GB2051382A

GB2051382A - Voltage Detecting Circuit

Info

Publication number: GB2051382A
Application number: GB8013958A
Authority: GB
Original assignee: Tokyo Shibaura Electric Co Ltd
Current assignee: Toshiba Corp
Priority date: 1979-04-27
Filing date: 1980-04-28
Publication date: 1981-01-14
Also published as: JPS55143471A; DE3016108A1; DE3016108C2; GB2051382B

Abstract

A reference voltage generating circuit (20, 24) includes a resistor (20) having one terminal connected to the positive terminal of a battery and a p-n junction (24) connected between the other terminal of the resistor and the negative terminal of the battery, and a detected voltage generating circuit (26, 28) includes a resistor (26) having one terminal connected to the negative terminal of the battery and a p-n junction (28) connected between the other terminal of the resistor (26) and the positive terminal of the battery. The junctions (24), (28) may be forward biassed diodes or zener diodes, and the resistors may be comprised of field-effect transistors or replaced by current sources. The detector may be used to monitor the voltage of a watch battery. <IMAGE>

Description

SPECIFICATION Voltage Detecting Circuit This invention relates to a voltage detecting circuit.

Recently, various electronic watches have been developed, and these electronic watches use a primary battery as drive source. Naturally, when the primary battery is used up to a certain extent so that its output voltage is reduced to a predetermined voltage, the electronic watch is subject to malfunction or stops its operation. In order to solve this problem, it is necessary to detect the reduction of the primary battery voltage to a predetermined voltage and replace the battery with a new one at this time. To this end, the circuit of the usual electronic watch includes a battery voltage detecting circuit, for instance, as shown in Fig. 1.

The battery voltage detecting circuit shown in Fig. 1 include a variable resistor 2 connected to the positive terminal of battery 4 and an Nchannel field-effect transistor (FET) 6 which has a current path connected between the variable resistor 2 and the negative terminal of the battery 4 and a gate connected to the positive terminal of the battery 4. It further includes a series combination of resistors 8 and 10 connected across the battery 4. A voltage V1 at the junction between the variable resistor 2 and FET 6 and a voltage V2 at the junction between the resistors 8 and 10 are applied to respective first and second input terminals of a comparator 12.

As shown in Fig. 2, when the output voltage of the battery 4 is sufficiently high, the detected voltage V1 at the junction between the variable resistor 2 and FET 6 is higher than the reference voltage V2 at the junction between the resistors 8 and 10, and the comparator 12 provides a low level output signal as shown in Fig. 3. When the output voltage of the battery 4 becomes lower than a predetermined value, the detected voltage V, becomes lower than the reference voltage V2 due to reduction of the resistance of the FET 6. As a result, the comparator 12 produces a high level output signal shown in Fig. 3. This high level output voltage is used to inform the user of the fact that the battery 4 has been used up to a greater extent.

In the voltage detecting circuit of Fig. 1 , the initial setting of the detected voltage V1 to an appropriate level with respect to the reference voltage V2, which is set by the resistors 8 and 10, through adjustment of the variable resistor 2 is important. Unless the initial setting of the detected voltage V1 is precisely made, the voltage detecting circuit cannot detect the reduction of the output voltage of the battery 4 to be lower than the predetermined level with high reliability.

For this reason, the adjustment of the variable resistor 2 is required to be done by the manufacturer.

Usually, it is possible to reduce the variable range of the variable resistor 2 by increasing the precision of the process for forming the individual circuit elements. Even in this case, however, it is necessary to set the detected voltage Vg to be in the neighborhood of the reference voltage V2. For this reason, it has been proposed to form a plurality of resistors having suitable resistances on a semiconductor substrate and appropriately combine these resistors when setting the initial value of the detected voltage V. However, this method, in which a plurality of resistors have to be formed on a single semiconductor chip, also has drawbacks in that these resistors increase the area of the chip and that they must be selectively coupled or separated.

An object of the invention is to provide a voltage detecting circuit, with which the initial state can be set without use of any variable resistor, and which is simple in construction.

As preferred form of the invention, there is provided a voltage detecting circuit, which comprises first and second power supply terminals, a first p-n junction circuit having one end connected to the first power supply terminal and including at least one p-n junction device, a first current source circuit connected between the other end of the first p-n junction circuit and the second power supply terminal, a second p-n junction circuit having one end connected to the second power supply terminal and including at least one p-n junction device, a second current source circuit connected between the other end of the second p-n junction circuit and the first power supply terminal and a comparator having a first input terminal connected to the junction between the first p-n junction circuit and first current source circuit and a second input terminal connected to the junction between the second pn junction circuit and second current source circuit.

This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a circuit diagram showing a prior art battery voltage detection circuit; Fig. 2 is a graph showing the service life of a battery in terms of the battery voltage; Fig. 3 is a graph showing the level change of an output signal produced from the battery voltage detection circuit shown in Fig. 1; Fig. 4 is a circuit diagram showing an embodiment of the battery voltage detection circuit according to the invention; Fig. 5 is a graph showing the relation between anode and cathode voltages of two diodes used in the voltage detecting circuit shown in Fig. 4 and the battery voltage;; Fig. 6 is a graph showing the level change of an output signal produced from the voltage detection circuit shown in Fig. 4 according to the change of the battery voltage; Figs. 7 to 9 are circuit diagrams showing other embodiments of the battery voltage detection circuit according to the invention; and Figs. 10 to 12 are circuit diagram showing specific circuit constructions of the battery voltage detection circuit shown in Fig. 4.

Fig. 4 shows an embodiment of the battery voltage detection circuit according to the invention. This circuit includes a resistor 20 connected to the positive terminal of battery 22, a diode 24 connected in a forward direction between the resistor 20 and the negative terminal of the battery 22, a resistor 26 connected to the negative terminal of the battery 22, and a diode 28 connected in a forward direction between the resistor 26 and the positive terminal of the battery 22. The junction 30 between the resistor 20 and diode 24 and the junction 32 between the resistor 26 and diode 28 are connected to respective first and second input terminals of a comparator 34.

In the circuit shown in Fig. 4 it is to be noted that in the series circuit including the resistor 20 and diode 24 the cathode of the diode 24 is connected to the negative terminal of the battery 22 so that the anode voltage of this diode 24 is taken out as a reference voltage V30 while in the series circuit including the diode 28 and resistor 26 and the anode of the diode 28 is connected to the positive terminal of the battery 22 so that the cathode voltage on this diode is applied as a detected voltage V32 to the comparator 34.

In the graph of Fig. 5, the relations of the reference and detected voltages with respect to the output voltage of the battery 22 are shown respectively by solid and broken lines. It will be seen from Fig. 5 that when the output voltage Vx of the battery 22 is higher than the forward voltage drop VF across the diode 24, the reference voltage V30 is substantially held at VF. Also, when the output voltage Vx of the battery 22 is higher than the forward voltage drop VF across the diode 28, the detected voltage V32 is substantially equal to a voltage (VXVF). This means that when the battery voltage Vx becomes 2VP the reference voltage V30 and detected voltage V32 becomes substantially equal to each other.In other words, the detected voltage V32 is higher than the reference voltage V30 when the battery voltage Vx is higher than 2VP and it becomes lower than the reference voltage V30 when the battery voltage Vx becomes lower than 2Vp.

Thus, the battery voltage detection circuit as shown in Fig. 4 may be designed such that, for instance, a relation VXC=2VF is met when the reduction of the battery voltage Vx to a critical value Vxc is detected. In this case, while the battery voltage Vx is higher than the threshold voltage Vx, i.e., 2VP the detected voltage V32 is higher than the reference voltage V30 as mentioned before. At this time, the comparator 34 produces a high level output signal as shown in Fig. 6. When the battery voltage Vx becomes lower than the critical voltage Vxc with gradual consumption of the battery 22, the detected voltage V32 becomes lower than the reference voltage V30, so that the comparator 34 produces a low level output signal as shown in Fig. 6.Thus, by coupling an alarm (not shown) to the battery voltage detection circuit so as to energize the alarm according to the level change of the output signal from the comparator 34, it is possible to reliably detect the proper timing of the replacement of the battery 22.

In the battery voltage detecting circuit shown in Fig. 4, the reference voltage V30 and detected voltage V32 are determined by the electric characteristics of the diodes 24 and 28 and the output voltage of the battery 22, so that unlike the prior art circuit there is no need of using a variable resistor for setting the detected voltage.

In addition, with the present semi-conductor manufacturing techniques diodes having desired electric characteristics can be manufactured at a relatively high precision, so that it is possible to obtain a desired critical voltage Vxc.

Fig. 7 shows another embodiment of the battery voltage detecting circuit according to the invention. The circuit of Fig. 7 is the same as the circuit of Fig. 4 except that zener diodes 34 and 38 are used in lieu of the diodes 24 and 28. In the voltage detecting circuit of Fig. 7, by denoting the zener voltages of the zener diodes 44 and 48 respectively by Vzl and Vz2, the reference voltage V, at the junction 50 between the resistor 20 and zener diode 44 is expressed substantially equal equal to Vz1, and the detected voltage V52 at the junction between the zener diode 48 and resistor 26 is expressed substantially equal to (VxVz2).

Thus, by setting the critical voltage Vxc to be equal to (Vz1+Vz2), a low level output signal can be produced from the comparator 34 at an instant when the battery voltage Vx is reduced to the critical voltage Vxc in the same manner as described before in connection with the voltage detecting circuit of Fig. 4, so that it is possible to detect the proper timing of the replacement of the battery 22.

Fig. 8 shows a further embodiment of the battery voltage detection circuit according to the invention. The circuit of Fig. 8 is the same as the circuit of Fig. 4 except that constant current sources 60 and 66 are used in lieu of the resistors 20 and 26. The circuit also operates in the same way as and has the same effects as the voltage detecting circuit of Fig. 4.

Fig. 9 shows a still further embodiment of the battery voltage detection circuit according to the invention. The circuit of Fig. 9 is the same as the voltage detecting circuit of Fig. 7 except that constant current sources 70 and 76 are used in lieu of the resistors 20 and 26. This circuit also operates in the same way as and has the same effects as the voltage detecting circuit of Fig. 4.

Figs. 10 to 12 show specific examples of the detailed circuit construction of the battery voltage detecting circuit shown in Fig. 4. In these circuits, field effect transistors (FET) 120 and 126 constitute the respective resistors 20 and 26 in the circuit of Fig. 4. The comparators used in these circuits are well-known, so they will be briefly described.

In the circuit of Fig. 10, the comparator 34 includes p-channel and n-channel FETs 130 and 132 having respective current paths connected in series across the battery 22 and p-channel and nchannel FETs 134 and 136 having respective current paths connected in series across the battery 22.

Here, when the battery voltage Vx is sufficiently high, the FET 1 30 which receives the reference voltage at its gate has a high resistance, while the FET 1 34 which receives the detected voltage at its gate has a low resistance.

At this time, the resistance of the FET 1 36 which has its gate connected to the source of the FET 1 30 is high, while the resistance of the FET 1 32 which has its gate connected to the source of the FET 1 34 is low. Thus, the output voltage of the comparator 34 is at a high level at this time.

When the battery voltage Vx becomes lower than the critical value Vxc, the resistance of the FETs 132 and 134 becomes high or than that of the FETs 130 and 136, so that a low level output signal is produced from the comparator 34..

In the voltage detecting circuit shown in Fig.

11, the comparator 34 is constituted by pchannel FETs 140 and 142 and n-channel FETs 144 and 1 46 having respective current paths connected in series across the battery 22 and pchannel FETs 150 and 152 and n-channel FETs 1 54 and 1 56 having respective current paths connected in series across the battery 22.

Here, when the battery voltage Vx is sufficiently high, the FET 1 52 and 1 54 respectively offer high and low resistances, so that the output signal from the comparator 34 is at a low level. Consequently, the FETs 140 and 146 have respective low and high resistances, so that a high voltage is coupled to the gate of the FETs 1 50 and 1 56. When the battery voltage Vx becomes lower than a predetermined value Vxc, the FETs 1 52 and 1 54 come up with respective low and high resistances, so that the comparator 34 produces a high level output signal.In this way, with this circuit with the reduction of the battery voltage Vx to be lower than the critical voltage Vxc a high level output signal is produced to drive an alarm (not shown) which is coupled to this circuit.

In the voltage detecting circuit shown in Fig.

12, the comparator 34 comprises a constant current source 1 60 connected to the positive terminal of the battery 22, a series combination of p-channel and n-channel FETs 1 62 and 1 64 and a series combination of p-channel and n-channel FETs 1 66 and 168, these series combinations being connected between the other terminal of the constant current source 1 60 and the negative terminal of the battery 22. Further, it includes a series combination of p-channel and' n-channel FETs 170 and 172 and a series combination of pchannel and n-channel FETs 174 and 176, these series combinations being connected across the battery 22.The gate of the FET 1 72 is connected to the gate and drain of the FET 1 64 and also to the gate of the FET 168, the gate of the FET 176 is connected to the drain of the FET 168, and the gate of the FET 174 is connected to the gate and source of the FET 170.

Here, when the battery voltage Vx is sufficiently high, a high voltage is applied to the gate of the FET 166, so that the FET 166 offers a higher resistance than that of the FET 1 62. Thus, high and low voltages are applied to the respective gates of the FETs 1 72 and 176, which thus offer respective high and low resistances, so that a high level signal is produced from the comparator 34. Whenthe battery voltage Vx becomes lower than the critical voltage Vx, the resistance of the FET 1 62 becomes higher than that of the FET 166. As a result, the FETs 1 72 and 176 come up with respective high and low resistances, so that a low level signal is produced from the comparator 34.

It will be clear from the circuit of Figs. 10 to 12 that the voltage detecting circuit according to the invention can be formed on a single chip and that the diodes 24 and 28 can be formed such that they have required characteristics for setting desired value of the critical voltage Vxc.

While the invention has been described above in conjunction with the preferred embodiments thereof, these embodiments are by no means limitative. For example, while the embodiments of Figs 4 and 8 have used two diodes 24 and 28 for setting the critical voltage Vxc, each of these diodes 24 and 28 may be replaced with a combination of series diodes. Of course the zener diodes 44 and 48 in the embodiments of Figs. 7 and 9 may each be replaced with a series circuit of zener diodes.

Further, it is possible to replace either one of the diodes 24 and 28 in the embodiments of Figs.

4 and 8 with a zener diode and also replace either one of the resistors 20 and 26 in the embodiments of Figs. 4and 7 with a constant current source.

Claims

1. A voltage detection circuit comprising: first and second power supply terminals; a first p-n junction circuit having one terminal connected to said first power supply terminal and including at least one p-n junction device; a first current source circuit connected between the other terminal of said first p-n junction circuit and said second power supply terminal; a second p-n junction circuit having one end connected to said second power supply terminal and including at least one p-n junction device; a second current source circuit connected between the other terminal of said second p-n junction circuit and said first power supply terminal; and a comparator circuit having a first input terminal connected to the junction between said first p-n junction circuit and said first current source circuit and a second input terminal connected to the junction between said second pn junction circuit and said second power supply terminal.

2. A voltage detection circuit according to claim 1 , wherein said first and second p-n junction circuits each include a diode connected in a forward direction.

3. A voltage detection circuit according to claim 1, wherein said first and second p-n junction circuits each include a zener diode connected in a reverse direction.

4. A voltage detection circuit according to claim 1,2 or 3, wherein said first and second current source circuits each include a resistive circuit element.

5. A voltage detection circuit according to claim 4, wherein said resistive circuit element is constituted by a field effect transistor

6. A voltage detection circuit according to claim 5, wherein said comparator circuit is constituted by a field effect transistor circuit.

7. A voltage detection circuit according to claim 4, wherein said resistive circuit element is constituted by a field effect transistor.

8. A voltage detection circuit according to claim 1,2 or 3, wherein said first and second current source circuits each include a constant current source.

9. A voltage detection circuit according to claim 8, wherein said first and second current source circuits each include a field effect transistor.

10. A voltage detection circuit according to claim 1, 2 or 3, wherein said comparator circuit produces an output signal when detecting that the voltage applied to its first input terminal is lower than the voltage applied to its second input terminal.

11. A voltage detecting circuit, substantially as hereinbefore described with reference to the accompanying drawings.