JP2005210641A - Voltage limiting circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a voltage limiting circuit which does not generate any error in an output voltage if an input voltage is lower than or equal to a limit voltage, and is capable of surely limiting the output voltage if the input voltage exceeds the limit voltage. <P>SOLUTION: A voltage limiting circuit 10 comprises: a branch line 20 which is branched from between a voltage input part 12 and an input port 14A which is a voltage output part, in a port input line 16; a first diode 22 which is provided in the branch line 20 forwardly to the downstream side; a clamp line 26 which is connected at the downstream side of the first diode 22 on the branch line 20, for applying a constant voltage (Vdd-Vf) to the connecting portion; and a first resistor 18 which is provided on the port input line 16 at the upper stream than a connecting portion 30 of the clamp line 26 in the branch line 20. The voltage limiting circuit does not generate any error in a port voltage Vp if an input voltage Vin is lower than or equal to the limit voltage, and limits the port voltage Vp to a power supply voltage Vdd if the input voltage Vin exceeds the limit voltage. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a voltage limiting circuit for limiting, for example, a voltage applied to an input port of a microcomputer.

  For example, various sensors are electrically connected to input ports of microcomputers (hereinafter simply referred to as microcomputers) incorporated in control circuits of various devices, and voltage signals are input as output signals of the various sensors. It is like that. A voltage limiting circuit is provided so that a voltage higher than the allowable voltage is not applied to the input port.

  Such a voltage limiting circuit will be described with reference to FIG. One end of a port input line 52 is connected to the input port of the microcomputer 50, and the other end of the port input line 52 is a voltage input unit 54 to which a voltage signal is input from a sensor (not shown). The port input line 52 is provided with a resistor 56, and a branch line 58 is connected to the port input line 52 downstream of the resistor 56 (on the input port side). A Zener diode 60 is provided on the branch line 58 in the reverse direction.

  When the input voltage (reverse voltage with respect to the zener diode 60) Vin of the voltage input unit 54 is equal to or lower than the zener voltage Vz, the zener diode 60 hardly causes a current (reverse current) to flow through itself. In order to prevent current from flowing to 58, the port voltage Vp applied to the input port of the microcomputer 50 (that is, the output voltage of the voltage limiting circuit) substantially matches the input voltage Vin of the voltage input unit 54. That is, as shown in FIG. 4, in the region where the input voltage Vin is equal to or lower than the zener voltage Vz, the port voltage Vp increases as the input voltage Vin increases.

  On the other hand, when the input voltage Vin of the voltage input unit 54 is larger than the Zener voltage Vz, the current flows through the Zener diode 60, that is, the branch line 58, so that the port voltage Vp is almost limited to the Zener voltage Vz (clamped). ) Therefore, if the Zener voltage Vz is set to the allowable input voltage Vpa of the input port, the input port is protected against overvoltage.

  However, because of the characteristics of the Zener diode 60, even if the input voltage Vin is in the region of the Zener voltage Vz or less, a minute current flows and a minute current flows through the branch line 58. Also, a voltage lower than the input voltage Vin is input to the input port of the microcomputer 50 due to a voltage drop due to the resistor 56 located upstream. That is, there is a problem that the Zener diode 60 causes an error in the port voltage Vp with respect to the input voltage Vin (sensor output) that should be input.

  As a countermeasure, when the Zener voltage Vz of the Zener diode 60 is set higher than the allowable input voltage Vpa of the input port, the port voltage Vp is set to the allowable input in a region where the input voltage Vin is relatively low as shown by a broken line in FIG. The voltage Vpa is exceeded, and there is a high possibility that the input port of the microcomputer 50 is destroyed. Even when the Zener voltage Vz is set to match the allowable input voltage Vpa of the input port, the output voltage gradually increases, that is, monotonously with the increase of the input voltage Vin exceeding the Zener voltage due to the characteristics of the Zener diode described above. Therefore, when the input voltage Vin is higher than the Zener voltage Vz, the port voltage Vp exceeds the allowable input voltage Vpa, and the input port may be destroyed.

  As described above, in the conventional voltage limiting circuit using the Zener diode 60, the error of the port voltage Vp when the input voltage Vin of the voltage input unit 54 is equal to or lower than the Zener voltage Vz and the voltage input unit 54 It has been difficult to achieve both overvoltage protection of the input port when the input voltage Vin exceeds the Zener voltage Vz.

  In consideration of the above fact, the present invention can reliably limit the output voltage when the input voltage exceeds the limit voltage without causing an error in the output voltage when the input voltage is equal to or lower than the limit voltage. The purpose is to obtain a voltage limiting circuit.

  In order to achieve the above object, the voltage limiting circuit according to the first aspect of the present invention is branched from between the voltage input portion and the voltage output portion of the voltage application line, and energization to the opposite side of the branch portion is allowed. A branch line provided with a first diode, a first resistor provided upstream of the branch line in the voltage application line, and a downstream of the first diode in the branch line, and connected to the connection portion A clamp line for applying a constant voltage.

  In the voltage limiting circuit according to claim 1, a voltage (hereinafter referred to as an input voltage) input to a voltage input unit of the voltage application line is a voltage obtained by adding a voltage drop of the first diode to a constant voltage of the clamp line (hereinafter, If the voltage is equal to or lower than the limit voltage, a constant voltage equal to or higher than the voltage obtained by subtracting the voltage drop of the first diode from the input voltage is applied to the clamp line connection portion (downstream of the first diode) in the branch line. No current flows through the first diode, that is, the branch line, and no voltage drop occurs in the voltage application line. For this reason, in the range where the input voltage is equal to or lower than the limit voltage, the input voltage is directly used as the output voltage of the voltage output unit, and an error in the output voltage with respect to the input voltage is prevented.

  On the other hand, when the input voltage is higher than the limit voltage, a current flows through the first diode, that is, the branch line. At this time, the voltage of the clamp line connecting portion in the branch line, that is, the voltage on the downstream side (opposite to the voltage application line) of the first diode is limited to a constant voltage of the clamp line. Since the voltage obtained by subtracting the forward voltage of the first diode from the first diode is clamped to a constant voltage of the clamp line corresponding to the Zener voltage, the output voltage of the voltage output unit in the voltage application line is set to the constant voltage. It corresponds to the voltage (hereinafter referred to as the limit voltage) to which the voltage drop is added. The constant voltage is applied from the outside of the branch line via the clamp line and does not fluctuate due to fluctuations in the input voltage. Therefore, the output voltage does not fluctuate even if the input voltage changes within a predetermined range. . Therefore, when the input voltage is within a predetermined range exceeding the limit voltage, the output voltage is prevented from exceeding the limit voltage.

  The predetermined range of the input voltage that can limit the output voltage in this way is a range in which the voltage applied to the clamp line connection portion in the branch line based on the input voltage is equal to or lower than the above-described constant voltage (the current is applied to the clamp line). It is set by providing a first resistor upstream of the branch line in the voltage application line. That is, the output voltage may be limited to the limit voltage as described above in a range where the voltage based on the input voltage after the current flows through the first diode and the voltage is dropped by the first resistor does not exceed the constant voltage of the clamp line. it can. Therefore, the input voltage range in which the output voltage is limited can be set to a desired range according to the resistance value of the first resistor. The first resistor only needs to fulfill the function of causing a voltage drop when a current flows, and may perform another function that does not hinder the above-described effects.

  Thus, in the voltage limiting circuit according to the first aspect, the output voltage is reliably limited when the input voltage exceeds the limit voltage without causing an error in the output voltage when the input voltage is equal to or lower than the limit voltage. be able to.

  A voltage limiting circuit according to a second aspect of the present invention is the voltage limiting circuit according to the first aspect, wherein the clamp line has substantially the same electrical characteristics as the first diode and is connected to the branch line. A second diode that allows energization is provided.

  In the voltage limiting circuit according to claim 2, since the second diode having substantially the same electrical characteristics as the first diode is provided in the clamp line, the forward voltage drop of the second diode is reduced from the voltage applied to the clamp line. The subtracted voltage is applied as a constant voltage to the connection site with the branch line. For this reason, the forward voltage drop of the first diode is offset by the forward voltage drop of the second diode. In addition, since the temperature characteristics of the first diode and the second diode are substantially the same, if they are arranged close to each other so as to operate in an almost isothermal environment, the temperature characteristics of the first diode are changed to the second diode. It is offset by the temperature characteristics of the diode. Therefore, the voltage applied to the clamp line may be set to match the desired limit voltage without considering the electrical characteristics (temperature characteristics) of the first diode, and the design of the voltage limit circuit is easy.

  A voltage limiting circuit according to a third aspect of the present invention is the voltage limiting circuit according to the first or second aspect, wherein the electric resistance value is lower than the first resistance downstream of the connection portion of the clamp line in the branch line. Is provided with a small second resistor.

  In the voltage limiting circuit according to the third aspect, the voltage at the connection portion of the clamp line in the branch line is held by the second resistor. And since the resistance value of this 2nd resistance is smaller than the resistance value of a 1st resistance, the range of the input voltage which can restrict | limit an output voltage to a limiting voltage is wide. That is, in the clamp line connection part of the branch line, the upper limit of the input voltage at which the voltage based on the input voltage after the voltage drop due to the first resistor is higher than the constant voltage is increased.

  More specifically, the resistance value of the first resistor is R1, the resistance value of the second resistor is R2, the constant voltage applied to the branch line through the clamp line is Vc, and the voltage drop of the first diode is Vf. Since the upper limit Vin of the input voltage is expressed by Vin = Vc × (R1 + R2) / R2 + Vf, the upper limit Vin of the input voltage can be set higher by reducing the resistance value R2 of the second resistor. . As a result, the output voltage can be limited to the limit voltage in a wide fluctuation range of the input voltage.

  As described above, the voltage limiting circuit according to the present invention reliably limits the output voltage when the input voltage exceeds the limiting voltage without causing an error in the output voltage when the input voltage is equal to or lower than the limiting voltage. It has an excellent effect of being able to.

  An example in which the voltage limiting circuit according to the embodiment of the present invention is applied to an input port of a microcomputer will be described with reference to FIGS.

  A circuit diagram of the voltage limiting circuit 10 is shown in FIG. As shown in this figure, the voltage limiting circuit 10 is applied to a port input line 16 as a voltage application line for connecting a voltage input unit 12 to which an output voltage of a sensor or the like (not shown) is applied and an input port 14A of the microcomputer 14. The port voltage Vp input to the input port 14A is limited to the allowable input voltage Vpa or less. Therefore, the connection portion of the port input line 16 with the input port 14A of the microcomputer 14 corresponds to the “voltage output portion” in the present invention. Hereinafter, the voltage limiting circuit 10 will be specifically described.

  The voltage limiting circuit 10 includes a first resistor 18 provided between the voltage input unit 12 and the input port 14 </ b> A in the port input line 16. Further, a branch line 20 is branched from the first resistor 18 in the port input line 16, that is, from the input port 14 </ b> A side with respect to the first resistor 18. The branch line 20 is provided with a first diode 22 and a second resistor 24 disposed downstream of the first diode 22.

  The first diode 22 is provided in the forward direction toward the downstream side of the branch line 20. As a result, a current can flow through the branch line 20 from the connection side (upstream) to the port input line 16 to the second resistor 24 side (downstream). The downstream end of the branch line 20 is grounded (not shown).

  Further, one end (downstream end) of a clamp line 26 serving as a clamp line is connected between the first diode 22 and the second resistor 24 in the branch line 20. A power supply voltage Vdd that is a constant voltage is applied to the other end of the clamp line 26. This power supply voltage Vdd is set in accordance with the allowable input voltage Vpa of the input port 14A of the microcomputer 14, and is applied from a power supply line common to the power supply voltage Vdd of the microcomputer 14 in this embodiment. In the following description, the connection part of the clamp line 26 in the branch line 20 is referred to as a clamp connection part 30.

  A second diode 28 is provided in the clamp line 26 in the forward direction toward the downstream. Accordingly, the clamp line 26 is configured to allow a current to flow from the power supply voltage Vdd side (upstream) to the clamp connection unit 30 side (downstream), and this current flows to the second resistor 24 of the branch line 20. It is like that. That is, the voltage of the clamp connection part 30 is held by the second resistor 24. In addition, since the first diode 22 is provided in the forward direction in the branch line 20, the current flowing through the clamp line 26 does not flow backward. Similarly, since the second diode 28 is provided in the forward direction in the clamp line 26, the current from the branch line 20 does not flow backward.

  The electrical characteristics including the temperature characteristics of the second diode 28 are substantially the same as the electrical characteristics including the temperature characteristics of the first diode 22. The first diode 22 and the second diode 28 are arranged in close proximity so that the operating temperatures are equal. In the present embodiment, the forward voltage drops Vf of the first diode 22 and the second diode 28 are both 0.6V.

  In the present embodiment, the resistance value R1 of the first resistor 18 is 20 KΩ, and the resistance value R2 of the second resistor 24 is 1 KΩ, which is sufficiently smaller than the resistance value R1 of the first resistor 18. Further, in the present embodiment, the power supply voltage Vdd, that is, the allowable input voltage Vpa of the input port 14A is constant at 5V.

  Next, the operation of the present embodiment will be described.

  In the voltage limiting circuit 10 configured as described above, an output voltage of a sensor or the like is input to the voltage input unit 12 of the port input line 16. In the region where the input voltage Vin is applied to the clamp line 26, that is, the allowable input voltage Vpa or less of the input port 14A, the input voltage Vin is supplied from the power supply voltage Vdd to the clamp connection 30 in the branch line 20 via the clamp line 26. Since a voltage (Vdd−Vf (≧ Vin−Vf)) obtained by subtracting the forward voltage drop Vf of the second diode 28 is applied, current flows through the branch line 20 as in the configuration using the conventional Zener diode. There is no end.

  Therefore, no current flows through the first resistor 18 of the port input line 16, and the input voltage Vin is input to the input port 14A of the microcomputer 14 as it is. That is, the port voltage Vp input to the input port 14A matches the input voltage Vin. Therefore, as shown in FIG. 2, in the region A where the input voltage Vin is equal to or lower than the power supply voltage Vdd, the port voltage Vp increases linearly as the input voltage Vin increases over the entire range of the region A. An error with respect to the input voltage Vin (sensor output or the like) is prevented from occurring in Vp.

  On the other hand, when the input voltage Vin is larger than the power supply voltage Vdd, a current flows through the first diode 22. In a region where the voltage of the clamp connection unit 30 based on the input voltage Vin is equal to or lower than the voltage of the clamp connection unit 30 based on the power supply voltage Vdd, a state in which a current flows also to the second diode 28 is maintained. Specifically, the voltage of the clamp connection 30 based on the input voltage Vin is (Vin−Vf) × R2 / (R1 + R2), and the voltage (Vdd−Vf) of the clamp connection 30 based on the power supply voltage Vdd. Therefore, in a region satisfying the following expression (1), a state where current flows through both the first diode 22 and the second diode 28 is maintained.

Vdd <Vin ≦ (Vdd−Vf) × (R1 + R2) / R2 + Vf (1)
In this region, the voltage of the clamp connection portion 30 is held at a constant voltage (Vdd−Vf) obtained by subtracting the forward voltage drop of the second diode 28 from the power supply voltage Vdd. In other words, from the input voltage Vin, Since the voltage (Vin−Vf) obtained by subtracting the forward voltage drop Vf of the diode 22 is clamped to the constant voltage (Vdd−Vf) corresponding to the Zener voltage, the port voltage Vp matches the power supply voltage Vdd. More specifically, the voltage (Vin−Vf) based on the input voltage is clamped to the constant voltage (Vdd−Vf) in the clamp connection unit 30 to be Vin−Vf = Vdd−Vf. 22 and the forward voltage drop Vf of the second diode 28 are canceled out to Vin = Vdd, and the port voltage Vp (= Vin) coincides with the power supply voltage Vdd.

  Since the power supply voltage Vdd is externally applied to the clamp connection portion 30 in the branch line 20 via the clamp line 26, the power supply voltage Vdd does not fluctuate due to fluctuations in the input voltage Vin unlike a conventional Zener diode. Therefore, as shown in FIG. 2, in the region B satisfying the above formula (1), the port voltage Vp is limited to the power supply voltage Vdd, that is, the allowable input voltage Vpa with high accuracy, and the allowable input that matches the power supply voltage Vdd. It is reliably prevented that the voltage Vpa is exceeded. Therefore, in the region B, the port voltage Vp does not exceed the allowable input voltage Vpa of the input port 14A, and the input port 14A of the microcomputer 14 is protected against overvoltage.

  Further, the range of the input voltage Vin that can limit the port voltage Vp to the power supply voltage Vdd is as follows. The resistance value R1 of the first resistor 18 is 20 KΩ, the resistance value R2 of the second resistor 24 is 1 KΩ, the power supply voltage Vdd is 5 V, When the forward voltage drop Vf = 0.6V of the first diode 22 is substituted into the equation (1) and calculated, 5V <Vin ≦ 93V is obtained. Therefore, the input port 14A is protected against overvoltage even when an input voltage Vin that is sufficiently higher than the allowable input voltage Vpa is applied.

  As described above, for example, when a vehicle battery voltage or the like is input to the voltage input unit 12 as a sensor output, the input voltage Vin is accurately input as the port voltage Vp when the input voltage Vin is in the range of 0V to 5V. Even when a battery voltage exceeding 5V is applied to the voltage input unit 12 for some reason, the port voltage Vp is set to the power supply voltage Vdd set in accordance with the allowable input voltage Vpa in a wide range of the input voltage Vin from 5V to 93V. The input port 14A of the microcomputer 14 is protected against overvoltage.

  When Vin> (Vdd−Vf) × (R1 + R2) / R2 + Vf (93V in the present embodiment), a state in which a current flows through the first diode 22 is maintained, while the second diode 28, that is, the clamp line 26, The current stops flowing (the second diode 28 is cut off), and the port voltage Vp increases with the increase of the input voltage Vin and exceeds the power supply voltage Vdd (= 5V) as in the region C shown in FIG. I understand that. Specifically, in the region C, the port voltage Vp is determined according to the voltage division ratio between the first resistor 18 and the second resistor 24, and in this embodiment, the forward voltage drop Vf of the first diode 22 is taken into consideration. Thus, it is expressed by the following formula (2), which exceeds the limit voltage (5 V) of the input port.

Vp = (R2 × Vin + R1 × Vf) / (R1 + R2) (2)
That is, if Vin = 93V is substituted into the equation (2), Vp = 5V is obtained. When Vin exceeds 93V, the port voltage Vp exceeds the set power supply voltage Vdd (= 5V) in accordance with the allowable voltage. You can see that.

  As described above, in the voltage limiting circuit 10 according to the present embodiment, the constant voltage based on the power supply voltage Vdd is applied to the downstream of the first diode 22 in the branch line 20 via the clamp line 26. When the input voltage Vin is equal to or lower than the power supply voltage Vdd that matches the allowable input voltage Vpa, the port voltage Vp is not limited, and when the input voltage Vin exceeds the power supply voltage Vdd, the port voltage Vp is surely limited. Can do.

  In particular, since the resistance value R2 of the second resistor 24 is sufficiently smaller than the resistance value R1 of the first resistor 18, the port voltage Vp calculated by the equation (1) matches the allowable input voltage Vpa. The range of the input voltage Vin that can be limited (clamped) to the power supply voltage Vdd or less, that is, the region B can be set wide. That is, the upper limit of the region B can be set higher than a configuration in which the port voltage gradually increases (monotonically increases) as the input voltage increases using a Zener diode as in the prior art. The input port 14A of the microcomputer 14 can be reliably protected against the input voltage Vin.

  In addition, since the second diode 28 provided in the clamp line 26 has substantially the same electrical characteristics as the first diode 22 of the branch line 20, in the region B, the forward voltage drop Vf of the first diode 22 is It is canceled out by the forward voltage drop Vf of the second diode 28, and the port voltage Vp is limited to the power supply voltage Vdd. That is, without considering the forward voltage drop Vf of the first diode 22, the voltage applied to the clamp line 26 (the power supply voltage Vdd in this embodiment) is set to match the allowable input voltage Vpa of the input port 14A. The circuit design of the voltage limiting circuit 10 is easy. In addition, since the first diode 22 and the second diode 28 are arranged close to each other and operate in an almost isothermal environment, the temperature characteristics are canceled out. Therefore, the temperature characteristics of the first diode 22 and the second diode 28 Therefore, the port voltage Vp can be accurately limited to the power supply voltage Vdd. In addition, it is easy to design the voltage limiting circuit 10 that can accurately limit the port voltage Vp to the power supply voltage Vdd.

  In the above embodiment, the voltage limiting circuit 10 is applied to the port input line 16 connected to the input port 14A of the microcomputer 14. However, the present invention is not limited to this, and the output voltage (port The present invention can be applied to any application that limits the voltage Vp). In addition, the voltage limiting circuit 10 is not limited to the application used across the area A and the area B shown in FIG. 2. For example, the application used only in the area A or the area B, the area B and the area C, It is also possible to apply to applications that are used across the board.

  In the above embodiment, the second diode 28 is provided in the clamp line 26. However, the present invention is not limited to this. For example, the clamp line 26 in which the second diode 28 is not provided is allowed. A voltage corresponding to a voltage (Vpa−Vf) obtained by subtracting the forward voltage drop Vf of the first diode 22 from the input voltage Vpa may be applied. Instead of the second diode 28, a resistor or the first diode 22 May be provided with diodes having different electrical characteristics.

  Further, the present invention is not limited by the above-described electrical characteristics of the first diode 22, the second diode 28, the first resistor 18, and the second resistor 24, the allowable input voltage Vpa, and the like. Needless to say, it can be implemented by appropriately changing the above. Therefore, the present invention is not limited to a preferable configuration in which the resistance value R2 of the second resistor 24 is smaller than the resistance value R1 of the first resistor 18. Further, the present invention is not limited to the configuration in which the voltage of the clamp connection portion 30 is held by the second resistor 24.

It is a circuit diagram of the voltage limiting circuit which concerns on embodiment of this invention. It is a diagram which shows the relationship between the input voltage of the voltage limiting circuit which concerns on embodiment of this invention, and the port voltage which is an output voltage. It is a circuit diagram of the conventional voltage limiting circuit. It is a diagram which shows the relationship between the port voltage which is the conventional input voltage and output voltage.

Explanation of symbols

10 Voltage Limiting Circuit 16 Port Input Line (Voltage Application Line)
18 First resistor 20 Branch line 22 First diode 24 Second resistor 26 Clamp line 28 Second diode

Claims (3)

A branch line provided with a first diode that branches from between a voltage input part and a voltage output part in the voltage application line, and that allows energization to the opposite side of the branch part;
A first resistor provided upstream of the branch line in the voltage application line;
A clamp line connected downstream of the first diode in the branch line for applying a constant voltage to the connection;
Voltage limiting circuit with   2. The second diode according to claim 1, wherein the clamp line is provided with a second diode that has substantially the same electrical characteristics as the first diode and allows energization to a connection side with the branch line. Voltage limiting circuit.   3. The voltage limiting circuit according to claim 1, wherein a second resistor having an electric resistance value smaller than that of the first resistor is provided downstream of the connection portion of the clamp line in the branch line.

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