Abstract:
There are provided a constant voltage circuit that outputs a stable constant voltage for an analog electronic clock, and an analog electronic clock featuring low current consumption and prolonged battery life. The constant voltage circuit has a first voltage holding circuit connected between the gate of an output transistor and an output terminal and a second voltage holding circuit connected between the gate of the output transistor and a ground terminal, and carries out control such that the second voltage holding circuit is enabled when the motor is operated.

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2013-020715 filed on Feb. 5, 2013, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a constant voltage circuit, which is capable of providing a stable constant voltage, and an analog electronic clock provided with the constant voltage circuit. 
     2. Background Art 
       FIG. 2  is a block diagram of an analog electronic clock. The analog electronic clock is comprised of a semiconductor device  81 , a crystal  80 , a battery  83 , and a motor  82 . The semiconductor device  81  is comprised of an oscillation circuit  811 , to which the crystal  80  is connected, a frequency division circuit  812 , a constant voltage circuit  810 , which outputs a constant voltage VREG for driving the oscillation circuit  811  and the frequency division circuit  812 , and an output circuit  813  which drives the motor  82 . 
     An analog electronic clock is required to minimize the frequency of replacing the battery thereof, so that the semiconductor device  81  is required to reduce current consumption. Methods that have been proposed for reducing the current consumption of the semiconductor device  81  include, for example, a method whereby to reduce the operating current of the constant voltage circuit  810  and a method whereby to intermittently operate the constant voltage circuit  810  (refer to, for example, Patent Document 1). 
       FIG. 6  is a block diagram of a conventional constant voltage circuit. The conventional constant voltage circuit includes a reference voltage circuit  22  that generates a reference voltage Vref, a differential amplifier circuit  23 , an output transistor  10 , a feedback circuit  21 , a holding circuit  40  composed of a capacitor, and a switch circuit  50 . 
     The conventional constant voltage circuit has the holding circuit  40 , which holds the gate voltage of the output transistor  10 , and reduces power consumption by intermittently operating the differential amplifier circuit  23  and the like. The operations of the differential amplifier circuit  23  and the feedback circuit  21  are interrupted by a signal Φ1 and the switch circuit  50  is turned off. At this time, the gate voltage of the output transistor  10  is held by the holding circuit  40  at a voltage before the switch circuit  50  was turned off. Unless a load current significantly varies, the constant voltage circuit is capable of outputting the constant voltage VREG. 
     [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-298523 
     However, due to the poor transient response characteristics of the differential amplifier circuit  23 , the foregoing conventional constant voltage circuit with the reduced current consumption is incapable of maintaining an output voltage in the case where a load current significantly varies. For example, when the motor of the analog electronic clock is driven and the battery voltage rapidly drops, the poor transient response characteristics of the differential amplifier circuit  23  leads to a reduced voltage between the gate and the source of the output transistor  10 , inconveniently causing the constant voltage VREG to vary. If the constant voltage VREG drops below an oscillation stop voltage VDOS of the oscillation circuit  811 , then the oscillation circuit  811  may lose its stability and stop oscillation. 
     SUMMARY OF THE INVENTION 
     The present invention has been made with a view toward solving the problems described above and provides a constant voltage circuit which consumes less current and which is capable of providing a stable constant voltage even if a battery voltage varies. 
     To solve the foregoing problems, a constant voltage circuit in accordance with the present invention includes: a first switch circuit connected between a differential amplifier circuit and a gate of an output transistor; a first holding circuit connected between the gate of the output transistor and an output terminal through a second switch circuit; and a second holding circuit connected between the gate of the output transistor and a power supply terminal through a third switch circuit, wherein the constant voltage circuit has a first operation mode in which the differential amplifier circuit and the first switch circuit are intermittently operated by a first control signal, and the second switch circuit is turned on and the third switch circuit is turned off by a second control signal, and a second operation mode in which the differential amplifier circuit and the first switch circuit are turned on by the first control signal, while the second switch circuit is turned off and the third switch circuit is turned on by the second control signal. 
     According to the present invention, a constant voltage circuit which consumes less current and performs a stable operation can be provided. This makes it possible to provide an analog electronic clock featuring a prolonged battery life. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram illustrating a constant voltage circuit according to a first embodiment; 
         FIG. 2  is a block diagram of an analog electronic clock; 
         FIG. 3  is an illustration of an operation of the analog electronic clock; 
         FIG. 4  is another illustration of the operation of the analog electronic clock; 
         FIG. 5  is a circuit diagram of a constant voltage circuit according to a second embodiment; and 
         FIG. 6  is a circuit diagram illustrating a conventional constant voltage circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following will describe embodiments of the present invention with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 2  is a block diagram of an analog electronic clock. The analog electronic clock is comprised of a semiconductor device  81 , a crystal  80 , the battery  83 , and a motor  82 . The semiconductor device  81  is comprised of an oscillation circuit  811  to which the crystal  80  is connected, a frequency division circuit  812 , a constant voltage circuit  810  that outputs a constant voltage VREG for driving the oscillation circuit  811  and the frequency division circuit  812 , and an output circuit  813  that drives the motor  82 . 
     The analog electronic clock operates on the basis of a power supply Vdd. In the following description, therefore, all circuits will be based on the power supply Vdd. 
     The oscillation circuit  811  oscillates the crystal  80 , which is external, at a stable frequency and outputs a clock signal of a fixed frequency. The frequency division circuit  812  divides the frequency of the clock signal of the oscillation circuit  811  and issues a signal of a required frequency. The output circuit  813  drives the motor  82  according to the signal from the frequency division circuit  812 . 
       FIG. 1  is a circuit diagram of the constant voltage circuit according to a first embodiment. 
     The constant voltage circuit  810  has a reference voltage circuit  22 , a differential amplifier circuit  23 , a switch circuit  50 , an output transistor  10 , a feedback circuit  21 , capacitors  40  and  60  constituting holding circuits, and switch circuits  51  and  52 . 
     The reference voltage circuit  22  generates a reference voltage Vref. The feedback circuit  21  divides a voltage VREG of the output terminal and outputs a feedback voltage VFB. The differential amplifier circuit  23  outputs a voltage V1 to the gate of the output transistor  10  such that the reference voltage Vref and the feedback voltage VFB become equal. Further, the differential amplifier circuit  23  and the feedback circuit  21  are controlled to be turned on/off by a signal Φ1. The switch circuit  50  is controlled to be turned on/off by the signal Φ1 in synchronization with the differential amplifier circuit  23 . 
     The holding circuit is comprised of the capacitor  40  connected between the gate of the output transistor  10  and the output terminal, the capacitor  60  connected between the gate of the output transistor  10  and a power supply terminal Vss, and the switch circuits  51  and  52 . When the switch circuit  50  is turned off, the holding circuit retains a voltage V1 of the gate of the output transistor  10  before the switch circuit  50  was turned off. 
     The constant voltage circuit  810  achieves reduced current consumption by intermittently operating the differential amplifier circuit  23  and the feedback circuit  21  by the signal Φ1 to reduce the operating current of the differential amplifier circuit  23 . 
     A description will now be given of the operation of the constant voltage circuit according to the present embodiment. 
     If the signal Φ1 and a signal Φ2 are both at a high level, then the constant voltage circuit  810  operates as a usual voltage regulator. The signal Φ1 is at the high level, so that the differential amplifier circuit  23  and the feedback circuit  21  operate, the switch circuit  50  is short-circuited, and the output terminal of the differential amplifier circuit  23  is connected to the gate of the output transistor  10 . The signal Φ2 is also at the high level, so that the switch circuit  51  opens, the switch circuit  52  is short-circuited, and the capacitor  60  of the holding circuit is active. 
     If the signal Φ1 and the signal Φ2 are both at a low level, then the constant voltage circuit  810  stops operating as the voltage regulator and is set to a low current consumption state. Since the signal Φ1 is at the low level, the differential amplifier circuit  23  and the feedback circuit  21  stop operating, the switch circuit  50  opens, and the output terminal of the differential amplifier circuit  23  is cut off from the gate of the output transistor  10 . The signal Φ2 is also at the low level, so that the switch circuit  51  is short-circuited, the switch circuit  52  opens, and the capacitor  40  of the holding circuit is active. 
     A description will now be given of an operation performed when the constant voltage circuit according to the present embodiment is applied to an analog electronic clock. 
       FIG. 3  is a timing chart of the operation of the analog electronic clock. A voltage VDD denotes a ground voltage, a voltage VDOS denotes an oscillation stop voltage of the oscillation circuit  811 , a voltage VREG is an output voltage of the constant voltage circuit  810 , and the voltage VSS denotes the power supply voltage. 
     The operation period of the analog electronic clock can be roughly divided into a period represented by the period from t1 to t2 and the period from t3 to t4, during which motor pulses are output (a motor operation mode), and the remaining period (a normal operation mode). The two periods are repeated at regular intervals. The signal Φ1 is a signal that controls an intermittent operation. The signal Φ1, for example, goes to the high level for stabilizing the constant voltage VREG in the motor operation mode, and continuously repeats the high level and the low level to carry out the low current consumption operation in the normal operation mode. The signal Φ2 is a signal for switching the operation mode. The signal Φ2 is controlled, for example, to be at the high level in the motor operation mode and to be at the low level in the normal operation mode. 
     In the normal operation mode, the signal Φ1 repeatedly switches between the high and the low levels at appropriate interval and duty, while the signal Φ2 remains at the low level. Hence, the constant voltage circuit  810  performs the intermittent operation. Further, the capacitor  40  of the holding circuit becomes active, thus enabling the constant voltage circuit  810  to output the constant voltage VREG even after the differential amplifier circuit  23  and the feedback circuit  21  stop their operations. 
     In the motor operation mode, the signal Φ1 and the signal Φ2 are both at the high level, and the constant voltage circuit  810  operates as the usual voltage regulator, the capacitor  60  being active. In the motor operation mode, a battery voltage |VSS| drops by a voltage value determined by the product of the load current of the motor  82  and the internal resistance of the battery  83 , and the source voltage of the output transistor  10  increases toward the VDD. At this time, the coupling operation of the capacitor  60  causes the gate voltage of the output transistor  10  to also increase toward the VDD, thus holding the voltage between the gate and the source of the output transistor  10  at a constant level. This enables the constant voltage circuit  810  to hold the constant voltage VREG at a constant level (the period of t1 to t2 in  FIG. 3 ). 
     As described above, the constant voltage circuit  810  is capable of stably outputting the constant voltage VREG also in the motor operation mode, thus permitting a stable oscillation output of the oscillation circuit  811 . 
       FIG. 4  is a detailed timing chart illustrating the switching of the operation mode. 
     When switching between the capacitor  40  and the capacitor  60  of the holding circuits, the stable operation of the constant voltage circuit is ensured only if the differential amplifier circuit  23  and the feedback circuit  21  are in operation, so that it is necessary to secure setup times tSU1 and tSU4. 
     Further, the stable operation of the constant voltage circuit is ensured only if the constant voltage circuit has securely switched the capacitor from the capacitor  40  to the capacitor  60  before the motor operation mode is started. It is necessary, therefore, to secure setup time tSU2. Similarly, it is necessary to secure setup time tSU3 when the signal Φ2 is switched from the high level to the low level. 
     Second Embodiment 
       FIG. 5  is a circuit diagram of a constant voltage circuit according to a second embodiment. Like constituent elements as those in the constant voltage circuit according to the first embodiment will be assigned like reference numerals, and the detailed descriptions thereof will be omitted. 
     The constant voltage circuit according to the second embodiment includes a precharge circuit for precharging a capacitor  60 . The precharge circuit includes a voltage follower circuit  70  and a switch circuit  53 . The voltage follower circuit  70  receives a gate voltage V1 of an output transistor  10  and outputs the gate voltage V1 to the capacitor  60 . The switch circuit  53  is short-circuited when a signal Φ2 is at a low level and opens when the signal Φ2 goes to a high level. 
     The constant voltage circuit according to the second embodiment short-circuits the switch circuit  53  to precharge the capacitor  60  to the gate voltage V1 of the output transistor  10  by the voltage follower circuit  70  in a normal operation mode. Thus, the gate voltage V1 of the output transistor  10  does not change when the signal Φ2 switches from the low level to the high level, enabling the constant voltage circuit to stably output a constant voltage VREG. 
     As described above, the constant voltage circuit according to the second embodiment is capable of outputting the constant voltage VREG that is further stable. 
     The voltage follower circuit  70  may be configured to perform an intermittent operation by an additional signal Φ3 to reduce current consumption. 
     Further, in the constant voltage circuit, a capacitor  40  may be also provided with a precharge circuit.