Voltage detection circuit

A voltage detection circuit of the invention is composed of the minimum needed number of circuit elements and that permits the temperature characteristic of the reference level for voltage detection to be set arbitrarily. The voltage detection circuit has a first transistor and a second transistor that have the emitters thereof connected together to form a differential pair, a voltage division circuit that divides the input voltage into a first division voltage and a second division voltage, that is connected directly to the base of the first transistor to apply the first division voltage thereto, and that is connected directly to the base of the second transistor to apply the second division voltage thereto, and a resistor that has one end thereof connected to the base of the second transistor and that has the other end thereof connected to the emitter of the second transistor. Whether the input voltage is equal to a predetermined level or not is checked based on the output from the differential pair.

This application is based on Japanese Patent Application No. 2003-099185 filed on Apr. 2, 2003, the contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a voltage detection circuit for checking whether an input voltage is equal to a predetermined level or not.

2. Description of the Prior Art

FIG. 4shows an example of the configuration of a conventional voltage detection circuit. In the voltage detection circuit shown inFIG. 4, when the voltage Vccapplied to an input terminal5is higher than a predetermined level Vsh, the voltage outputted from a terminal4is equal to the voltage Vcc; when the voltage Vccapplied to the input terminal5is lower than the predetermined level Vsh, the voltage outputted from the terminal4is equal to zero. Moreover, in the voltage detection circuit shown inFIG. 4, the voltage division factor of the voltage division circuit composed of resistors r1to r3and a diode-connected transistor Tr1, the base-emitter voltage of a transistor Tr4, the base-emitter voltage of a transistor Tr5, the resistance of a resistor r4, and the resistance of a resistor r5are so set that the temperature coefficient of the predetermined level Vshis equal to zero. Incidentally, the voltage detection circuit shown inFIG. 4is disclosed in Japanese Patent Registered No. 3218641.

As described above, in the voltage detection circuit shown inFIG. 4, the voltage division factor of the voltage division circuit composed of the resistors r1to r3and the diode-connected transistor Tr1, the base-emitter voltage of the transistor Tr4, the base-emitter voltage of the transistor Tr5, the resistance of the resistor r4, and the resistance of the resistor r5are so set that the temperature coefficient of the predetermined level Vshis equal to zero. This means that the voltage detection circuit shown inFIG. 4is absolutely required to be provided with the resistors r1to r3, the transistor Tr1, the transistor Tr4, the transistor Tr5, the resistor r4, and the resistor r5.

As a result, the voltage detection circuit shown inFIG. 4, in which the temperature coefficient of the predetermined level Vshused as the reference level for voltage detection can be made equal to zero, requires a larger number of circuit elements than a voltage detection circuit in which the reference level for voltage detection varies with temperature. Since an increase in the number of circuit elements constituting a circuit hampers its cost reduction and miniaturization, it is desirable to minimize such an increase in the number of circuit elements used. However, the voltage detection circuit shown inFIG. 4is not composed of the minimum needed number of circuit elements.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a voltage detection circuit that can be composed of the minimum needed number of circuit elements and that permits the temperature characteristic of the reference level for voltage detection to be set arbitrarily.

To achieve the above object, according to the present invention, a voltage detection circuit is provided with: a first transistor and a second transistor that have the emitters thereof connected together to form a differential pair; a voltage division circuit that divides the input voltage into a first division voltage and a second division voltage, that is connected directly to the base of the first transistor to apply the first division voltage thereto, and that is connected directly to the base of the second transistor to apply the second division voltage thereto; and a resistor that has one end thereof connected to the base of the second transistor and that has the other end thereof connected to the emitter of the second transistor. Here, whether the input voltage is equal to a predetermined level or not is checked based on the output from the differential pair.

With this configuration, the temperature characteristic of the predetermined level (the reference level for voltage detection) can be set arbitrarily by appropriately setting the voltage division factor of the voltage division circuit, the base-emitter voltage of the first transistor, the base-emitter voltage of the second transistor, and the resistance of the resistor that has one end thereof connected to the base of the second transistor and that has the other end thereof connected to the emitter of the second transistor. Moreover, since the voltage division circuit is connected directly to the base of the first transistor and to the base of the second transistor, the voltage detection circuit can be composed of the minimum needed number of circuit elements.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1shows an example of the configuration of a voltage detection circuit embodying the invention. The voltage detection circuit shown inFIG. 1is composed of an input terminal1, a constant current source2, an output terminal3, PNP-type transistors Q1to Q3, NPN-type transistors Q4to Q6, and resistors R1to R4.

The input terminal1is connected to the emitter of the transistor Q3, and is also connected through the constant current source2to the emitter of the transistor Q1and to the emitter of the transistor Q2. The collector of the transistor Q3is connected, through a serial circuit composed of the resistors R1to R3, to ground. The collector and base of the transistor Q3are short-circuited together.

The base of the transistor Q1is connected directly to the node at which the resistors R1and R2are connected together, and the base of the transistor Q2is connected directly to the node at which the resistors R2and R3are connected together. The emitter and base of the transistor Q2are connected together through the resistor R4.

The collector of the transistor Q1is connected to the collector of the transistor Q4and to the base of the transistor Q6, and the collector of the transistor Q2is connected to the collector of the transistor Q5. The collector and base of the transistor Q5is short-circuited together, and the base of the transistor Q4and the base of the transistor Q5are connected together. The emitter of the transistor Q4and the emitter of the transistor Q5are connected together and are connected to ground.

The collector of the transistor Q6is connected to the output terminal3, and the emitter of the transistor Q6is connected to ground.

Configured as described above, the voltage detection circuit shown inFIG. 1operates as follows. When the voltage applied to the input terminal1is lower than a threshold level VS, the potential difference across the resistor R2is so low that the transistor Q1is kept on and the transistor Q2is kept off. This keeps the transistor Q6on, and thus the output terminal3is kept at the ground potential. By contrast, when the voltage applied to the input terminal1is higher than the threshold level VS, the potential difference across the resistor R2is so high that both the transistors Q1and Q2are kept on. This keeps the transistor Q6off, and thus the output terminal3is kept in an open state. In this operation of the voltage detection circuit shown inFIG. 1, the threshold level VSserves as a reference level for voltage detection.

Next, a description will be given of the temperature characteristic of the threshold level VS, i.e., the reference level for voltage detection. Here, let the difference between the base-emitter voltage of the transistor Q1and the base-emitter voltage of the transistor Q2as observed when the collector currents of the transistors Q1and Q2are in a state of equilibrium be ΔVBE. The voltage ΔVBEis produced by making the emitter current density of the transistor Q1and the emitter current density of the transistor Q2different from each other. The emitter current densities of the transistors Q1and Q2can be made different, for example, by giving the transistors Q1and Q2different emitter areas.

Let the base-emitter voltage of the transistor Q3be VF1, and let the base-emitter voltage of the transistor Q2be VF2. Let the current that flows through the resistor R2be I1, let the current that flows from the node at which the resistor R4and the base of the transistor Q2are connected together to the node at which the resistors R2and R3are connected together be I2, and let the resistances of the resistors R1, R2, R3, and R4be R1, R2, R3, and R4, respectively.

If it is assumed that the base currents of the transistors Q1and Q2can be ignored, threshold level VS, the current I1, and the current I2are given respectively by Equations (1) to (3) below.
VS=VF1+(R1+R2)·I1+R3·(I1+I2)   (1)
I1=ΔVBE/R2(2)
I2=VF2/R4(3)

When Equations (1) to (3) are combined together, the threshold level VSis given by Equation (4) below.
VS=VF1+(R1+R2)·ΔVBE/R2+R3·(ΔVBE/R2+VF2/R4)  (4)

When the transistors Q2and Q3are given identical characteristics so that their base-emitter voltages VF1and VF2, respectively, are equal, then the equation VF1=VF2=VFholds. Hence, Equation (4) above can be rearranged to Equation (5) below.

When Equation (5) above is partially differentiated with respect to the absolute temperature T, Equation (6) below is obtained.

In Equation (6), the first term of the right side has a positive value, and the second term of the right side has a negative value. Thus, by appropriately setting the base-emitter voltages of the transistors Q1to Q3and the resistances R1to R4, it is possible to set the temperature coefficient ∂VS/∂T of the threshold level VSto be any of a positive arbitrary value, a negative arbitrary value, and zero. Normally, the base-emitter voltages of the transistors Q1to Q3and the resistances R1to R4are so set that the temperature coefficient ∂VS/∂T of the threshold level VSis equal to zero. In a case where the circuit that is connected to the voltage detection circuit shown inFIG. 1has a temperature characteristic, the base-emitter voltages of the transistors Q1to Q3and the resistances R1to R4may be so set that the temperature coefficient ∂VS/∂T of the threshold level VScancels out the temperature characteristic of that circuit.

Next, a description will be given of why it is preferable to use PNP-type transistors as the differential pair transistors for voltage detection (i.e., the transistors Q1and Q2inFIG. 1) and as the diode-connected transistor provided in the voltage division circuit (i.e., the transistor Q3inFIG. 1). When an NPN-type transistor is formed in a low-concentration N-type epitaxial layer, it has a vertical structure in which an emitter layer, a baser layer, and a collector layer are vertically arranged as shown inFIG. 2. By contrast, when a PNP-type transistor is formed in a low-concentration N-type epitaxial layer, it has a horizontal structure in which an emitter layer, a baser layer, and a collector layer are horizontally arranged as shown inFIG. 3. InFIGS. 2 and 3, the following symbols are used: “B” represents a base contact; “C” represents a collector contact; “E” represents an emitter contact; “N+” represents a high-concentration N-type diffusion layer; “N−” represents a low-concentration N-type epitaxial layer; and “P+” represents a high-concentration P-type diffusion layer.

In the NPN-type transistor shown inFIG. 2, since the base layer is a high-concentration P-type diffusion layer, it has a small resistive component. By contrast, in the PNP-type transistor shown inFIG. 3, since the base layer is a low-concentration N-type epitaxial layer, it has a large resistive component. Whereas the resistive component in the base layer exhibits a positive temperature characteristic, the base-emitter junction potential exhibits a negative temperature characteristic. Thus, the temperature-related variation of the base-emitter voltage is smaller in a PNP-type transistor, in which the base layer resistive component is large, than in an NPN-type transistor, in which the base layer resistive component is small. For this reason, it is easier to set the temperature coefficient of the reference level for voltage detection to be equal to zero when PNP-transistors are used as the differential pair transistors for voltage detection (i.e., the transistors Q1and Q2inFIG. 1) and as the diode-connected transistor provided in the voltage division circuit (i.e., the transistor Q3inFIG. 1) than when NPN-type transistor are used instead.

The voltage detection circuit shown inFIG. 1is typically incorporated in a semiconductor integrated circuit device, which is fabricated through a combination of various processes such as film formation, lithography, etching, and impurity doping. Here, it is preferable that the resistors R1to R4be formed simultaneously by the same process. By forming the resistors R1to R4by the same process, even if the individual resistances R1to R4deviate from their design values, it is possible to minimize the deviations in the ratios between them (for example R1/R2). This also helps, as will be clear from Equation (6) noted above, to minimize the deviation in the temperature coefficient ∂VS/∂T of the threshold level VSfrom its set value.

In the voltage detection circuit shown inFIG. 1, between the input terminal1and the resistor R1is provided the transistor Q3having the base and collector thereof short-circuited together. It is, however, also possible to connect the input terminal1directly to the resistor R1and connect the transistor Q3having the base and collector thereof short-circuited together between the resistors R1and R2. It is also possible to use the transistor Q3as the reference source of a current mirror circuit. For example, it is possible to use as the constant current source2a PNP-type transistor that together with the transistor Q3forms a current mirror circuit.