Patent Publication Number: US-11662761-B2

Title: Reference voltage circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefits of Japanese application no. 2020-182127, filed on Oct. 30, 2020. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
     BACKGROUND 
     Technical Field 
     The present invention relates to a reference voltage circuit. 
     Description of Related Art 
     A reference voltage circuit using NPN transistors has been proposed (see, for example, Japanese Laid-Open No. 2005-182113). 
     The reference voltage circuit described in Japanese Laid-Open No. 2005-182113 and shown in  FIG.  5    includes a first NPN transistor Q 41 , a second NPN transistor Q 42 , an operational amplifier OP, and resistors  41 ,  42 ,  43  and  44 , in which a reference voltage with no temperature characteristic is obtained by applying currents of the same value to the first NPN transistor Q 41  and the second NPN transistor Q 42  and adjusting (trimming) the resistor  44 . 
       FIG.  6    is a schematic cross-sectional view of an NPN transistor. The NPN transistor is composed of an emitter  31 , a base  32 , and a collector  33 . When the NPN transistor is formed on a PSUB board  34 , the NPN transistor has a parasitic diode  35  between the collector  33  and the PSUB board  34  as shown in  FIG.  7   . Through this parasitic diode  35 , a part of the current that should originally flow through the NPN transistor at a high temperature flows as a leakage current of the parasitic diode  35 . 
     Further, in the reference voltage circuit of  FIG.  5   , the size of the first NPN transistor Q 41  is set to be larger than the size of the second NPN transistor Q 42 . The same applies to the sizes of the parasitic diodes, and therefore the size of the parasitic diode of the first NPN transistor Q 41  is larger than the size of the parasitic diode of the second NPN transistor Q 42 . In addition, the leakage current increases with the size of the parasitic diode. Thus, the leakage current flowing through the parasitic diode is larger in the first NPN transistor Q 41  than in the second NPN transistor Q 42 . The currents flowing through the first NPN transistor Q 41  and the second NPN transistor Q 42  may deviate from the same current value originally set at a high temperature, and the reference voltage circuit of  FIG.  5    has large temperature dependence. 
     SUMMARY 
     The present invention has an object to provide a reference voltage circuit having little temperature dependence. 
     A reference voltage circuit according to one aspect of the present invention includes: a first NPN transistor having a collector and a base shorted and diode-connected; a second NPN transistor having a collector and a base shorted and diode-connected, and having an emitter connected to a first potential node, and the second NPN transistor operating at a higher current density than the first NPN transistor; a first resistor connected in series with the first NPN transistor; a second resistor having one end connected to a circuit in which the first NPN transistor and the first resistor are connected in series; a third resistor having one end connected to the collector of the second NPN transistor; a connection point to which the other end of the second resistor and the other end of the third resistor are connected; an arithmetic amplifier circuit having an inverting input terminal connected to one end of the second resistor, a non-inverting input terminal connected to one end of the third resistor, and an output terminal connected to the connection point; and a current supply circuit connected to the collector of the first NPN transistor. 
     According to the present invention, a reference voltage having little temperature dependence can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a circuit diagram showing a first configuration example of a reference voltage circuit according to an embodiment of the present invention. 
         FIG.  2    is a circuit diagram showing a second configuration example of a reference voltage circuit according to the embodiment. 
         FIG.  3    is a circuit diagram showing a third configuration example of a reference voltage circuit according to the embodiment. 
         FIG.  4    is a circuit diagram showing a fourth configuration example of a reference voltage circuit according to the embodiment. 
         FIG.  5    is a circuit diagram showing an example of a conventional reference voltage circuit having NPN transistors. 
         FIG.  6    is a cross-sectional view showing a structure of a general NPN transistor. 
         FIG.  7    is a circuit diagram showing an equivalent circuit of a general NPN transistor. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, a reference voltage circuit according to an embodiment of the present invention will be described with reference to the drawings. 
       FIG.  1    is a circuit diagram of a reference voltage circuit  10  which is an example (first configuration example) of the reference voltage circuit according to an embodiment of the present invention. The reference voltage circuit  10  includes a conventional reference voltage circuit  20  and a current supply circuit  21 . 
     The conventional reference voltage circuit  20  includes NPN transistors  1  and  2 , resistors  3 ,  4 , and  5 , an operational amplifier  6 , and an OUT terminal. Here, the NPN transistor  2  is a transistor that has a larger transistor size than the NPN transistor  1 . The resistor  4  and the resistor  5  have the same resistance value. The current supply circuit  21  includes an NPN transistor  7  and P-channel type MOS transistors  8  and  9 . 
     Connection of the conventional reference voltage circuit  20  will be described. A base terminal and a collector terminal of the NPN transistor  1  are connected to each other and are connected to one end of the resistor  4 . An emitter terminal is connected to a GND power supply. A base terminal and a collector terminal of the NPN transistor  2  are connected to each other and are connected to one end of the resistor  5 . An emitter terminal is connected to a GND power supply via the resistor  3 . Further, the base terminal and the collector terminal of the NPN transistor  2  are connected to a drain terminal of the P-channel type MOS transistor  9  of the current supply circuit  21 . The other end of the resistor  4  and the other end of the resistor  5  are connected to a connection point  17 . The operational amplifier  6  has a non-inverting input terminal connected to the collector terminal of the NPN transistor  1 , an inverting input terminal connected to the collector terminal of the NPN transistor  2 , and an output terminal connected to the connection point  17  and the OUT terminal. The description of a power supply of the operational amplifier  6  will be omitted. 
     Connection of the current supply circuit  21  will be described. The P-channel type MOS transistor  8  has a source terminal connected to a VDD power supply, and a gate terminal connected to the drain terminal, a gate terminal of the P-channel type MOS transistor  9 , and a collector terminal of the NPN transistor  7 . The P-channel type MOS transistor  9  has a source terminal connected to the VDD power supply, the gate terminal connected to the gate terminal of the P-channel type MOS transistor  8 , and the drain terminal connected to the collector terminal of the NPN transistor  2  of the conventional reference voltage circuit  20 . The NPN transistor  7  has the collector terminal connected to the drain terminal of the P-channel type MOS transistor  8 , and a base terminal connected to the emitter terminal and a GND power supply. The P-channel type MOS transistor  8  and the P-channel type MOS transistor  9  form a current mirror circuit. 
     An operation of the conventional reference voltage circuit  20  will be described. The operational amplifier  6  amplifies a voltage of a difference between a voltage, which is obtained by adding a voltage generated in the resistor  3  and a base-emitter voltage VBE 2  of the NPN transistor  2 , and a base-emitter voltage VBE 1  of the NPN transistor  1 , and applies an output voltage of the operational amplifier  6  to the resistor  4  and the resistor  5 . 
     Here, when the output voltage of the operational amplifier  6  is lower than a specified value, the current flowing through the resistor  4  and the resistor  5  is smaller than a specified value. Here, the resistance values of the resistor  4  and the resistor  5  are set relatively large, and the voltage drop values of the resistor  4  and the resistor  5  are set to be larger than the base-emitter voltage VBE 1  of the NPN transistor  1  and the base-emitter voltage VBE 2  of the NPN transistor  2 . The base-emitter voltage VBE 1  of the NPN transistor  1  and the base-emitter voltage VBE 2  of the NPN transistor  2  have substantially the same values as the specified value. Therefore, assuming that the resistance value of the resistor  3  is a resistance value R 3  and the current flowing through the resistor  3  is a current value IR 3 , the input potential of the non-inverting input terminal of the operational amplifier  6  is determined by the voltage VBE 1 , and the input potential of the inverting input terminal is determined by the voltage VBE 2 +the resistance value R 3 ×the current value IR 3 . Since the current value IR 3  is lower than the one when the output voltage is the specified value, the input voltage of the non-inverting input terminal becomes lower than the input potential of the inverting input terminal, and the output voltage of the operational amplifier  6  operates so as to go up and rises to a steady value. 
     When the output voltage of the operational amplifier  6  is higher than the specified value, the voltage generated in the resistor  3  becomes higher, and for the same reason as explained above, the input voltage of the inverting input terminal of the operational amplifier  6  becomes higher than the input voltage of the non-inverting input terminal, and the output voltage of the operational amplifier drops to a steady value. 
     When the operation of the reference voltage circuit  20  enters a stable state, the input voltages of the non-inverting input terminal and the inverting input terminal of the operational amplifier  6  have the same potential. Therefore, currents of the same value flow through the NPN transistor  1  and the NPN transistor  2 . As described above, the transistor size of the NPN transistor  2  is larger than the transistor size of the NPN transistor  1 . The NPN transistor  1  operates at a larger current density than the NPN transistor  2 . A difference voltage ΔVBE between the base-emitter voltage VBE 1  of the NPN transistor  1  and the base-emitter voltage VBE 2  of the NPN transistor  2  is expressed by the following equation.
 
ΔVBE=VBE1−VBE2=( KT/q )×ln  N   [Formula 1]
 
     Here, K is the Boltzmann constant, T is the absolute temperature, q is the charge amount, and N is the ratio of the transistor sizes of the NPN transistor  1  and the NPN transistor  2 . 
     Therefore, a current of the voltage ΔVBE/the resistance value R 3  flows through the resistor  3 , and the current also flows through the resistor  5 . Since currents of the same value flow through the NPN transistor  1  and the NPN transistor  2 , and currents of the same value flow through the resistor  4  and the resistor  5 , the output voltage of the operational amplifier  6  is expressed by the following equation.
 
 V OUT=VBE1+(ΔVBE/ R 3)× R 4  [Formula 2]
 
     Here, R 4  is the resistance value of the resistor  4 . Since the value of the voltage ΔVBE is proportional to the absolute temperature T as shown in the previous equation, the voltage ΔVBE increases as the temperature rises, but since the voltage VBE 1  decreases as the temperature rises, it is possible to generate a reference voltage with no temperature characteristic by appropriately selecting the resistance values of the resistors  3 ,  4 , and  5 . 
     When the reference voltage circuit is built in an integrated circuit, the NPN transistors may be formed on a PSUB board.  FIG.  6    shows a cross-sectional view of an NPN transistor formed on a PSUB board. Further,  FIG.  7    shows an equivalent circuit of an NPN transistor formed on a PSUB board. 
     In the NPN transistor formed on a PSUB board  34 , the first N-type diffusion layer serves as a collector  33 , the P-type diffusion layer serves as a base  32 , and the second N-type diffusion layer serves as an emitter  31 . At the same time, a parasitic diode  35  is formed by the PSUB board  34  and the first N-type diffusion layer which is the collector  33 . 
     Since a reverse bias voltage is applied during the operation of the NPN transistor, the parasitic diode  35  usually has no effect on the operation of the NPN transistor. However, in the parasitic diode  35  to which the reverse bias voltage is applied, a minute leakage current flows from the cathode to the anode. The leakage current flowing through the parasitic diode  35  has temperature dependence, and a larger leakage current flows at a higher temperature. 
     In the conventional reference voltage circuit  20  shown in  FIG.  1   , both the NPN transistor  1  and the NPN transistor  2  have parasitic diodes, and a part of the current flowing through each of the NPN transistor  1  and the NPN transistor  2  flows to the GND power supply via the parasitic diode. Here, since the transistor size of the NPN transistor  2  is larger than the transistor size of the NPN transistor  1 , the parasitic diode of the NPN transistor  2  also has a larger diode size than the parasitic diode of the NPN transistor  1 . 
     In order to generate a reference voltage with little temperature dependence, it is required for the NPN transistor  1  and the NPN transistor  2  to have equal currents flowing therethrough. However, since the parasitic diode existing in the NPN transistor  2  has a larger diode size than the NPN transistor  1 , the leakage current flowing through the parasitic diode at a high temperature is also large. At a high temperature, the current flowing through the NPN transistor  2  decreases more than the current flowing through the NPN transistor  1 . As a result, there is a difference between the currents flowing through the NPN transistor  1  and the NPN transistor  2 . The conventional reference voltage circuit formed on the PSUB board cannot generate a reference voltage having little temperature dependence, and the generated reference voltage has temperature dependence. 
     Therefore, in this embodiment, the current supply circuit  21  is connected to the collector of the NPN transistor  2 . The NPN transistor  7  of the current supply circuit  21  has a parasitic diode, and a leakage current flows in the same manner as in the NPN transistor  2 . The current supply circuit  21  supplies the leakage current flowing through the NPN transistor  7  to the collector of the NPN transistor  2  via the current mirror circuit formed by the P-channel type MOS transistor  8  and the P-channel type MOS transistor  9 . 
     By adjusting the transistor size of the NPN transistor  7  and the mirror ratio of the current mirror circuit, the currents flowing through the NPN transistor  1  and the NPN transistor  2  can be set to be equal. Specifically, the transistor size adjustment of the NPN transistor  7  can be realized by connecting a plurality of NPN transistors in parallel to form the NPN transistor  7  and, if necessary, separating a part of the plurality of transistors from the circuit by trimming or the like. Similarly, the adjustment of the mirror ratio of the current mirror circuit can be realized by connecting a plurality of P-channel type MOS transistors in parallel to form one transistor that constitutes the current mirror circuit and, if necessary, separating a part of the plurality of P-channel type MOS transistors from the circuit by trimming or the like. 
     Here, the resistor  3  is connected between the NPN transistor  2  and the GND power supply, but like a reference voltage circuit  11  of a second configuration example shown in  FIG.  2   , the resistor  3  may be connected between the resistor  5  and the NPN transistor  2 , the inverting input terminal of the operational amplifier  6  may be connected to the connection point of the resistor  3  and the resistor  5 , the current supply circuit  21  may be connected to the collector of the NPN transistor  2  as in  FIG.  1   , and the emitter of the NPN transistor  2  may be connected to the GND power supply. 
     Further, the NPN transistor  7  may be a diode  7   a  as in a reference voltage circuit  12  of a third configuration example shown in  FIG.  3   . The diode  7   a  has a cathode terminal connected to the drain terminal of the P-channel type MOS transistor  8 , and an anode terminal connected to the GND power supply. The diode  7   a  is provided with only the parasitic diode of the NPN transistor  7 , and a leakage current similar to the leakage current of the NPN transistor  7  flows. 
     Further, the resistor  4  and the resistor  5  may be constituted by a resistor  14 , a resistor  15 , and a resistor  16  as in a reference voltage circuit  13  of a fourth configuration example shown in  FIG.  4   . One end of the resistor  14  is connected to the collector terminal of the NPN transistor  1 , and the other end is connected to a connection point  18 . One end of the resistor  15  is connected to the collector terminal of the NPN transistor  2 , and the other end is connected to the connection point  18 . One end of the resistor  16  is connected to the connection point  18 , and the other end is connected to the output terminal of the operational amplifier  6 . The fourth configuration example is a configuration in which a part of the resistor  4  and the resistor  5  are replaced with the resistor  16 . 
     The reference voltage circuit  10  of this embodiment includes the conventional reference voltage circuit  20  and the current supply circuit  21 . By compensating the leakage current flowing through the parasitic diode of the NPN transistor  2  with the current supply circuit  21 , the reference voltage circuit  10  can set the currents flowing through the main body of the NPN transistor  1  and the main body of the NPN transistor  2  that generate the reference voltage to be the same regardless of the temperature, and can generate the reference voltage having little temperature dependence. 
     Nevertheless, the present invention is not limited to the embodiments as described above. At the implementation stage, the present invention can be implemented in various forms other than the above-described examples, and various omissions, replacements, and changes can be made without departing from the gist of the present invention. For example, each switch described in the embodiments of the present invention may be constituted by a PMOS transistor or an NMOS transistor. These embodiments and modifications thereof are included in the scope and gist of the present invention, and are also included in the scope of the present invention defined in the claims and the equivalent scope thereof.