Power Supply Device With Voltage Dropping Means

The invention relates to a power supply device for an electronic calculator or timekeeping device using a battery as a power source, such as a desk-top type calculator and a wristwatch.

The electronic device using a battery as the power source, for example, a wristwatch, uses a lithium battery with the battery voltage (about 3 V) two times as high as the conventional one. The output voltage of the lithium battery is dropped to a proper voltage and the related electronic circuit is driven by the dropped voltage. In this way, the lifetime of the battery is prolonged and the battery replacing work is saved.

To be more specific, in this type electronic timepiece, 3 V of a lithium battery is dropped to 1.5 V, for example, by a voltage drop circuit. The dropped voltage is supplied to a logic circuit including an oscillator, a frequency divider and a time counting circuit for producing hour data. The hour data outputted from the logic circuit is visualized by a display unit using, for example, liquid crystal elements.

Generally, this type device has a lamp circuit to display the time at night and a sounding device to sound an alarm. The load circuit of those devices are directly coupled with the lithium battery as a drive power source of the logic circuit, in order to obtain more effective time display and alarm sounding.

As described above, in this type of timepiece, the logic circuit is driven by the dropped 1.5 V voltage, so that the lifetime of the battery is two times that of the battery when the logic circuit is driven by the 3 V power source not dropped, if the power loss arising from the voltage drop is not considered. In this point, this power supply system is excellent.

In this power supply system, the voltage to be applied as a drive voltage to the logic circuit is formed by dropping the voltage from the lithium battery by a voltage drop circuit. If the voltage of the lithium battery drops, the dropped voltage from the voltage drop circuit also drops, as a matter of course. This causes various troubles, such as stopping of oscillation of the oscillator and an erroneous operation in the frequency divider and the counter. When the lamp circuit is driven by a switch to energize the lamp, a large amount of current flows therethrough to considerably drop the output voltage of the lithium power source, resulting in the above-mentioned troubles. A great voltage drop also occurs when the sounding device such as a buzzer is driven.

The power consumption by the voltage drop circuit is utterly useless. The voltage drop oscillator for dropping the voltage of the battery is indispensable for the power supply system as mentioned above. The voltage drop by this circuit is not negligible.

Accordingly, an object of the invention is to provide a power supply system which can drop voltage with low power consumption and prevent the logic circuit from stopping or erroneously operating when the power source voltage drops.

To achieve the above object, there is provided a power supply device with voltage dropping means comprising a battery power source, voltage dropping means for dropping the output voltage of the battery power source, logic circuit means to which a dropped voltage outputted from the voltage dropping means is applied as a drive voltage, and supplied voltage control means which supplies the output voltage of the battery power source to the logic circuit means when the output voltage of the battery power source drops.

With such a construction, the power supply device of the present invention drives the electronic device with low power consumption. Further, when the battery voltage drops due to the driving of a heavy load such as an illumination lamp, a variation of the drive voltage of the load circuit is restrained within a given range to reliably prevent erroneous operation and a stoppage of the circuit operation.

FIG. 1 illustrates a circuit diagram of a power supply system for an electronic wristwatch which is an embodiment of the invention;

FIG. 2 is a time chart useful in explaining the operation of the power supply system shown on FIG. 2;

FIG. 3 is a circuit diagram of a voltage switching circuit applied for another embodiment of the invention; and

FIG. 4 is a time chart illustrating how the circuit of FIG. 3 operates.

In FIG. 1 illustrating a power supply system of an electronic wristwatch, the output voltage V1 of a lithium battery 1 is supplied to a load circuit such as an illumination lamp circuit 3 which provides a signal l with high level when a lamp switch 2 is closed or when a lamp is lit. The output voltage V1 is also connected to the source of an N-channel transistor 4a (referred to as a transistor). The output from the drain of the transistor 4a is applied through the source of a transistor 4b and the drain with a capacitance 4c to a common signal generating circuit 5 to be described later and a voltage switching circuit 6 as a dropped voltage V2. The connection point between the drain of the transistor 4a and the source of the transistor 4b is connected to the output of an inverter 4e, through a capacitor 4d with the same capacitance as that of the capacitor 4c. The input of the inverter 4e is supplied with a clock signal for a voltage drop of 512 Hz from a logic circuit 7 to be described later. When the clock signal of 512 Hz is at its high level, the output of the inverter 4e is coupled with a signal with low level, or the voltage V2. When it is at its low level, the output is coupled with a signal of high level, or ground. The clock signal of 512 Hz is applied through an inverter 4f to the gate of the transistor 4b and through an inverter 4g to the gate of the transistor 4a. The common signal generating circuit 5 supplies common signals X and Y to two common electrodes (not shown) of a liquid display unit 8. The common signal generating circuit 5 is supplied with three reference voltages, the output voltage V1 from the lithium battery 1, the dropped voltage V2 and ground voltage. The circuit 5 combines signals f1 and f2 with given frequencies, e.g. 32 Hz and 64 Hz, to compose the common signals X and Y having the voltages V1 and V2, and the ground voltage.

The voltage switching circuit 6 is so arranged that the dropped voltage V2 from a voltage drop circuit 4 is coupled with the drain of a transistor 6a, the voltage from the source of the transistor 6a is applied to the logic circuit 7 as a drive voltage V3 and is coupled with the output voltage V1 of the lithium battery 1 through the drain and source of a transistor 6b. The gate of the transistor 6b is supplied with a voltage switch signal from the logic circuit 7. The voltage switch signal is also applied to the gate of the transistor 6a, through an inverter 6c.

The logic circuit 7 frequency-divides an oscillating signal from a crystal oscillating circuit 7a. The clock signal with one second period produced from the frequency divider 7a is counted by a time counter 7c. The hour, minute, and second data produced by the time counter 7c is decoded by a decoder 7d of which the output is applied to a segment signal drive circuit 9 supplied with the voltage V1. The segment signal drive circuit 9 dynamically displays the hour data by the liquid display unit 8. To this end, the circuit 9 is supplied with the signal f1 used to compose the common signals X and Y from the frequency dividing circuit 7b. The frequency dividing circuit 7b supplies the clock signal of 512 Hz as a voltage drop clock signal to the voltage drop circuit 4, and supplies the pulse signal with one second period to a delay circuit 7e receiving the signal l applied from the illumination lamp circuit 3 when the lamp is lit. The output signal from the delay circuit 7e, together with the signal l, is applied to the voltage switch circuit 6 through an OR circuit 7f, as the voltage change voltage.

In the above-described circuit construction, since the clock signal of 512 Hz is not applied to the voltage drop circuit 4, the voltage drop circuit 4 does not operate and thus the logic circuit 7 also does not operate. Under this condition, if the lamp switch 2 is turned on, the illumination lamp circuit 3 produces a signal l of high level which in turn is applied through the OR circuit 7f to the transistor 6b thereby to turn on the transistor 6b. As a result, the output voltage V1 of the lithium battery 1 is applied through the transistor 6b to the logic circuit 7, so that the logic circuit 7 starts its operation and the clock signal of 512 Hz is applied to the voltage drop circuit 4 which in turn produces the dropped voltage V1. Then, when the lamp switch 2 is turned off, the output signal from an OR circuit 7f becomes low in level after a given time lapse while the output signal from the inverter 6c becomes high in level. Accordingly, the dropped voltage V2 is applied to the logic circuit 7 through the transistor 6a. Subsequently, when the lamp circuit 3 is in its non-operating condition, the dropped voltage V2 outputted from the voltage drop circuit 4 is applied through the transistor 6a to the logic circuit as the drive voltage V3. The voltage drop circuit 4 operates such that, when the clock signal of 512 Hz is high at its level, the transistor 4b is OFF while the transistor 4a is ON, and the output of the inverter 4e is coupled with the dropped voltage V2. Through its operation, the capacitors 4c and 4d are connected in series and those capacitances are charged by the voltage V1 of the lithium battery 1. Then, when the clock signal for voltage drop becomes low in level, the transistor 4b is turned on while the transistor 4a is turned off. The inverter is grounded at the output, so that the capacitors 4c and 4d are connected in parallel between the voltage drop output and ground. As a result, the voltages charged in the capacitors 4c and 4d, i.e. the voltage which is half of the voltage V1 of the lithium battery 1, is produced from the parallel capacitors.

When the lamp switch 2 is turned on, large current flows into the lamp, so that the output voltage V1 of the lithium battery 1 drops as shown in FIG. 2(C) and the dropped voltage V2 also greatly reduces. At this time, the lamp circuit 3 produces a signal l of high level which is delayed by the delay circuit 7e by a given time and then is applied to the OR circuit 7f. Upon receipt of the signal, the OR circuit 7f produces a signal as shown in FIG. 2(B). The output signal from the OR circuit 7f turns on the transistor 6b in the voltage switching circuit 6 and at the same time the transistor 6a is turned off. Therefore, the output voltage V1 of the lithium battery 1 is applied to the logic circuit 7 as the drive voltage. When the output signal from the OR circuit 7f becomes low in level, the transistor 6a is turned on while the transistor 6b is turned off, like the case before the lamp is driven. Then, the dropped voltage V2 appears instead of the drive voltage V3. In the above-mentioned embodiment, when the illumination lamp circuit 3 as a load circuit to cause the drop of the output voltage of the battery 1 is driven, the drive voltage of the logic circuit 7 being supplied with the dropped voltage V2 is changed to the output voltage of the battery 1. Therefore, the power supply system of the invention is free from the oscillation stoppage or erroneous operation. If the output voltage V1 from the battery 1 is returned to the dropped voltage V2 immediately after the lighting of the illumination lamp ends, the dropped voltage V2 is still low since the voltage of the battery 1 is insufficiently restored. Accordingly, there is still a possibility that oscillation stoppage and erroneous operation takes place. However, as described above, the embodiment of the invention is so designed that, with provision of the delay circuit 7e, the voltage is restored after the voltage of the lithium battery 1 is sufficiently restored. Therefore, the power supply system of the invention ensures reliable operations of the oscillator and the remaining circuits.

When a variation of the voltage V3 applied to the logic circuit is excessive, the circuit elements of the logic circuit 7 are deteriorated to possibly cause a problem in the circuit. The value of the voltage V1, however, may be reduced by inserting a resistor at the source side of the transistor 6b or increasing the resistance between the gate and the source of the transistor 6b. The voltage switching circuit 10 as shown in FIG. 3 may be used. During a period that the illumination lamp lights, the output voltage V1 is outputted through the transistor 10a and then during a period that the OR circuit 7f produces a signal of high level, the output voltage is produced with a restricted low voltage, through the transistor 10b and the resistor R1. When the output voltage of the OR circuit 7f becomes low in level, the dropped voltage V2 is outputted through the transistor 10c. In this way, the voltage of V3 may be restricted low as shown in FIG. 4(C).

The above embodiment is so designed that the drive voltage to the entire logic circuit 7 is switched. However, this is not essential. For example, the voltage switching may be applied to only the drive voltage to the crystal oscillator 7a as a basic portion of the electronic timepiece.

In addition to the electronic device for electronically and optically displaying the hour by a liquid crystal display, invention is applicable for electronic devices for displaying the hour in analogue manner with the combination of a needle and a step-motor or a desk-top type calculator. Further, any suitable value is allowable for the battery voltage V1 and the dropped voltage V2. The heavy load circuit for dropping the battery voltage is not limited to the illumination lamp but it may be a sounding device such as a buzzer.