GB1587028A

GB1587028A - Voltage comparator

Info

Publication number: GB1587028A
Application number: GB1398578A
Authority: GB
Original assignee: Seiko Instruments Inc
Current assignee: Seiko Instruments Inc
Priority date: 1977-04-23
Filing date: 1978-04-10
Publication date: 1981-03-25
Also published as: DE2817602A1; FR2388333A1

Description

(54) A VOLTAGE COMPARATOR (71) We, KABUSHIKI KAISHA DAINI SEIKOSHA, a Japanese Company of 6-31-1 Kameido, Koto-ku, Tokyo, Japan, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to voltage comparators.

In a conventional voltage comparator an AC-DC transducer including a normal operational amplifier is used. However, such a voltage comparator needs a relatively high power source i.e. of the order of 15 volts and so is not suitable for use in an electronic system operated by a relatively low voltage power source.

Electronic timepieces have a relatively low power consumption and use integrated circuitry so that a conventional voltage comparator is not suitable. Moreover, it is difficult to design a conventional voltage comparator which will compare voltages in the region of the voltage of the power source and which is efficient as far as power consumption is concerned.

According to one aspect of the present invention there is provided a voltage comparator comprising: a first pair of MOSFETs consisting of a P-MOSFET and an N-MOSFET whose drains are connected by a passive connection; a second pair of MOSFETs consisting of a P-MOSFET and an N-MOSFET connected in series; a pair of input terminals one of which is connected to the gate of one of the MOSFETs of the first pair and the other of which is connected to the gate of the corresponding one of the MOSFETs of the second pair; an output terminal connected between the MOSFETs of the second pair, the gate of the other MOSFET of the first pair being connected to the gate of the other MOSFET of the second pair; and a common junction between the connection between the drains of the MOSFETs of the first pair and the connection between the gate of the other MOSFET of the first pair and the gate of the other MOSFET of the second pair.

In one embodiment, the voltage comparator includes means for applying a voltage which is proportional to the voltage applied across the first pair of MOSFETs to at least one MOSFET of the second pair of MOSFETs so as to produce a comparison of the voltage signals applied to the first and second input.

The voltage dividing means are preferably connected to apply said voltage to at least one of the MOSFETs of the first pair of MOSFETs.

The voltage dividing means may include a MOSFET. In the preferred embodiment the voltage dividing means comprises a series arrangement consisting of a resistor or capacitor and said MOSFET.

According to a further aspect of the present invention there is provided a voltage comparator arrangement comprising: at least two voltage comparators as recited above having different operating input voltages; and voltage discriminating means, at least one input of the voltage comparators being applied to the voltage discriminating means, at least one voltage comparator being selectively operated by the output of said other voltage comparator.

The voltage discriminating means may be a CMOS inverter.

According to a further aspect of the pr.esent invention there is provided a battery life detecting circuit, for example, for an electronic timepiece, including a voltage comparator as recited above or a voltage comparator arrangement as recited above.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 is a circuit diagram of a first embodiment of a voltage comparator according to the present invention; Figures 2 to 5 illustrate graphically the voltage-current characteristics of the voltage comparator of Figure 1; Figure 6 shows an experimental circuit including the voltage comparator of Figure 1; Figure 7 shows graphically input and output characteristics of the voltage comparator of Figure 1; Figure 8 illustrates the voltage comparator of Figure 1 used in a battery life detecting circuit; Figure 9 is a block diagram of a voltage comparator arrangement employing voltage comparators according to the present invention Figure 10 is a circuit diagram of the voltage comparator arrangement of Figure 9;; Figure 11 illustrates the operation of the voltage comparator arrangement shown in Figure 10; and Figure 12 is a circuit diagram of a voltage comparator according to the present invention forming part of the voltage comparator arrangement of Figure 10.

Referring now to Figure 1, a first embodiment of a voltage comparator according to the present invention has a power source terminal 1 which is connected to a relatively high voltage side of a power source, and to the source of P-MOSFETs 5, 7. The gate and the drain of the P-MOSFET 5 are connected to each other, a common junction 9 being connected to the gates of the P-MOSFETs 5, 7 and the drain of an N-MOSFET 6. Thus the drain of the P-MOSFET 5 and the N-MOSFET 6 are directly connected by a passive connection.

A first input terminal 2 of the voltage comparator is connected to the gate of the N-MOSFET 6 whose source is grounded i.e. is connected to the relatively low voltage side of the power source.

A second input terminal 3 of the voltage comparator is connected to the gate of an N MOSFET 8 whose source is grounded. The drain of the N-MOSFET 8 is connected to the drain of the P-MOSFET 7 and to an output terminal 4.

The characteristics of the P-MOSFET 5 are shown by curve (5) in Figure 2 and are determined by the following formulae: ID = Kp (VGs-VTP)2 . . . (1) where ID is the drain current, Kp is a constant, VGS is the voltage between the gate and the source, and VTP is the threshold voltage On the other hand, the characteristics of the N-MOSFET 6 are shown by curve (6) in Figure 2 and are determined by the following formulae:: ID = K,(2(VGs-VTN)VsD-VsA2 } .. (2) when VGS VTN2 > VSD ID = KN (VGs-VTN) when VGS-VTN S VSD where ID is the drain current, Kp is a constant, VGS is the voltage between the gate and the source VSD is the voltage between the source and the drain, and VTN is the threshold voltage.

Thus, the operating point at the common junction 9 is the point where the drain current ID given by formulae (1) and (2) becomes equal. This is shown by point (P) in Figure 2 where an electric current I2 is applied to the P-MOSFET 5 and the N-MOSFET 6 and the voltage at the common junction 9 is V2.

The N-MOSFET 8 has the same characteristics as the N-MOSFET 6 and these are illustrated by curve (8) in Figure 3 and are determined by formulae similar to formulae (2).

The characteristics of the P-MOSFET 7 are illustrated by curve (7) in Figure 3.

When a voltage Vi2 applied to the terminal 2 is greater than a voltage Vi3 applied to the terminal 3 (Vj2 < Vj3) a voltage V4 of the output terminal 4 becomes substantially equal to a voltage E applied to the terminal 1 as shown in Figure 3. This occurs because the saturation current of the P-MOSFET 7 and the N-MOSFET 8 become equal as indicated in Figure 3.

On the other hand if Vj2 < VjS, the voltage V4 becomes substantially zero as indicated in Figure 5. If V2 = Vj3, the voltage V4 becomes substantially equal to E/2 as indicated in Figure 4.

Figure 6 shows an experimental circuit including the voltage comparator of Figure 1 and Figure 7 illustrates its operating characteristics for three different voltages, namely, 3, 4, 5 volts. A voltage V/2 is obtained by dividing the voltage Vi2 by a voltage divider consisting of two resistors R. The curves shown in Figure 7 illustrate the change in output voltage V0 for change in input voltage Vil. The voltage divider may, instead of being a resistance bridge as shown be a CMOS integrated circuit e.g. No. CD4007 made by RCA. As will be appreciated from Figure 7, the voltage comparator works stably with change in operating voltage V.

The voltage comparator of Figure 1 has a relatively low power consumption and a relatively low operating voltage and may be constructed of CMOS integrated circuitry. It is not necessary to arrange a characteristic in a mutual P and N-MOSFET without a characteristic of VTH and K of P and N-MOSFET so that the circuitry may advantageously be of integrated circuit construction.

Figure 8 shows the voltage comparator of Figure 1 employed in a battery life detecting circuit of an electronic timepiece, the output voltage being inverted when a power source produces a voltage equal to (1 + R2 ) x (VTP + VTN } R1 The voltage comparator of Figure 1 may be used as an amplifying circuit by a way of setting a bias voltage and it is applicable to a large number of different types of circuitry.

The present embodiment, may, it will be appreciated be modified by using P-MOSFETs in place of the N-MOSFETs and vice versa.

Figures 9 and 10 illustrate a voltage comparator arrangement having a positive input terminal 10, a negative input terminal 11, an output terminal 12, a positive power source terminal 13 and a negative power source terminal 14. The operation of the voltage comparator arrangement is illustrated by the following truth table: Input Output Vl > V2 H Vl < V2 L where, V1 is the voltage of terminal 10 and V2 is the voltage of terminal 11.

The terminal 10 is connected to the positive input terminals of a first voltage comparator 15 according to the present invention and a second voltage comparator 16 according to the present invention. The terminal 10 is also connected to the gate of a P-MOSFET 17 and the gate of an N-MOSFET 18 via an inverter 20. The source of the P-MOSFET 17 is connected to the terminal 13 and the source of the N-MOSFET 18 is connected to the terminal 14.

Therefore, a voltage is not applied to positive and negative power sides of the comparator 16, so that it is inoperative. On the other hand, a voltage is applied to the positive power source side of the comparator 15 when the P-MOSFET 17 and an N-MOSFET 19 are conductive so that the comparator 15 is rendered normally operative.

Thus, when the voltage at the terminal 10 is lower than the inverted voltage Vl at the output of the inverter 20, the comparator 15 is operative whilst its input voltage is between the threshold voltage VTN of the N-MOSFET 19 and the positive voltage VD of the power source. The comparator 16 is operative whilst its input voltage is between the negative voltage of the power source and the threshold voltage VTP of a P-MOSFET 21. This situation is illustrated in Figure 11, Figure 11 (a) illustrating the operational region of the comparator 15 and Figure 11 (b) the operational region of the comparator 16.

An output of the inverter 20 is connected to the gate of the N-MOSFET 19 and a gate of the P-MOSFET 21 via an inverter 22. The source of the N-MOSFET 19 is connected to the terminal 14 and the source of the P-MOSFET 21 is connected to the terminal 13. The terminal 11 is connected to the negative terminals of the comparators 15, 16. The terminal 12 is connected to the output terminals of the comparators 15, 16. The positive power source side of the comparator 15 is connected to the drain of the N-MOSFET 19. The positive power source side of the comparator 16 is connected to the drain of the P-MOSFET 21 and the negative source side is connected to the drain of the N-MOSFET 18.

When the voltage which is applied to the terminal 10 is higher than an inverted voltage Vl at the output of the inverter 20, the output of the inverter 20 is level L, the N-MOSFET 18 turns OFF and the P-MOSFET 17 turns ON. The output of the inverter 22 becomes level H so that the N-MOSFETT 19 turns ON and the P-MOSFET 21 turns OFF.

Voltage VL iS given by the formula:

where KP is the conductive constant of the P-MOSFET 21 and KN is the conductive constant of the N-MOSFET 19.

Therefore, the voltage VI is preferable for an operational range of the comparators 15, 16 by separating the input of the inverter 20.

The comparator 15 is of the same construction as shown in Figure 1 and operates in the same manner as described with reference to Figures 2 to 5.

Figure 12 is a circuit diagram of the comparator 16. The comparator 16 is similar in construction to the voltage comparator of Figure 1 except that P-MOSFETs are used in place of N-MOSFETs and vice versa and the power source is oppositely connected so that the same operation as that of the comparator 15 is achieved. Consequently, a detailed description of the comparator 16 is considered unnecessary.

The voltage comparator arrangement of Figure 10 has a relatively low operating voltage and relatively low power consumption and the range of input voltages becomes the voltage range of the power source. Moreover so that the comparator can easily be constructed of integrated circuitry.

WHAT WE CLAIM IS: 1. A voltage comparator comprising: a first pair of MOSFETs consisting of a P-MOSFET and an N-MOSFET whose drains are connected by a passive connection; a second pair of MOSFETs consisting of a P-MOSFET and an N-MOSFET connected in series; a pair of input terminals one of which is connected to the gate of one of the MOSFETs of the first pair and the other of which is connected to the gate of the corresponding one of the MOSFETs of the second pair; an output terminal connected between the MOSFETs of the second pair, the gate of the other MOSFET of the first pair being connected to the gate of the other MOSFET of the second pair; and a common junction between the connection between the drains of the MOSFETs of the first pair and the connection between the gate of the other MOSFET of the first pair and the gate of the other MOSFET of the second pair.

2. A voltage comparator as claimed in claim 1 including voltage dividing means for applying a voltage which is proportional to the voltage applied across the first pair of MOSFETs to at least one MOSFET of the second pair of MOSFETs so as to produce a comparison of the voltage signals applied to the first and second input.

3. A voltage comparator as claimed in claim 2 in which the voltage dividing means are connected to apply said voltage to at least one of the MOSFETs of the first pair of MOSFETs.

4. A voltage comparator as claimed in claim 2 or 3 in which the voltage dividing means includes a MOSFET.

5. A voltage comparator as claimed in claim 4 in which the voltage dividing means comprises a series arrangement consisting of a resistor or capacitor and said MOSFET.

6. A voltage comparator substantially as described with reference to and as shown in Figures 1 to 8 or 13 to 16 of the accompanying drawings.

7. A voltage comparator arrangement comprising: at least two voltage comparators as claimed in any preceding claim having different operating input voltages; and voltage discriminating means, at least one input of the voltage comparators being applied to the voltage discriminating means, at least one voltage comparator being selectively operated by the output of said other voltage comparator.

8. A voltage comparator arrangement as claimed in claim 7 in which the voltage discriminating means is a CMOS inverter.

9. A voltage comparator arrangement substantially as herein described with reference to and as shown in Figures 9 to 12 of the accompanying drawings.

10. A battery life detecting circuit including a voltage comparator as claimed in any of claims 1 to 6 or a voltage comparator arrangement as claimed in claim 7 or 8.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. so that the N-MOSFETT 19 turns ON and the P-MOSFET 21 turns OFF. Voltage VL iS given by the formula: where KP is the conductive constant of the P-MOSFET 21 and KN is the conductive constant of the N-MOSFET 19. Therefore, the voltage VI is preferable for an operational range of the comparators 15, 16 by separating the input of the inverter 20. The comparator 15 is of the same construction as shown in Figure 1 and operates in the same manner as described with reference to Figures 2 to 5. Figure 12 is a circuit diagram of the comparator 16. The comparator 16 is similar in construction to the voltage comparator of Figure 1 except that P-MOSFETs are used in place of N-MOSFETs and vice versa and the power source is oppositely connected so that the same operation as that of the comparator 15 is achieved. Consequently, a detailed description of the comparator 16 is considered unnecessary. The voltage comparator arrangement of Figure 10 has a relatively low operating voltage and relatively low power consumption and the range of input voltages becomes the voltage range of the power source. Moreover so that the comparator can easily be constructed of integrated circuitry. WHAT WE CLAIM IS:

1. A voltage comparator comprising: a first pair of MOSFETs consisting of a P-MOSFET and an N-MOSFET whose drains are connected by a passive connection; a second pair of MOSFETs consisting of a P-MOSFET and an N-MOSFET connected in series; a pair of input terminals one of which is connected to the gate of one of the MOSFETs of the first pair and the other of which is connected to the gate of the corresponding one of the MOSFETs of the second pair; an output terminal connected between the MOSFETs of the second pair, the gate of the other MOSFET of the first pair being connected to the gate of the other MOSFET of the second pair; and a common junction between the connection between the drains of the MOSFETs of the first pair and the connection between the gate of the other MOSFET of the first pair and the gate of the other MOSFET of the second pair.

11. An electronic timepiece having a battery life detecting circuit as claimed in claim 10.