the PN junction  401  are connected in series between a power source and the ground, and a node therebetween is connected to a non-inverting input terminal of the amplifier  39   a.  The amplifier  39   a  has an output terminal connected to a gate terminal of the transistor  33   a  and an inverting input terminal connected to a source terminal of the transistor  33   a.  The transistors  35  and  33   a  and the resistor  34   a  are connected in series between the power source and the ground. The transistors  35  and  11  are current-mirror-connected. 
         [0054]    The current source  31   b  and the PN junction  402  are connected in series between the power source and the ground, and a node therebetween is connected to a non-inverting input terminal of the amplifier  39   b.  The amplifier  39   b  has an output terminal connected to a gate terminal of the transistor  33   b  and an inverting input terminal connected to a source terminal of the transistor  33   b.  The transistors  11   33   b  and the resistor  34   b  are connected in series between the power source and the ground. 
         [0055]    The transistor  33   a  and the resistor  34   a  allow a current Ia which is based on a voltage Vpn 1  generated at the PN junction  401  to flow. The transistor  33   b  and the resistor  34   b  allow a current Ib which is based on a voltage Vpn 2  generated at the PN junction  402  to flow. 
         [0056]    In this case, as a voltage Vk based on the PN junctions, a voltage VA generated at the PN junction  401  is used. As a current Ik based on a voltage difference of the two PN junctions, a current obtained by subtracting the current Ib from the current Ia is used. From the above, the current Ik obtained by subtracting the current Ib from the current Ia is a current based on the voltage difference of the two PN junctions. 
         [0057]    Even by the reference voltage circuit of the third embodiment having the configuration illustrated in  FIG. 3  described above, the same effect as in the reference voltage circuit of the first embodiment can be obtained. 
         [0058]    Note that, the reference voltage circuit of the third embodiment has a configuration in which the voltage Vk is subjected to impedance conversion by the amplifier  12 , but, in the case where the impedance of the voltage Vk is low, the voltage Vk may be connected to the resistor  14  directly. 
         [0059]    Further, in the reference voltage circuit of the third embodiment, as the voltage Vk based on the PN junctions, the voltage VA generated at the PN junction  401  is used, but a voltage VB or another voltage may be used.