Abstract:
Provided are a constant voltage circuit configured to, when a power supply voltage is low, detect a leakage current to output a stable voltage at a power supply voltage level, and a crystal oscillation circuit using the constant voltage circuit. The constant voltage circuit includes a leakage current detection circuit including a PMOS transistor for monitoring a leakage current, which has a gate and a source being grounded. When a leakage current is detected, even with a constant voltage power supply, a voltage sufficient for turning on an output transistor of the constant voltage circuit can be applied to a gate of the output transistor.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2015-022427 filed on Feb. 6, 2015, 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 an oscillation device including a crystal oscillation circuit, and more particularly, to a constant voltage circuit configured to detect a leakage current when a power supply voltage is low to enable a stable voltage output at a power supply voltage level. 
         [0004]    2. Description of the Related Art 
         [0005]      FIG. 5  is a block diagram of a related-art oscillation device  100  that is widely used in a clock, electronic equipment, and the like. The related-art oscillation device  100  includes a constant voltage circuit  10  configured to generate a constant output voltage VREG from an input voltage, and a crystal oscillation circuit  20  configured to oscillate a crystal unit XTAL with the generated constant voltage VREG. Note that, in the oscillation device  100 , a ground potential is denoted as VDD and a power supply voltage is denoted as VSS. 
         [0006]    In the oscillation device  100 , in order to reduce a current consumption, it is important to reduce a voltage for driving the crystal oscillation circuit  20  as much as possible. Therefore, the constant voltage circuit  10  is formed so as to output a predetermined constant voltage VREG even when the power supply voltage is equal to or higher than a predetermined voltage. On the other hand, the crystal oscillation circuit  20  has an oscillation stop voltage VDOS that is determined by oscillation characteristics of the crystal unit XTAL, an oscillation inverter, a load capacitance, and the like. Therefore, in the constant voltage circuit  10 , it is necessary that an absolute value |VREG| of the constant voltage VREG be larger than an absolute value |VDOS| of the oscillation stop voltage VDOS. 
         [0007]      FIG. 6  is a circuit diagram for illustrating the constant voltage circuit  10  of the related-art oscillation device  100 . The constant voltage circuit  10  includes a reference voltage circuit  101 , a differential amplifier circuit  102 , and an output circuit  103 . 
         [0008]    In the reference voltage circuit  101 , a constant current IREF flows to a PMOS transistor MP 1  from a depletion type NMOS transistor MD 1  as a constant current source to generate a reference voltage VREF. In the differential amplifier circuit  102 , the reference voltage VREF is input to an inverting input terminal, and a feedback voltage FB is input to a non-inverting input terminal. The differential amplifier circuit  102  controls a gate voltage of an output transistor MN 5  connected to an output terminal NO 2  so that the reference voltage VREF and the feedback voltage FB are equal to each other. Therefore, the absolute value |VREG| of the constant voltage output of the constant voltage circuit  10  is a sum of an absolute value |VREF| of the reference voltage and a gate-source voltage Vgs of an NMOS transistor MN 6 . 
         [0009]    When the power supply voltage is low, a voltage at the ground potential VDD level is applied to a gate of the output transistor MN 5 , and thus, the output voltage VREG of the related-art constant voltage circuit  10  is equal to the power supply voltage VSS (see, for example, Japanese Patent Application Laid-open No. 2001-312320). 
         [0010]    However, when a threshold voltage of the MOS transistor is lower than a predetermined value due to a high temperature, manufacture variations, and the like, and when a leakage current of the MOS transistor increases, a drain-source voltage Vds of the PMOS transistor MP 1  becomes lower, and a gate-source voltage Vgs of a PMOS transistor MP 3  cannot be secured. Further, when the ground potential VDD cannot be sufficiently applied to a gate of an output transistor MN 5 , or, when the ground potential VDD cannot be sufficiently applied to the gate of the output transistor MN 5  due to a leakage current through an NMOS transistor MN 3 , a gate-source voltage Vgs of the output transistor MN 5  cannot be secured and the output transistor MN 5  is turned off. It follows that the relationship between the absolute value |VREG| of the constant voltage and an absolute value |VSS| of the power supply voltage is not |VREG|=|VSS| but |VREG|&lt;|VSS|. When |VREG| is smaller than an absolute value |VDOS| of an oscillation stop voltage, the crystal oscillation circuit  20  cannot operate. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention has been made in view of the problem described above, and an object of the present invention is to provide a constant voltage circuit that is not affected by a leakage current and that enables a stable voltage output at a power supply voltage level when a power supply voltage is low, and to provide a crystal oscillation circuit using the constant voltage circuit. 
         [0012]    In order to solve the above-mentioned problem, according to one embodiment of the present invention, the following constant voltage circuit is constructed. 
         [0013]    There is provided a constant voltage circuit, including: a differential amplifier circuit to which a reference voltage and a feedback voltage are input; an output transistor configured to output a constant voltage to an output terminal of the constant voltage circuit using an output voltage of the differential amplifier circuit; and a leakage current detection circuit configured to detect a leakage current of a transistor, in which, when the leakage current detection circuit detects that the leakage current exceeds a predetermined value, the leakage current detection circuit increases a gate-source voltage of the output transistor. 
         [0014]    According to the one embodiment of the present invention, with the constant voltage circuit, when the leakage current of the transistor exceeds the predetermined value, the leakage current detection circuit may apply a voltage sufficient for turning on the output transistor to a gate thereof. Therefore, a stable voltage at a power supply voltage level may be output. The present invention is particularly effective in a process in which a threshold voltage of a MOS transistor is decreased for the purpose of reducing a constant voltage output to reduce a current consumption, or in an oscillation circuit in which a channel length of a MOS transistor is reduced for the purpose of reducing a chip area of an IC. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a circuit diagram for illustrating a constant voltage circuit according to a first embodiment of the present invention. 
           [0016]      FIG. 2  is a circuit diagram for illustrating a leakage current detection circuit according to the first embodiment. 
           [0017]      FIG. 3  is a circuit diagram for illustrating an inner structure of a constant voltage circuit according to a second embodiment of the present invention. 
           [0018]      FIG. 4  is a circuit diagram for illustrating a leakage current detection circuit according to the second embodiment. 
           [0019]      FIG. 5  is a schematic diagram for illustrating an oscillation device in which a constant voltage circuit is used of the present invention. 
           [0020]      FIG. 6  is a circuit diagram for illustrating a constant voltage circuit of a related-art oscillation device. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]      FIG. 1  is a circuit diagram for illustrating a constant voltage circuit  10  according to a first embodiment of the present invention. The constant voltage circuit  10  includes a reference voltage circuit  101 , a differential amplifier circuit  102 , an output circuit  103 , and a leakage current detection circuit  30 . 
         [0022]    The reference voltage circuit  101  includes a depletion type NMOS transistor MD 1 , NMOS transistors MN 1  and MN 2 , and a PMOS transistor MP 1 . The depletion type NMOS transistor MD 1  operates as a constant current source. The NMOS transistor MN 1  and the NMOS transistor MN 2  are current mirror connected, and a constant current IREF also flows through the NMOS transistor MN 2 . A gate and a drain of the PMOS transistor MP 1  are connected to each other, and a source of the PMOS transistor MP 1  is grounded. Therefore, in the reference voltage circuit  101 , the constant current IREF flows through the PMOS transistor MP 1  from the depletion type NMOS transistor MD 1  as the constant current source to generate a reference voltage VREF. 
         [0023]    The differential amplifier circuit  102  includes a PMOS transistor MP 2 , PMOS transistors MP 3  and MP 4  that form a differential pair, and NMOS transistors MN 3  and MN 4  that form a current mirror. The constant current IREF flows through the PMOS transistor MP 2  that forms a current mirror with the PMOS transistor MP 1  as an operating current. 
         [0024]    The output circuit  103  includes a PMOS transistor MP 5  and output transistors MN 5  and MN 6 . A gate of the output transistor MN 5  is connected to a drain of the PMOS transistor MP 3  as an output NO 3  of the differential amplifier circuit  102 , a source of the output transistor MN 5  is connected to the power supply voltage VSS, and a drain of the output transistor MN 5  is connected to an output VREG of the constant voltage circuit  10 . A gate of the NMOS transistor MN 6  is connected to a drain thereof to be an input to the PMOS transistor MP 4  as a non-inverting input terminal of the differential amplifier circuit  102 , and a source of the NMOS transistor MN 6  is connected to the output VREG of the constant voltage circuit  10 . A gate of the PMOS transistor MP 5  is connected to the gate and the drain of the PMOS transistor MP 1 , and a source of the PMOS transistor MP 5  is grounded. The constant current IREF flows through the PMOS transistor MP 5  that forms a current mirror with the PMOS transistor MP 1 . 
         [0025]    The reference voltage VREF is input to a gate of the PMOS transistor MP 3  as an inverting input terminal in the differential pair in the differential amplifier circuit  102 . A drain voltage of the PMOS transistor MP 5 , that is, a feedback voltage FB, is input to a gate of the PMOS transistor MP 4  as the non-inverting input terminal. The output NO 3  of the differential amplifier circuit  102  is input to the gate of the output transistor MN 5 , and thus, the gate potential NO 3  of the output transistor MN 5  is controlled so that the reference voltage VREF and the feedback voltage FB finally become equal to each other. Therefore, an absolute value |VREG| of the constant voltage output of the constant voltage circuit  10  is a sum of an absolute value (VREF) of the reference voltage and a gate-source voltage Vgs of the NMOS transistor MN 6 , which is expressed as |VREG|=|VREF|+Vgs=α|Vtp|+βVtn, where Vtp is a threshold voltage of the PMOS transistor, Vtn is a threshold voltage of the NMOS transistor, and α and β are predetermined constants. On the other hand, when the power supply voltage is low in a normal state in which a leakage current does not flow, the PMOS transistors MP 2  and MP 3  can secure sufficient gate-source voltages Vgs to be turned on, and thus, the gate potential of the output transistor MN 5  is a ground potential VDD. A gate-source voltage Vgs of the output transistor MN 5  can be secured to turn on the output transistor MN 5 . Thus, |VREG|=|VSS| is achieved. 
         [0026]    The leakage current detection circuit  30  is connected between the reference voltage VREF and the output NO 3  of the differential amplifier circuit  102 .  FIG. 2  is a circuit diagram for illustrating the leakage current detection circuit  30 . 
         [0027]    The leakage current detection circuit  30  includes a PMOS transistor MPL 1  for monitoring a leakage current, PMOS transistors MP 6 , MP 7 , and MP 8 , and NMOS transistors MN 7  and MN 8 . A gate and a source of the PMOS transistor MPL 1  for monitoring a leakage current are grounded, and a drain of the PMOS transistor MPL 1  for monitoring a leakage current is connected to a source of the PMOS transistor MP 8 . A gate of the PMOS transistor MP 8  is connected to the reference voltage VREF, and a drain of the PMOS transistor MP 8  is connected to a drain of the NMOS transistor MN 8 . A gate of the NMOS transistor MN 8  is connected to the reference voltage VREF, and a source of the NMOS transistor MN 8  is connected to the power supply voltage VSS. A gate of the PMOS transistor MP 7  is connected to the reference voltage VREF, a source of the PMOS transistor MP 7  is grounded, and a drain of the PMOS transistor MP 7  is connected to a drain of the NMOS transistor MN 7 . A gate of the NMOS transistor MN 7  is connected to the drain of the PMOS transistor MP 8  and the drain of the NMOS transistor MN 8 , and a source of the NMOS transistor MN 7  is connected to the power supply voltage VSS. A gate of the PMOS transistor MP 6  is connected to the drain of the PMOS transistor MP 7  and the drain of the NMOS transistor MN 7 , a source of the PMOS transistor MP 6  is grounded, and a drain of the PMOS transistor MP 6  is connected to the gate NO 3  of the output transistor MN 5  of the constant voltage circuit  10 . 
         [0028]    Operation of the constant voltage circuit  10  of the oscillation device  100  according to the first embodiment is now described. 
         [0029]    The gate and the source of the PMOS transistor MPL 1  for monitoring a leakage current of the leakage current detection circuit  30  are grounded, and thus, the PMOS transistor MPL 1  for monitoring a leakage current is normally off. Further, the PMOS transistor MPL 1  for monitoring a leakage current appropriately adjusts an element size in accordance with a predetermined amount of a leakage current. The predetermined amount of a leakage current herein refers to a condition with which, when the threshold voltage of the MOS transistor is lower than a predetermined value due to an operating environment at a high temperature, manufacture variations, and the like, and when the power supply voltage is low, a drain-source voltage Vds of the PMOS transistor MP 1  of the constant voltage circuit  10  becomes lower and a gate-source voltage Vgs of the PMOS transistor MP 3  cannot be secured. 
         [0030]    When the leakage current exceeds the predetermined amount, a leakage current flows through the PMOS transistor MPL 1  for monitoring a leakage current. The gate potential of the PMOS transistor MP 8  is the reference voltage VREF, and thus, the flow of the leakage current through the PMOS transistor MPL 1  for monitoring a leakage current enables a flow of a smaller one of the leakage current and the constant current IREF. 
         [0031]    On the other hand, the NMOS transistor MN 8  has, similarly to the PMOS transistor MP 8 , a gate potential that is the reference voltage VREF and a source potential that is the power supply voltage VSS. In other words, both the PMOS transistor MP 8  and the NMOS transistor MN 8  are on. Therefore, the transistor that has a higher current driving capability exceeds the other transistor, and applies the potential thereof to the gate of the NMOS transistor MN 7  of the following stage. When the power supply voltage is low, it is difficult to sufficiently secure a gate-source voltage Vgs of the NMOS transistor MN 8 , and thus, a current driving capability of the PMOS transistor MP 8  exceeds that of the NMOS transistor MN 8 , and the PMOS transistor MP 8  applies the ground potential VDD to the gate of the NMOS transistor MN 7  of the following stage. 
         [0032]    On the other hand, the PMOS transistor MP 7  has a gate potential that is the reference voltage VREF and a source potential that is the ground potential VDD, and thus, causes the constant current IREF to flow therethrough. In other words, both the PMOS transistor MP 7  and the NMOS transistor MN 7  are on. When the power supply voltage is low, through adjustment of the element size so that the NMOS transistor MN 7  may have a higher current driving capability than the PMOS transistor MP 7 , the power supply voltage VSS is applied to the gate of the PMOS transistor MP 6  of the following stage. 
         [0033]    Therefore, when the power supply voltage is low, and when the leakage current exceeds the predetermined amount, the PMOS transistor MP 6  is turned on, the gate potential NO 3  of the output transistor MN 5  becomes the ground potential VDD, and the gate-source voltage Vgs of the output transistor MN 5  can be secured to turn on the output transistor MN 5 . Thus, |VREG|=|VSS| is achieved. 
         [0034]    When a leakage current exceeding the predetermined amount does not flow, the PMOS transistor MPL 1  for monitoring a leakage current is off, and thus, even when the PMOS transistor MP 8  is on, a current cannot flow. The source potential of the NMOS transistor MN 8  is the power supply voltage VSS, and thus, the on state of the NMOS transistor MN 8  applies the power supply voltage VSS to the gate of the NMOS transistor MN 7  of the following stage. Next, the gate potential of the NMOS transistor MN 7  is the power supply voltage VSS, and thus, the NMOS transistor MN 7  is off. The gate potential of the PMOS transistor MP 7  is the reference voltage VREF, and the source potential of the PMOS transistor MP 7  is the ground potential VDD, and thus, the PMOS transistor MP 7  is on. Therefore, the ground potential VDD is applied to the gate of the PMOS transistor MP 6  of the following stage. 
         [0035]    Therefore, when a leakage current exceeding the predetermined amount does not flow, the PMOS transistor MP 6  is off, and thus, the leakage current detection circuit  30  does not operate and does not affect the operation of the constant voltage circuit  10 . Further, the leakage current detection circuit  30  does not have a path to cause a current to flow therethrough when not operated, and thus, a current consumption of the constant voltage circuit  10  that is realized according to the present invention does not increase compared with that of the related-art constant voltage circuit  10 . 
         [0036]    Next, the constant voltage circuit  10  according to a second embodiment of the present invention is described. 
         [0037]      FIG. 3  is a circuit diagram for illustrating the constant voltage circuit  10  according to the second embodiment. The constant voltage circuit  10  includes the reference voltage circuit  101 , the differential amplifier circuit  102 , the output circuit  103 , and a leakage current detection circuit  40 . 
         [0038]    The leakage current detection circuit  40  is connected between a reference voltage NO 1  as a source potential of the depletion type NMOS transistor MD 1  forming a constant current source, and the output NO 3  of the differential amplifier circuit  102 .  FIG. 4  is a circuit diagram for illustrating the leakage current detection circuit  40 . 
         [0039]    The leakage current detection circuit  40  includes an NMOS transistor MNL 1  for monitoring a leakage current, PMOS transistors MP 12 , MP 9 , MP 10 , and MP 11 , and NMOS transistors MN 9 , MN 10 , and MN 11 . A gate and a source of the NMOS transistor MNL 1  for monitoring a leakage current are connected to the power supply voltage VSS, and a drain of the NMOS transistor MNL 1  for monitoring a leakage current is connected to a source of the NMOS transistor MN 11 . A gate of the NMOS transistor MN 11  is connected to the reference voltage NO 1  and a drain of the NMOS transistor MN 11  is connected to a drain of the PMOS transistor MP 11 . A gate of the PMOS transistor MP 11  is connected to the reference voltage NO 1  and a source of the PMOS transistor MP 11  is grounded. A gate of the NMOS transistor MN 10  is connected to the reference voltage NO 1  a source of the NMOS transistor MN 10  is connected to the power supply voltage VSS, and a drain of the NMOS transistor MN 10  is connected to a drain of the PMOS transistor MP 10 . A gate of the PMOS transistor MP 10  is connected to the drain of the PMOS transistor MP 11  and the drain of the NMOS transistor MN 11 , and a source of the PMOS transistor MP 10  is grounded. A gate of the NMOS transistor MN 9  is connected to the drain of the PMOS transistor MP 10  and the drain of the NMOS transistor MN 10 , a source of the NMOS transistor MN 9  is connected to the power supply voltage VSS, and a drain of the NMOS transistor MN 9  is connected to a drain of the PMOS transistor MP 9 . A gate of the PMOS transistor MP 9  is connected to the drain of the PMOS transistor MP 10  and the drain of the NMOS transistor MN 10 , and a source of the PMOS transistor MP 9  is grounded. A gate of the PMOS transistor MP 12  is connected to the drain of the PMOS transistor MP 9  and the drain of the NMOS transistor MN 9 , a source of the PMOS transistor MP 12  is grounded, and a drain of the PMOS transistor MP 12  is connected to the gate NO 3  of the output transistor MN 5  of the constant voltage circuit  10 . 
         [0040]    Operation of the constant voltage circuit  10  of the oscillation device  100  according to the second embodiment is now described. 
         [0041]    The gate and the source of the NMOS transistor MNL 1  for monitoring a leakage current of the leakage current detection circuit  40  are connected to the power supply voltage VSS, and thus, the NMOS transistor MNL 1  for monitoring a leakage current is normally off. Further, the NMOS transistor MNL 1  for monitoring a leakage current appropriately adjusts an element size in accordance with a predetermined amount of a leakage current. The predetermined amount of a leakage current herein refers to a condition with which, when the threshold voltage of the MOS transistor is lower than a predetermined value due to an operating environment at a high temperature, manufacture variations, and the like, and when the power supply voltage is low, a flow of the leakage current through the NMOS transistor MN 3  of the constant voltage circuit  10  pulls the gate potential of the output transistor MN 5  to the power supply voltage VSS side. 
         [0042]    When the leakage current exceeds the predetermined amount, a leakage current flows through the NMOS transistor MNL 1  for monitoring a leakage current. The gate potential of the NMOS transistor MN 11  is the reference voltage NO 1 , and thus, the flow of the leakage current through the NMOS transistor MNL 1  for monitoring a leakage current enables a flow of a smaller one of the leakage current and the constant current IREF. 
         [0043]    On the other hand, the PMOS transistor MP 11  has, similarly to the NMOS transistor MN 11 , a gate potential that is the reference voltage NO 1  and a source potential that is the ground potential VDD. In other words, both the NMOS transistor MN 11  and the PMOS transistor MP 11  are on. Therefore, the transistor that has a higher current driving capability exceeds the other transistor, and applies the potential thereof to the gate of the PMOS transistor MP 10  of the following stage. When the power supply voltage is low, it is difficult to sufficiently secure a gate-source voltage Vgs of the PMOS transistor MP 11 , and thus, a current driving capability of the NMOS transistor MN 11  exceeds that of the PMOS transistor MP 11 , and the NMOS transistor MN 11  applies the power supply voltage VSS to the gate of the PMOS transistor MP 10  of the following stage. 
         [0044]    On the other hand, the NMOS transistor MN 10  has a gate potential that is the reference voltage NO 1  and a source potential that is the power supply voltage VSS, and thus, causes the constant current IREF to flow therethrough. In other words, both the NMOS transistor MN 10  and the PMOS transistor MP 10  are on. When the power supply voltage is low, through adjustment of the element size so that the PMOS transistor MP 10  may have a higher current driving capability than the NMOS transistor MN 10 , the ground potential VDD is applied to the gate of the PMOS transistor MP 9  and the NMOS transistor MN 9  of the following stage. The NMOS transistor MN 9  is on, and thus, the power supply voltage VSS is applied to the gate of the PMOS transistor MP 12  of the following stage. 
         [0045]    Therefore, when the power supply voltage is low, and when the leakage current exceeds the predetermined amount, the PMOS transistor MP 12  is turned on, the gate potential NO 3  of the output transistor MN 5  becomes the ground potential VDD, and the gate-source voltage Vgs of the output transistor MN 5  can be secured to turn on the output transistor MN 5 . Thus, IVREGHVSS 1  is achieved. 
         [0046]    When a leakage current exceeding the predetermined amount does not flow, the NMOS transistor MNL 1  for monitoring a leakage current is off, and thus, even when the NMOS transistor MN 11  is on, a current cannot flow. The source potential of the PMOS transistor MP 11  is the ground potential VDD, and thus, the on state of the PMOS transistor MP 11  applies the ground potential VDD to the gate of the PMOS transistor MP 10  of the following stage. Next, the gate potential of the PMOS transistor MP 10  is the ground potential VDD, and thus, the PMOS transistor MP 10  is off. The gate potential of the NMOS transistor MN 10  is the reference voltage NO 1 , and the source potential of the NMOS transistor MN 10  is the power supply voltage VSS, and thus, the NMOS transistor MN 10  is on. Therefore, the power supply voltage VSS is applied to the gate of the PMOS transistor MP 9  and the gate of the NMOS transistor MN 9  of the following stage. The PMOS transistor MP 9  is on, and thus, the ground potential VDD is applied to the gate of the PMOS transistor MP 12  of the following stage. 
         [0047]    Therefore, when a leakage current exceeding the predetermined amount does not flow, the PMOS transistor MP 12  is off, and thus, the leakage current detection circuit  40  does not operate and does not affect the operation of the constant voltage circuit  10 . Further, the leakage current detection circuit  40  does not have a path to cause a current to flow therethrough when not operated, and thus, a current consumption of the constant voltage circuit  10  that is realized according to the present invention does not increase compared with that of the related-art constant voltage circuit  10 . 
         [0048]    Embodiments of the present invention are described above, but the present invention is not limited to those embodiments and to oscillation devices, and the present invention can be implemented in various modes that fall within the gist thereof.