Abstract:
A reference voltage generating circuit is provided that includes: a differential amplifier amplifying an input signal and generate a reference voltage; an output amplifier amplifying the reference voltage and outputting the amplified reference voltage; and a startup circuit connected between an output terminal of the differential amplifier and an input terminal of the output amplifier. The startup circuit has a first switch and a second switch. The first switch connects the output terminal of the differential amplifier and the input terminal of the output amplifier according to a voltage of an output terminal of the output amplifier. The second switch connects the input terminal and the output terminal of the output amplifier according to a voltage of the output terminal of the differential amplifier.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2004-053459, filed on Feb. 27, 2004, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a reference voltage generating circuit having a startup circuit. 
     2. Description of the Related Art 
       FIG. 2  shows a configuration example of a reference voltage generating circuit  200  and a startup circuit  210  thereof according to a prior art. 
     First, the configuration of the reference voltage generating circuit  200  will be described. A resistor  111  is connected between a gate of a p-channel MOS field-effect transistor (hereinafter, referred to as FET)  123  and an output terminal  140 . A resistor  112  is connected between the gate of the MOSFET  123  and an anode of a diode  113 . A cathode of the diode  113  is connected to a ground potential. A resistor  114  is connected between a gate of a p-channel MOSFET  124  and the output terminal  140 . A diode  115  has an anode connected to the gate of the MOSFET  124  and a cathode connected to the ground potential. 
     A differential amplifier  120  has a direct-current power supply  121  and MOSFETs  122  to  126 . The direct-current power supply  121  has an anode connected to a gate of the p-channel MOSFET  122  and a cathode connected to the ground potential. The p-channel MOSFET  122  has a source connected to a power supply potential and a drain connected to a junction point between sources of the p-channel MOSFETs  123 ,  124 . Gates of the n-channel MOSFETs  125 ,  126  are connected to each other, and a junction point therebetween is connected to a drain of the MOSFET  123 . The n-channel MOSFET  125  has a drain connected to the drain of the MOSFET  123  and a source connected to the ground potential. The n-channel MOSFET  126  has a drain connected to a drain of the MOSFET  124  and a source connected to the ground potential. 
     An output amplifier  130  has MOSFETs  131 ,  132 . The p-channel MOSFET  131  has a gate connected to the anode of the direct-current power supply  121 , a source connected to a power supply potential, and a drain connected to the output terminal  140 . The n-channel MOSFET  132  has a gate connected to the drain of the MOSFET  124 , a drain connected to the output terminal  140 , and a source connected to the ground potential. 
     Next, the configuration of the startup circuit  210  will be described. A direct-current power supply  211  has an anode connected to a gate of a p-channel MOSFET  212  and a cathode connected to the ground potential. The p-channel MOSFET  212  has a source connected to a power supply potential and a drain connected to a drain of an n-channel MOSFET  213 . The n-channel MOSFET  213  has a gate connected to the output terminal  140  and a source connected to the ground potential. Gates of p-channel MOSFETs  214 ,  216  are connected to each other and a junction point therebetween is connected to a drain of the MOSFET  214 . The MOSFET  214  has a source connected to the power supply potential and a drain connected to a drain of an n-channel MOSFET  215 . The MOSFET  215  has a gate connected to the drain of the MOSFET  212  and a source connected to the ground potential. A MOSFET  216  has a source connected to the power supply potential and a drain connected to a gate of the MOSFET  124 . 
       FIG. 3A  shows output voltages  302   a ,  302   b  of the output terminal  140  of the reference voltage generating circuit  200  when the startup circuit  210  is not provided. The horizontal axis represents time [t] after power application, and the vertical axis represents voltage [V]. After the power application, a power supply voltage  301  gradually increases and is stabilized at Vcc [V] before long. The reference voltage generating circuit  200  outputs one of two kinds of the output voltages  302   a ,  302   b  according to manufacturing variation or the like. It is not possible to assure which one of two kinds of the output voltages  302   a ,  302   b  is outputted. As a result, the output voltage (reference voltage) of the reference voltage generating circuit  200  has two stabilization points 0 [V] and Vo [V]. Vo [V] is a desired stabilization point of the output voltage and 0 [V] is an undesired stabilization point of the output voltage. 
       FIG. 3B , which corresponds to  FIG. 3A , shows the output voltage  302   a  of the output terminal  140  when the startup circuit  210  is connected to the reference voltage generating circuit  200 . The startup circuit  210  is capable of leading the output voltage of the reference voltage generating circuit  200  to the one desired stabilization point Vo [V] out of the two stabilization points 0 [V] and Vo [V]. This can ensure that the reference voltage generating circuit  200  outputs the desired output voltage  302   a  in accordance with the increase in the power supply voltage  301 . 
     Next, the operation of the reference voltage generating circuit  200  without the startup circuit  210  will be described. The reference voltage generating circuit  200  has a two-stage amplification circuit constituted of the differential amplifier  120  and the output amplifier  130 . When an offset does not exist in the differential amplifier  120 , the output voltage  302   a  is outputted, whereas, when the offset exists, the output voltage  302   b  is outputted. Specifically, in the differential amplifier  120 , the MOSFETs  125 ,  126  function as current mirrors, and a voltage according to the output voltage of the output terminal  140  is feedback-inputted to the gates of the differential input MOSFETs  123 ,  124  of the differential amplifier  120 , so that the MOSFETs  123 ,  124  are feedback-controlled to have equal drain voltages. When the drain voltages of the differential input MOSFETs  123 ,  124  are the same, which means that no offset exists, the output voltage  302   a  is outputted. On the other hand, when an offset of difference in drain voltage between the MOSFETs  123 ,  124  by a predetermined voltage (for example, 10 mV) or more exists due to manufacturing variation or the like, the output voltage  302   b  is outputted. 
     When the output voltage  302   a  in  FIG. 3A  is outputted, the power supply voltage  301  gradually increases after the power application, so that a voltage of the direct-current power supply  121  also gradually increases. Then, the output voltage  302   a  of the output terminal  140  also gradually increases and before long, the output voltage  302   a  is kept at the reference voltage Vo. 
     On the other hand, when the output voltage  302   b  in  FIG. 3A  is outputted, the power supply voltage  301  gradually increases after the power application and the output voltage  302   b  of the output terminal  140  also increases. Here, an offset exists in the differential amplifier  120 , resulting in difference in drain voltage between the MOSFETs  123 ,  124 . For example, the drain voltage of the MOSFET  123  becomes a low voltage as a minus offset, while the drain voltage of the MOSFET  124  becomes a high voltage as a plus offset. Then, the n-channel MOSFET  132  operates so as to turn on, so that the output voltage  302   b  of the output terminal  140  decreases. The decrease in the output voltage  302   b  causes by feedback the decrease in gate voltages of the input MOSFETs  123 ,  124  of the differential amplifier  120 , and then the drain voltage of the p-channel MOSFET  124  increases. By this feedback loop, the output voltage  302   b  gradually decreases to stabilize at 0 [V] before long. 
     Next, the operation of the reference voltage generating circuit  200  with the startup circuit  210  connected thereto will be described. In this case, the output voltage  302   a  in  FIG. 3B  is outputted. After the power application, the power supply voltage  301  and the output voltage  302   a  gradually increase. The voltage  302   a  of the output terminal  140  of the reference voltage generating circuit  200  is monitored at the gate of the n-channel MOSFET  213  that is an input of the startup circuit  210 . When the output voltage  302   a  of the reference voltage generating circuit  200  is lower than a threshold voltage of the n-channel MOSFET  213 , the MOSFET  213  turns off. At this time, the gate voltage of the n-channel MOSFET  215  increases since the p-channel MOSFET  212  is a constant-current source, so that the MOSFET  215  turns on and a current flows therethrough. This current also flows through the p-channel MOSFET  214 . Then, the same current also flows through the p-channel MOSFET  216  that has a current-mirror relation with the p-channel MOSFET  214 . The current flowing through the p-channel MOSFET  216  increases the gate voltage of the input MOSFET  124  of the differential amplifier in the reference voltage generating circuit  200 . As a result, a gate voltage of the n-channel MOSFET  132  lowers to increase the output voltage  302   a  of the reference voltage generating circuit  200 . 
     When the output voltage  302   a  of the reference voltage generating circuit  200  increases to be equal to or more than the threshold voltage of the n-channel MOSFET  213  in the startup circuit  210 , the n-channel MOSFET  213  turns on and the n-channel MOSFET  215  turns off due to the decrease in its gate voltage. Accordingly, the p-channel MOSFET  216  also turns off, so that the startup circuit  210  is disconnected from the reference voltage generating circuit  200  and the startup operation is stopped. Thereafter, by the operation of the reference voltage generating circuit  200 , the output voltage  302   a  reaches the stabilization point Vo [V]. As described above, the startup circuit  210  operates so as to increase the output voltage  302   a  when the output voltage  302   a  of the reference voltage generating circuit  200  is low, and when the output voltage  302   a  becomes sufficiently high, it operates so as to stop the startup operation. 
     The following patent document 1 also discloses in  FIG. 5  a startup circuit of a reference voltage generating circuit. 
     (Patent Document 1) Japanese Patent Application Laid-open No. 2000-181554 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a reference voltage generating circuit including a startup circuit which requires only a small number of components and whose circuit is simple and easy to design. 
     It is another object of the present invention to provide a reference voltage generating circuit including a startup circuit without any unnecessary current flowing therethrough, and thus achieving reduction in power consumption. 
     According to one of the aspects of the present invention, a reference voltage generating circuit is provided that includes: a differential amplifier amplifying an input signal and generate a reference voltage; an output amplifier amplifying the reference voltage and outputting the amplified reference voltage; and a startup circuit connected between an output terminal of the differential amplifier and an input terminal of the output amplifier. The startup circuit has a first switch and a second switch. The first switch connects the output terminal of the differential amplifier and the input terminal of the output amplifier to each other according to a voltage of an output terminal of the output amplifier. The second switch connects the input terminal and the output terminal of the output amplifier according to a voltage of the output terminal of the differential amplifier. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram showing a configuration example of a reference voltage generating circuit including a startup circuit according to an embodiment of the present invention; 
         FIG. 2  is a circuit diagram showing a configuration example of a reference voltage generating circuit and its startup circuit according to a prior art; and 
         FIG. 3A  and  FIG. 3B  are charts to explain the operation of the reference voltage generating circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The startup circuit  210  shown in  FIG. 2  uses 5 MOSFETs, which is a large number of components. Further, since the n-channel MOSFET  213  determines the voltage for starting or stopping the startup operation based on its threshold voltage, the freedom degree of the size is low, resulting in difficulty in size adjustment. Further, the n-channel MOSFET  213  and the p-channel MOSFET  212  turn on simultaneously at the normal operation time after the startup operation is stopped, so that an unnecessary current flows through the MOSFETs  212 ,  213 . 
       FIG. 1  shows a configuration example of a reference voltage generating circuit according to an embodiment of the present invention. The reference voltage generating circuit includes a startup circuit  100 . Portions except the startup circuit  100  in the reference voltage generating circuit are the same as those in the reference voltage generating circuit  200  in  FIG. 2 . 
     First, the portions except the startup circuit  100  will be described. A resistor  111  is connected between a gate of a p-channel MOS field-effect transistor (hereinafter, referred to as FET)  123  and an output terminal  140 . A resistor  112  is connected between the gate of the MOSFET  123  and an anode of a diode  113 . A cathode of the diode  113  is connected to a ground potential. A resistor  114  is connected between a gate of a p-channel MOSFET  124  and the output terminal  140 . A diode  115  has an anode connected to the gate of the MOSFET  124  and a cathode connected to the ground potential. 
     A differential amplifier  120  has a direct-current power supply  121  and MOSFETs  122  to  126 . Input terminals of the differential amplifier  120  are gates of p-channel MOSFETs  123 ,  124 . An output terminal of the differential amplifier  120  is a junction point between a drain of the p-channel MOSFET  124  and a drain of the n-channel MOSFET  126 . The direct-current power supply  121  has an anode connected to a gate of the p-channel MOSFET  122  and a cathode connected to the ground potential. The p-channel MOSFET  122  has a source connected to a power supply potential and a drain connected to a junction point between sources of the p-channel MOSFETs  123 ,  124 . Gates of the n-channel MOSFETs  125 ,  126  are connected to each other, and a junction point therebetween is connected to a drain of the MOSFET  123 . The n-channel MOSFET  125  has a drain connected to the drain of the MOSFET  123  and a source connected to the ground potential. The n-channel MOSFET  126  has a drain connected to the drain of the MOSFET  124  and a source connected to the ground potential. 
     An output amplifier  130  has MOSFETs  131 ,  132 . An input terminal of the output amplifier  130  is a gate of the n-channel MOSFET  132 . The output terminal  140  of the reference voltage generating circuit also serves as an output terminal of the output amplifier  130 . The p-channel MOSFET  131  has a gate connected to the anode of the direct-current power supply  121 , a source connected to a power supply potential, and a drain connected to the output terminal  140 . The n-channel MOSFET  132  has a gate connected to the startup circuit  100 , a drain connected to the output terminal  140 , and a source connected to the ground potential. 
     Next, the configuration of the startup circuit  100  will be described. The startup circuit  100  has n-channel MOSFETs  101 ,  102 . The n-channel MOSFET  101  has a gate connected to the output terminal  140 , a source connected to the junction point (the output terminal of the differential amplifier  120 ) between the drains of the MOSFETs  124 ,  126 , and a drain connected to the gate (the input terminal of the output amplifier  130 ) of the n-channel MOSFET  132 . The n-channel MOSFET  102  has a gate connected to the junction point (the output terminal of the differential amplifier  120 ) between the drains of the MOSFETs  124 ,  126 , a source connected to the output terminal  140 , and a drain connected to the gate (the input terminal of the output amplifier  130 ) of the n-channel MOSFET  132 . 
     Eliminating the startup circuit  100  and short-circuiting the output terminal of the differential amplifier  120  and the input terminal of the output amplifier  130  will result in the same circuit as the reference voltage generating circuit  200  in  FIG. 2 . In this case, the reference voltage generating circuit outputs one of two kinds of output voltages  302   a ,  302   b  as shown in  FIG. 3A  according to manufacturing variation or the like. It is not possible to assure which one of two kinds of the output voltages  302   a ,  302   b  is outputted. As a result, the output voltage (reference voltage) of the reference voltage generating circuit has two stabilization points 0 [V] and Vo [v]. Vo [V] is a desired stabilization point of the output voltage and 0 [V] is an undesired stabilization point of the output voltage. 
     When the startup circuit  100  is provided in the reference voltage generating circuit as shown in  FIG. 1 , the startup circuit  100  is capable of leading the output voltage of the reference voltage generating circuit to the one desired stabilization point Vo [V] out of the two stabilization points 0 [V] and Vo [V] as shown in  FIG. 3B . This can ensure that the reference voltage generating circuit outputs the desired output voltage  302   a  in accordance with the increase in the power supply voltage  301 . 
     Next, the operation of the reference voltage generating circuit including the startup circuit  100  will be described. The output voltage of this reference voltage generating circuit has two stabilization points, for example, 0 [V] and Vo=1.2 [V]. A power supply voltage Vcc is, for example, 3 [V], 2.5 [V], 1.8 [V], or the like. When the startup circuit  100  is provided, the output voltage  302   a  in  FIG. 3B  is outputted. After the power application, the power supply voltage  301  gradually increases, but the output voltage  302   a  remains 0 [V]. Since the gate of the n-channel MOSFET  132  is in a floating state, electric charges remain stagnant at this gate, so that the MOSFET  132  is sometimes on. When the MOSFET  132  is on, the output voltage  302   a  remains 0 [V]. When the output voltage  302   a  of the output terminal  140  is 0 [V], the n-channel MOSFET  101  is off. The voltage of the output terminal of the differential amplifier  120  increases and the n-channel MOSFET  102  turns on before long. Then, a gate voltage of the n-channel MOSFET  132  turns to 0 [V], so that the n-channel MOSFET  132  turns off. Thereafter, the output voltage  302   a  of the output terminal  140  increases toward 1.2 [V] that is the desired stabilization point. When the output voltage  302   a  increases, the n-channel MOSFET  101  turns on before long and the voltage of the output terminal of the differential amplifier  120  gradually lowers, so that the n-channel MOSFET  102  turns off. The MOSFET  101  turns on and the MOSFET  102  turns off, so that the startup operation is stopped, which means that the reference voltage generating circuit of this embodiment turns to the same circuit as the reference voltage generating circuit  200  in  FIG. 2 . Thereafter, the output voltage  302   a  of the reference voltage generating circuit of this embodiment reaches the stabilization point 1.2 [V] as in the operation of the reference voltage generating circuit  200  in  FIG. 2 . 
     As described above, in the startup circuit  100 , the MOSFET  101  turns off in a first period of time after the power application, and thereafter is kept on. The MOSFET  102  turns on in a second period of time after the power application and thereafter is kept off. 
     The reference voltage generating circuit of this embodiment has a two-stage amplification circuit constituted of the differential amplifier  120  to amplify an input signal and the output amplifier  130  to amplify and output the reference voltage. The startup circuit  100  is connected between the output terminal of the differential amplifier  120  and the input terminal of the output amplifier  130 , and has the first switch (MOSFET)  101  and the second switch (MOSFET)  102 . The first and second switches  101 ,  102  are, for example, n-channel MOSFETs as described above. The first switch  101  connects the output terminal of the differential amplifier  120  and the input terminal of the output amplifier  130  according to the voltage of the output terminal  140  of the output amplifier  130 . The second switch  102  connects the input terminal and the output terminal  140  of the output amplifier  130  according to the voltage of the output terminal of the differential amplifier  120 . 
     Further, since the differential amplifier  120  and the output amplifier  130  perform amplification, the MOSFETs in the differential amplifier  120  and the output amplifier  130  are relatively large in size. On the other hand, the MOSFETs  101 ,  102  in the startup circuit  100  may be smaller in size than the MOSFETs in the differential amplifier  120  and the output amplifier  130  since no large current flows therethrough. 
     The use of the startup circuit  100  for the reference voltage generating circuit makes it possible to lead the output voltage of the output amplifier  130  to the one desired stabilization point out of the plural stabilization points. Further, since the startup circuit  100  is realized by two elements, namely, the MOSFETs  101 ,  102 , the number of components thereof is small and its circuit is simple and easy to design. Moreover, downsizing is achieved since the MOSFETs  101 ,  102  may be small in size. Further, in the startup circuit  210  in  FIG. 2 , an unnecessary current flows through the MOSFETs  212 ,  213  in the normal operation after the startup operation is finished, as described above. On the other hand, such an unnecessary current does not flow in the startup circuit  100  of this embodiment, which allows reduction in power consumption. 
     The use of a startup circuit in a reference voltage generating circuit makes it possible to lead an output voltage of an output amplifier to one desired stabilization point out of a plurality of stabilization points. Further, since the startup circuit is realized by a first switch and a second switch, the number of components thereof is small and its circuit is simple and easy to design. Moreover, no unnecessary current flows in the startup circuit, which allows reduction in power consumption. 
     The present embodiments are to be considered in all respects as illustrative and no restrictive, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof.