Abstract:
To provide a crystal oscillation circuit low in current consumption and stably short in oscillation start time. A crystal oscillation circuit is equipped with a crystal vibrator, a feedback resistor, a bias circuit, a constant voltage circuit, and an oscillation inverter configured by a constant current inverter. The oscillation inverter is configured so as to be controlled by currents based on input signals from the bias circuit and the crystal vibrator and driven by an output voltage of the constant voltage circuit.

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2014-179420 filed on Sep. 3, 2014, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a crystal oscillation circuit which is low in current consumption and stably short in oscillation start time. 
     Background Art 
     As a crystal oscillation circuit used in an electronic timepiece or the like, there has been known such a configuration as shown in Patent Document 1.  FIG. 5  is illustrated in a range not departing from the crystal oscillation circuit shown in Patent Document 1. 
     The crystal oscillation circuit  109  is comprised of PMOS transistors P 31  and P 32 , NMOS transistors N 31  and N 32 , capacitors C 1 , C 2 , C 3  and C 4 , a feedback resistor  29 , a constant current source  49 , a constant voltage circuit  19 , and a crystal vibrator  69 . 
     An oscillation inverter configured by the PMOS transistor P 31  and the NMOS transistor N 31  is controlled by a current I 9  which allows an operating current to flow through the constant current source  49 . Thus, the crystal oscillation circuit is capable of reducing current consumption by reducing the current I 9 . Further, an amplitude limiter circuit comprised of the PMOS transistor P 32  and the NMOS transistor N 32  is capable of reducing current consumption of the crystal oscillation circuit by limiting the amplitude of a terminal XOUT. Furthermore, it is possible to reduce the current consumption of the crystal oscillation circuit by driving the crystal oscillation circuit by a constant voltage VREG outputted from the constant voltage circuit  19 . Besides, the crystal oscillation circuit has also an effect in that an oscillation start time is made quick by the amplitude limiter circuit. 
     [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-134347 
     SUMMARY OF THE INVENTION 
     The related art crystal oscillation circuit is however accompanied by the following problems. 
     There is a possibility that when the current I 9  is made small, the crystal oscillation circuit will not be capable of oscillating. Further, there is a possibility that since the cutoff frequency of a high pass filter parasitically configured by the capacitor C 2  and the constant current source  49  increases when the current I 9  is made large, the crystal oscillation circuit will not be capable of oscillating. Therefore, the current I 9  needed to be optimized. Further, the crystal oscillation circuit was accompanied by a drawback that when the current I 9  varied, the oscillation start time became long. 
     In order to solve the related art problems, a crystal oscillation circuit of the present invention is configured as follows: 
     The crystal oscillation circuit is equipped with a crystal vibrator, a feedback resistor, a bias circuit, a constant voltage circuit, and an oscillation inverter configured by a constant current inverter. The oscillation inverter is controlled by currents based on input signals from the bias circuit and the crystal vibrator and driven by an output voltage of the constant voltage circuit. 
     According to the crystal oscillation circuit of the present invention, an advantageous effect is brought about in that the crystal oscillation circuit is low in current consumption and stably short in oscillation start time even if variations in process occur. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram illustrating a crystal oscillation circuit of the present embodiment; 
         FIG. 2  is a circuit diagram illustrating one example of a constant voltage circuit of the crystal oscillation circuit of the present embodiment; 
         FIG. 3  is a diagram illustrating the operation of the crystal oscillation circuit of the present embodiment; 
         FIG. 4  is a circuit diagram illustrating another example of the constant voltage circuit of the crystal oscillation circuit of the present embodiment; and 
         FIG. 5  is a circuit diagram illustrating a related art crystal oscillation circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present embodiment will hereinafter be described with reference to the accompanying drawings. 
       FIG. 1  is a circuit diagram illustrating a crystal oscillation circuit of the present embodiment. 
     The crystal oscillation circuit  100  is comprised of PMOS transistors P 1  and P 2 , NMOS transistors N 1  and N 2 , capacitors CP, CN, CC, CG and CD, a feedback resistor  20 , resistors RP and RN, a bias circuit  50 , a constant voltage circuit  10 , and a crystal vibrator  60 . The bias circuit  50  is comprised of constant current sources  40  and  41 , PMOS transistors P 3  and P 4 , and NMOS transistors N 3  and N 4 . 
       FIG. 2  is a circuit diagram illustrating one example of the constant voltage circuit of the crystal oscillation circuit of the present embodiment. 
     The constant voltage circuit  10  is comprised of a constant current source  42 , PMOS transistors P 11 , P 12 , and P 13 , NMOS transistors N 10 , N 11 , N 12 , and N 13 , and a differential amplifier circuit  30 . 
     A description will be made about connections of the crystal oscillation circuit of the present embodiment. 
     The PMOS transistor P 2  has a source connected to a drain of the PMOS transistor P 1 , a gate connected to a node VG, and a drain connected to a node XOUT. The PMOS transistor P 1  has a source connected to a power supply VDD and a gate connected to a node VP 1 . The NMOS transistor N 2  has a source connected to a drain of the NMOS transistor N 1 , a gate connected to the node VG, and a drain connected to the node XOUT. The NMOS transistor N 1  has a source connected to an output terminal (node VREG) of the constant voltage circuit  10  and a gate connected to a node VN 1 . The feedback resistor  20  has one end connected to the node VG and the other end connected to the node XOUT. The capacitor CC has one end connected to a node XIN and the other end connected to the node VG. The capacitor CP has one end connected to the node XIN and the other end connected to the node VP 1 . The capacitor CN has one end connected to the node XIN and the other end connected to the node VN 1 . The capacitor CG has one end connected to the node XIN and the other end connected to the power supply VDD. The capacitor CD has one end connected to the node XOUT and the other end connected to the power supply VDD. The resistor RP has one end connected to the node VP 1  and the other end connected to an output terminal (node VP 0 ) of the bias circuit  50 . The resistor RN has one end connected to the node VN 1  and the other end connected to an output terminal (node VN 0 ) of the bias circuit  50 . The crystal vibrator  60  has one end connected to the node XIN and the other end connected to the node XOUT. 
     A description will be made about connections of the bias circuit  50 . 
     The constant current source  40  has one end connected to the power supply VDD and the other end connected to the node VN 0 . The constant current source  41  has one end connected to the power supply VDD and the other end connected to a source of the PMOS transistor P 4 . The PMOS transistor P 4  has a drain connected to the node VN 0  and a gate to which a signal S 1  is inputted. The NMOS transistor N 3  has a source connected to the node VREG and a gate and drain connected to the node VN 0 . The NMOS transistor N 4  has a source connected to the node VREG, a gate connected to the node VN 0 , and a drain connected to the node VP 0 . The PMOS transistor P 3  has a source connected to the power supply VDD and a gate and drain connected to the node VP 0 . 
     A description will be made about connections of the constant voltage circuit  10 . 
     The constant current source  42  has one end connected to the power supply VDD and the other end connected to a gate and drain of the NMOS transistor N 10 . The NMOS transistor N 10  has a source connected to a power supply VSS. The NMOS transistor N 11  has a source connected to the power supply VSS, a gate connected to the gate of the NMOS transistor N 10  and a drain connected to a node VP 3 . The PMOS transistor P 11  has a source connected to the power supply VDD and a gate and drain connected to the node VP 3 . The PMOS transistor P 12  has a source connected to the power supply VDD, a gate connected to the node VP 3 , and a drain connected to a node VN 3 . The NMOS transistor N 12  has a source connected to the node VREG and a gate and drain connected to the node VN 3 . The NMOS transistor N 13  has a source connected to the power supply VSS, a gate connected to an output terminal (node VOUT) of the differential amplifier circuit  30 , and a drain connected to the node VREG. The PMOS transistor P 13  has a source connected to the power supply VDD, a drain connected to the node VOUT, and a gate to which the signal S 1  is inputted. 
     The PMOS transistors P 1  and P 2  and the NMOS transistors N 1  and N 2  configure an oscillation inverter. The current which flows through the oscillation inverter is a drive current. 
     The bias circuit  50  outputs voltages VP 0  and VN 0  for determining gate voltages of the PMOS transistor P 1  and the NMOS transistor N 1 . Further, for example, the PMOS transistors P 3  and P 4  and the NMOS transistors N 3  and N 4  are respectively configured in the same size, and a current I 1  of the constant current source  40  flows through each transistor. The current flowing through each transistor is however not limited to the current I 1  in particular, but may be set as appropriate to satisfy such functions as described below. 
     The constant voltage circuit  10  outputs a voltage VREG with the power supply VDD as the reference from the output terminal at normal operation and outputs the voltage of the power supply VSS at the start of oscillation. The voltage VREG is a voltage proportional to the sum of threshold voltages VTH of the PMOS transistor P 11  and the NMOS transistor N 12  by the function of the differential amplifier circuit  30 . The constant voltage circuit  10  shown in  FIG. 2  is one example, but not limited to it if there is provided a circuit which outputs such a voltage VREG as described above. 
     A description will be made about the operation of the crystal oscillation circuit of the present embodiment configured as described above. 
     [At Normal Operation] 
       FIG. 3  is a diagram illustrating the operation of the crystal oscillation circuit of the present embodiment. 
     The signal S 1  is at a High level at normal operation. Since the PMOS transistor P 13  is turned OFF, the constant voltage circuit  10  outputs the voltage VREG to the output terminal thereof. Thus, the voltages of the nodes XIN and XOUT of the crystal oscillation circuit  100  vibrate about a voltage VREG/2. Since the signal S 1  is at the High level, the PMOS transistor P 4  of the bias circuit  50  is OFF. Therefore, the node VN 0  assumes a voltage determined by the current I 1  of the constant current source  40  and a threshold voltage VTH of the NMOS transistor N 3 . The node VN 1  is connected to the node VN 0  through the resistor RN and coupled to the node XIN by the capacitor CN. Therefore, the voltage of the node VN 1  vibrates in the same phase as the node XIN about the voltage of the node VN 0 . Likewise, the node VP 0  assumes a voltage determined by the current I 1  of the constant current source  40  and a threshold voltage VTH of the PMOS transistor P 3 . Since the node VP 1  is connected to the node VP 0  through the resistor RP and coupled to the node XIN by the capacitor CP, the voltage of the node VP 1  vibrates in the same phase as the node XIN about the voltage of the node VP 0 . 
     When the voltage of the node XOUT is closest to the voltage of the power supply VDD, i.e., the PMOS transistor P 2  is ON, the voltage of the node VP 1  is lower than the voltage of the node VP 0 . Thus, the current made to flow by the PMOS transistor P 1  becomes more than the current I 1 . Further, since the voltage of the node VN 1  is also lower than the voltage of the node VN 0 , the current made to flow by the NMOS transistor N 1  is smaller than the current I 1 . 
     Further, when the voltage of the node XOUT is closest to the voltage VREG, i.e., the NMOS transistor N 2  is ON, the voltage of the node VN 1  is higher than the voltage of the node VN 0 . Thus, the current made to flow by the NMOS transistor N 1  becomes more than the current I 1 . Further, since the voltage of the node VP 1  is also higher than the voltage of the node VP 0 , the current made to flow by the PMOS transistor P 1  becomes smaller than the current I 1 . 
     Thus, it is possible to reduce a through current while optimally operating the oscillation inverter configured by the PMOS transistors P 1  and P 2  and the NMOS transistors N 1  and N 2  as a constant current inverter. Further, since the current I 1  can be minimized, current consumption of the bias circuit  50  can also be reduced, thus making it possible to reduce current consumption of the crystal oscillation circuit. 
     Further, since the voltage of the node XOUT which serves as the output of the oscillation inverter vibrates about the voltage VREG/2, the current to charge or discharge the capacitor CD and the crystal vibrator  60  depends on the voltage VREG. Thus, the charging and discharging current is minimized by reducing the voltage VREG to thereby enable the current consumption of the crystal oscillation circuit to be reduced. The constant voltage VREG is however required to be set so as not to fall below an oscillation stop voltage. 
     [At the Start of Oscillation] 
     At the oscillation start, the signal S 1  is set to a Low level for a preset period. 
     Since the PMOS transistor P 4  of the bias circuit  50  is turned ON when the signal S 1  is brought to the Low level, the driving current of the crystal oscillation circuit  100  becomes the sum of the current I 1  of the constant current source  40  and a current I 2  of the constant current source  41 . Increasing the driving current makes the currents flowing through the PMOS transistor P 1  and the NMOS transistor N 1  sufficiently large. Thus, the oscillation inverter is operated like a CMOS inverter configured by the PMOS transistor P 2  and the NMOS transistor N 2  without operating as the constant current inverter. Accordingly, the crystal oscillation circuit  100  enables the oscillation start time to be made quick stably. 
     Since the PMOS transistor P 13  of the constant voltage circuit  10  is turned ON when the signal S 1  becomes the Low level, the NMOS transistor N 13  is turned ON so that the voltage of the power supply VSS is outputted to the output terminal of the constant voltage circuit  10 . Thus, since the driving voltage of the oscillation inverter becomes the voltage between the power supply VDD and the power supply VSS, the oscillation start time can stably be made quick. 
     As described above, the crystal oscillation circuit  100  enables the oscillation start time to be stably made quick by increasing the driving current and voltage of the oscillation inverter at the oscillation start more than at the normal operation. Thus, since it is possible to reduce the driving current of the oscillation inverter and lower its driving voltage during the oscillation operation, current consumption can be reduced without sacrificing the oscillation start time. 
     Incidentally, although a description has been made about the configuration in which the driving current at the oscillation start is increased by the constant current source  41  and the PMOS transistor P 4 , another circuit configuration may be used. For example, the mirror ratios between the NMOS transistors N 3  and N 1  and the PMOS transistors P 1  and P 2  which respectively form current mirrors may be changed at the oscillation start. Further, at the oscillation start, the node VN 1  may be connected to the power supply VDD and the node VP 1  may be connected to the power supply VSS. 
       FIG. 4  is a circuit diagram illustrating another example of the constant voltage circuit of the crystal oscillation circuit of the present embodiment. 
     The constant voltage circuit  11  is equivalent to one in which the PMOS transistor P 13  is deleted from the constant voltage circuit  10  and an NMOS transistor N 14  and a SW 70  are further added. 
     The NMOS transistor N 14  has a source connected to a node VN 5  and a gate and drain connected to a node VN 4 . The SW 70  has one end connected to the node VN 4 , the other end connected to the node VN 5 , and a control terminal to which a signal S 1  is inputted. The SW 70  is turned ON when the signal S 1  is at a High level and turned OFF when the signal S 1  is at a Low level, for example. 
     [At Normal Operation] 
     Since the signal S 1  is at the High level at the normal operation, the SW 70  is kept ON. Thus, the constant voltage circuit  11  is set to an operation similar to the normal time of the constant voltage circuit  10 . 
     [At Oscillation Start] 
     At the start of oscillation, the signal S 1  is brought to a Low level for a preset period. 
     Since the signal S 1  is at the Low level, the SW 70  of the constant voltage circuit  11  is OFF. Thus, a voltage VREG proportional to the sum of the threshold voltages VTH of the PMOS transistor P 11 , the NMOS transistor N 12 , and the NMOS transistor N 14  is outputted to an output terminal of the constant voltage circuit  11 . Since the voltage VREG is increased by the threshold voltage VTH of the NMOS transistor N 14  as compared with the voltage at the normal operation, the oscillation start time can be made quick. 
     The crystal oscillation circuit has a possibility of making a transition to high-frequency oscillation when the driving voltage is high, but brings about an advantageous effect in that since the voltage VREG outputted from the constant voltage circuit  11  is a voltage larger by the threshold voltage VTH of the NMOS transistor N 14  than the output voltage at the normal operation, high-frequency oscillation can be prevented and the oscillation start time is made quick. 
     As described above, the crystal oscillation circuit  100  is capable of making the oscillation start time quick stably by increasing the driving current and voltage of the oscillation inverter at the oscillation start more than at the normal operation. Thus, since it is possible to reduce the driving current of the oscillation inverter and lower its driving voltage upon the normal operation, current consumption can be reduced without sacrificing the oscillation start time. Accordingly, the crystal oscillation circuit of the present invention is most suitable for an electronic timepiece or the like which requires a crystal oscillation circuit low in current consumption and stably short in oscillation start time. 
     Incidentally, the configuration of the crystal oscillation circuit of the present embodiment is one example but can be modified within the scope not departing from the scope of claims. 
     Further, the bias circuit  50  may be shared as a partial circuit of the constant voltage circuit  10 . For example, the nodes VP 0  and VN 0  of the crystal oscillation circuit  100  are respectively connected to the nodes VP 3  and VN 3  of the constant voltage circuit  10 . Being configured in this way enables a reduction in chip area. 
     Further, as the resistors RP and RN, a transmission gate, a voltage follower circuit, etc. may be used in place of their resistive elements. 
     Furthermore, although a description has been made about the case where both of the driving current and voltage are changed at the oscillation start, either of them may be changed.