Voltage Detection Circuit Using Threshold as Reference Voltage for Detecting Input Voltage

The present invention relates to a voltage detection circuit for detecting an input voltage having a predetermined level by comparing the input voltage with a reference voltage, and more particularly, to a voltage detection circuit using the threshold voltage of a semiconductor active element such as a transistor as the reference voltage.

In general, a comparison circuit is employed as a voltage detection circuit for detecting input voltage, and the comparison output from the comparison circuit is used as a voltage detection output. An input voltage to be detected is applied to one input terminal of the comparison circuit, while a reference voltage obtained by using the power supply voltage or a constant voltage element (e.g., a zener diode) is supplied to the other input terminal. Such a voltage detection circuit offers an advantage that the voltage level for detecting input voltage can be made to vary by adjusting the reference voltage. However, it is hard to maintain the predetermined value of the reference voltage which relies upon the power supply voltage or a constant voltage of a constant voltage element, when the power supply voltage transiently changes at the time of the turning on or off of the power, or when the power supply voltage is so low that it becomes impossible for the constant voltage element to maintain a fixed voltage.

Instead of using the reference voltage having the mentioned drawback, a threshold voltage of a semiconductor active element such as transistor may be employed as the reference voltage to be compared with the input voltage to be detected. The threshold voltage is such a voltage that the semiconductor active element is activated by a voltage higher than that. Since the threshold voltage is fixed and independent of the power supply voltage, the aforementioned problem ocurring when the power supply voltage is in a transient state can be eliminated. By attenuating the input voltage by means of a resistance-dividing circuit, adjusting the attenuation ratio of the dividing circuit and comparing the attenuated input voltage with the threshold voltage, it is possible to vary the voltage level for detecting the input voltage. However, although the threshold voltage is constant with respect to the power supply voltage, it has a temperature coefficient. In other words, the threshold voltage varies in response to changes in the ambient temperature. Consequently, the input voltage detection level varies with temperature, and it is impossible to expect a reliable detection operation.

It is, therefore, an object of the present invention to provide a voltage detection circuit using the threshold voltage wherein the effect of the temperature coefficient of the threshold voltage is diminished.

It is another object of the present invention to provide a voltage detection circuit having an arrangement suited to a semiconductor integrated circuit, wherein the variation in the input voltage detection level resulting from the ambient temperature change is reduced.

A voltage detection circuit in accordance with the present invention includes a circuit having a semiconductor active element, with a threshold voltage. This circuit is further provided with a voltage input terminal and an output terminal. An input voltage to be detected is supplied to the voltage input terminal. When the input voltage having a greater voltage amplitude than this threshold voltage is supplied to the voltage input terminal, the semiconductor active element is activated to produce a predetermined output signal at the output terminal. The threshold voltage at the aforementioned circuit comprises a first voltage component corresponding to the threshold of the semiconductor active element and a second voltage component having a temperature coefficient opposite to the temperature coefficient of the first voltage component.

Thus, the input voltage is supplied to the voltage input terminal of the circuit having a semiconductor active element, and compared with the threshold voltage. This threshold voltage is fixed irrespective of the state of the power supply voltage since it is not obtained by using the power supply voltage. Further, since the threshold voltage comprises the first voltage component corresponding to the threshold of the semiconductor active element and the second voltage component having a temperature coefficient opposite to the temperature coefficient of the first voltage component, the variation of the threshold voltage of the circuit; resulting from the ambient temperature change can be reduced. If the absolute values of the temperature coefficients of the first voltage component and the second voltage component are equalized, the threshold voltage becomes substantially fixed irrespective of the variations in the ambient temperature.

Above and other objects, features and advantages of the present invention will be more apparent from the following description taken in connection with the accompanying drawings:

FIG. 1 is a circuit diagram illustrating a voltage detection circuit according to the prior art, which compares the input voltage with a voltage using the threshold voltage of a semiconductor active element.

FIG. 2 is a circuit diagram of a voltage detection circuit in accordance with a preferred embodiment of the present invention.

FIG. 3 is a circuit diagram of a voltage detection circuit in accordance with another preferred embodiment of the present invention.

Referring now to FIG. 1, a voltage detection circuit according to prior art will be described hereinafter. An input voltage to be detected is supplied between a detection terminal 1 and a reference potential terminal 3 (a ground terminal in FIG. 1). Resistors R.sub.1 and R.sub.2 are connected in series between the terminals 1 and 3. A base-emitter path of a transistor Q.sub.1 is connected in parallel with the resistor R.sub.2, and its collector is connected to the voltage input terminal 1 via a resistor R.sub.3. The collector of the transistor Q.sub.1 is further connected to the base of a transistor Q.sub.2. The emitter of the transistor Q.sub.2 is connected to the terminal 3, while its collector is connected to an output terminal 2.

When the base-emitter threshold voltage of the transistor Q.sub.1 when turned on is taken as V.sub.thQ1, the threshold voltage V.sub.thd of the detection terminal 1 is expressed by the following formula: ##EQU1## When the voltage at the detection terminal 1, i.e., the input PG,8 voltage V.sub.d, is less than V.sub.thd, the base-emitter voltage V.sub.BEQ1 is less than the threshold voltage V.sub.thQ1 (V.sub.BEQ1 <V.sub.thQ1), so that the transistor Q.sub.1 is cut off. Consequently, the transistor Q.sub.2 is made to conduct, and the output terminal 2 takes a low level. When the voltage V.sub.d at the input terminal 1 is greater than or equal to V.sub.thd the base-emitter voltage V.sub.BEQ1 becomes greater than the threshold voltage V.sub.thQ1 (V.sub.BEQ1 .gtoreq.V.sub.thQ1), so that the transistor Q.sub.1 is made to conduct, while the transistor Q.sub.2 is cut off. Consequently, the output terminal 2 is in an open condition. Thus, it is possible to detect the input voltage supplied to the detection terminal 1 by comparing it with the threshold voltage V.sub.thd of this circuit using the threshold V.sub.thQ1 of the transistor Q.sub.1. Since the threshold voltage V.sub.thd has a fixed level with respect to the power supply voltage, is evident from the formula (1), it is possible to attain a desired detection operation irrespective of the state of the power supply voltage.

In the prior art circuit shown in FIG. 1, however, there is a drawback that there are large temperature variations with respect to the threshold voltage V.sub.thd at the detection terminal 1. In other words, the temperature coefficient .differential.V.sub.thd /.differential.T of V.sub.thd is expressed from the formula (1): ##EQU2## Here, the temperature coefficient .differential.V.sub.thQ1 /.differential.T of the base-emitter threshold voltage of the transistor Q.sub.1 is approximately -2 mV/.degree. K. Accordingly, the temperature coefficient .differential.V.sub.thd /.differential.T of the threshold voltage V.sub.thd the detection input terminal 1 takes a negative temperature coefficient, and its value takes a value of several mV/.degree. K. As a specific example, if it is assumed that R.sub.1 =16 k.OMEGA., and R.sub.2 =4 k.OMEGA., then .differential.V.sub.thd /.differential.T=-10 mV/.degree. K.

Referring now to FIG. 2, a preferred embodiment of the present invention will be described hereinafter. The voltage to be detected is supplied between the voltage detection terminal (input terminal) 1 and the reference potential (GND) terminal 3. Resistors R.sub.10 and R.sub.11 are connected in series between the terminals 1 and 3. The base of a transistor Q.sub.9 is connected to the junction between R.sub.10 and R.sub.11, and its collector is connected to the detection terminal 1. The emitter of the transistor Q.sub.9 is connected to the terminal 3 via a resistor R.sub.7 and a diode-connected transistor Q.sub.6. The emitter of the transistor Q.sub.9 is further connected to the collector of a transistor Q.sub.7 via a resistor R.sub.8, and the base of the transistor Q.sub.7 is connected to the base of the transistor Q.sub.6. The emitter of the transistor Q.sub.7 is connected to the ground terminal 3 via a resistor R.sub.9. The collector of the transistor Q.sub.7 is connected to the base of a transistor Q.sub.8 , and its emitter is connected to the terminal 3. The collector of the transistor Q.sub.8 is connected to an output terminal 2. This circuit is made in the form of a semiconductor integrated circuit, and therefore, each transistor operates at the same junction temperature. The threshold voltage V.sub.thf of the detection terminal 1 at the time when the transistor Q.sub.8 is made to conduct is expressed by the following formula (3). Further the following formulas (4) through (10) hold with respect to the voltages and currents at each operating points. ##EQU3## where V.sub.BQ9 : base voltage of transistor Q.sub.9 ;

V.sub.EQ9 : emitter voltage of transistor Q.sub.9 ;

V.sub.BEQ9, V.sub.BEQ6, V.sub.BEQ7 : respective base-emitter voltages of transistors Q.sub.9, Q.sub.6, and Q.sub.7 ;

V.sub.thQ8 base-emitter threshold voltage at the time when transistor Q.sub.8 becomes conductive;

I.sub.SQ6, I.sub.SQ7, I.sub.SQ8 : respective saturated currents of transistors Q.sub.6, Q.sub.7 and Q.sub.8 ;

I.sub.Q6, I.sub.Q8 respective collector currents at transistors Q.sub.6 and Q.sub.8 ;

I.sub.Q7 : emitter current of transistor Q.sub.7 ;

R.sub.7, R.sub.8, R.sub.9, R.sub.10, R.sub.11 respective resistance values of resistors R.sub.7, R.sub.8, R.sub.9, R.sub.10, R.sub.11 ;

q: electric charge of electrons;

K: Boltzmann's constant;

T: absolute temperature (.degree. K.);

When the base-emitter junction area of the transistor Q.sub.7 is a times as large as that of the transistor Q.sub.6, the following formula holds:

Using the formulas (8), (9) and (11) for the formula (7), I.sub.Q7 is obtained by: ##EQU4##

Since the operating point is selected in such a way that V.sub.BEQ6 and V.sub.thQ8 become substantially equal when the transistor Q.sub.8 is made to conduct, the formula (12) can be expressed as follows: ##EQU5## Substituting the formulas (4), (5) and (13) into the formula (3), ##EQU6##

When the voltage V.sub.f at the detection terminal 1 meets the condition of V.sub.f<V.sub.thf, the relationship of the I.sub.Q6 <I.sub.Q7 is satisfied between currents I.sub.Q6 and I.sub.Q7 flowing through the transistors Q.sub.6 and Q.sub.7, with the result that the base-emitter voltage V.sub.BEQ8 at the transistor Q.sub.8 is less than the threshold voltage V.sub.thQ8 (V.sub.BEQ8 <V.sub.thQ8). Hence, the transistor Q.sub.8 is cut off, and the output terminal 2 is opened. On the other hand, when the voltage V.sub.f at the detection terminal 1 is greater than or equal to V.sub.thf, I.sub.Q6 is greater than or equal to I.sub.Q7, so that V.sub.BEQ8 is greater than or equal to V.sub.thQ8 (V.sub.BEQ8 .gtoreq.V.sub.thQ8). Consequently, the transistor Q.sub.8 is conducting and the output terminal 2 is at a low level. Thus, it is possible to detect the input voltage V.sub.f applied to the terminal 1 by comparing it with the threshold voltage V.sub.thf. The temperature coefficient of the threshold voltage V.sub.thf at the detection terminal 1 is expressed as follows: ##EQU7## If appropriate values are selected for resistance ratios R.sub.8 /R.sub.9 and R.sub.8 /R.sub.7 as well as the emitter area ratio a of the transistor Q.sub.6 to the transistor Q.sub.7 in such a way that the following formula is obtained from the right side of the formula (15), ##STR1## then it is possible to bring the temperature coefficient .differential.V.sub.thf /.differential.T of the threshold voltage V.sub.thf at the detection terminal 1 to zero. As a specific example, if selections are made as: R.sub.7 =R.sub.8 =29 k.OMEGA., R.sub.9 =1 k.OMEGA., a=5, R.sub.10 =4 k.OMEGA., and R.sub.11 =8 k.OMEGA., the .differential.V.sub.thf /.differential.T=38 .mu.V/.degree. K. due to the fact that both .differential.V.sub.BEQ9 /.differential.T and .differential.V.sub.thQ8 /.differential.T are approximately -2 mV/.degree. K. Thus, the temperature coefficient becomes extremely small.

As described above, the threshold voltage V.sub.thf of the circuit shown in FIG. 2 is determined by the base-emitter voltage V.sub.BEQ9 of the transistor Q.sub.9, the threshold voltage V.sub.thQ8 of the transistor Q.sub.8, the voltage drop V.sub.R8 across the resistor R.sub.8, and the resistance ratio of the resistors R.sub.10 to R.sub.11, and is independent of the power supply voltage. Accordingly, it is possible to detect the input voltage supplied to the detection terminal 1 regardless of the transient state of the power supply voltage or a considerable power supply voltage drop. Also, since the threshold voltage V.sub.thf is variable by adjusting the resistance ratio of the resistor R.sub.10 to the resistor R.sub.11, it is possible to adjust the detection level of the input voltage. Furthermore, since the temperature coefficient of the threshold voltage V.sub.thf is determined by negative temperature coefficients of the base-emitter voltage V.sub.BEQ9 of the transistor Q.sub.9 and the threshold voltage V.sub.thQ8 of the transistor Q.sub.8, and by a positive temperature coefficient of the voltage drop V.sub.R8 of the resistor R.sub.8, the variation in the threshold voltage V.sub.thf caused by the ambient temperature change can be reduced. Hence, the variation in the detection level of the input voltage is suppressed. If the negative and positive temperature coefficients are made to be the same level, the threshold voltage V.sub.thf becomes substantially constant irrespective of the ambient temperature change.

The reason why the voltage drop V.sub.R8 across the resistor R.sub.8 has a positive temperature coefficient is that the transistors Q.sub.6 and Q.sub.7 operate at different current densities. Since the resistor R.sub.9 is connected to the emitter of the transistor Q.sub.7, the transistors Q.sub.6 and Q.sub.7 operate at different current densities.

FIG. 3 shows another preferred embodiment of the present invention. The circuit shown in FIG. 3 has a hysteresis characteristic as to an input voltage level at which the state of the output terminal 2 is inverted, to compensate the detection operation with respect to the noise included in the input voltage.

A detection circuit, which detects the input voltage to be detected by comparing the same with the threshold, performs a stable detection operation without being affected by the power supply voltage. Therefore, such a detection circuit is often used in detecting the state of the power supply voltage. More specifically, the power supply voltage falls in a transient state when the power is turned on or off. Accordingly, for instance, in an amplifier circuit, a switching regulator and other electronic circuits, noise signals are sometimes supplied to a load owing to the transient state of the power supply voltage. Hence, it is desirable to deactivate operations of such circuits or to cut the supply of signals to the load in a transient state such as during the turning on or off of the power. For this purpose, the detection circuit shown in FIG. 2 employs a voltage corresponding to the power supply voltage as the input voltage supplied to the terminal 1. In the detection circuit shown in FIG. 2, the voltage corresponding to the power supply voltage is compared with the threshold voltage V.sub.thf expressed by the formula (14), and the state of the output terminal 2 undergoes change according to their relationship of magnitude. The threshold voltage V.sub.thf can be established irrespective to the power supply voltage. Therefore, by establishing such a threshold V.sub.thf as would eliminate the transient state of the power supply voltage, the circuit of FIG. 2 utilizes an output signal from the terminal 2 to control the operations of the above circuits or to cut the supply of signals to the load.

In the detection circuit shown in FIG. 2, however, there is only one threshold V.sub.thf that would invert the state at the output terminal 2. Therefore, when the voltage such as the power supply voltage in the transient state is supplied, the state of the output signal at terminal 2 is inverted periodically due to large noise components. A voltage detection circuit that resolves this problem is shown in FIG. 3.

Turning back to FIG. 3, the same functional elements as those used in FIG. 2 are shown by the same references and their description will be omitted. In FIG. 3 the collector of the transistor Q.sub.8 is not connected to the output terminal 2, but connected to the detection terminal 1 via a resistor 12. The collector of the transistor Q.sub.8 is further connected to the bases of transistors Q.sub.10 and Q.sub.11 via resistors R.sub.14 and R.sub.15, respectively. The emitter of the transistor Q.sub.10 is grounded, while its collector is connected to the output terminal 2. The emitter of the transistor Q.sub.11 is connected to the terminal 3, while its collector is connected to the junction point between the resistors R.sub.10 and R.sub.11 via a resistor R.sub.16.

In the circuit arrangement shown in FIG. 3, when the input voltage is smaller than the threshold, the transistor Q.sub.8 is cut off to turn the transistors Q.sub.10 and Q.sub.11 on. Therefore, the output terminal 2 takes a low level. At this time, the resistor R.sub.16 is grounded by the transistor Q.sub.11. On the other hand, when the input voltage is greater than the threshold voltage, the transistor Q.sub.8 is made to conduct, so that the transistors Q.sub.10 and Q.sub.11 are off. Hence the output terminal 2 is opened. Also, the resistor R.sub.16 is opened.

In a circuit having such an arrangement, if the threshold voltage at the detection terminal 1 appearing when the transistor Q.sub.8 is shifted to its conductive state from a cut-off state is taken as V.sub.thf-1, and that appearing when the transistor Q.sub.8 turns to a closed state from a conductive state as V.sub.thf-2, then V.sub.thf-1 and V.sub.thf-2 can be expressed as follows: ##EQU8## where R.sub.11 // R.sup.16 : parallel resistance value of resistors R.sub.11. and R.sub.16

V.sub.BQ9 : base voltage at transistor Q.sub.9

Here, the base voltage V.sub.BQ9 at the transistor Q.sub.9 is equal in respective cases, and can be given as follows from the formulas (4), (5) and (13). ##EQU9##

Therefore, if the hysteresis width is taken as .DELTA.V.sub.thf, from the formulas (17) and (18) it is possible to establish: ##EQU10## Hence, this hysteresis characteristic becomes advantageous to such a voltage detection circuit that detects the power supply voltage having a transient phonemenon.

Also, with respect to the temperature coefficient of the threshold voltage at the detection terminal 1 in FIG. 3, it is possible to make it small by selecting appropriate values for resistance ratios R.sub.8 /R.sub.9 and R.sub.8 /R.sub.7 and the emitter area ratio of the transistor Q.sub.6 to the transistor Q.sub.7, as already mentioned in the preferred embodiment shown in FIG. 2.

In this embodiment, there is only one output terminal. However, a voltage detection circuit having a plurality of output terminals can be obtained by connecting a plurality of circuits having the same arrangement as that comprised of the resistor R.sub.14 and the transistor Q.sub.10 to the collector of the transistor Q.sub.8 in parallel with the circuit of R.sub.14 and Q.sub.10 and with each other and coupling output terminals to collectors of the respective transistors.