JPH10108359A - Input protective circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an input protective circuit which avoids the delay of a signal or the influence on the signal value, while protecting an internal circuit from high voltage. SOLUTION: A channel is turned on and it can transmit a signal when the input voltage from outside is at a specified value or under, by connecting the drain electrode of a depression type n-channel JFET 6 whose gate electrode is earthed to an external input terminal 3, and the source electrode to an electrode 2 for input of an internal circuit 1, and when the drain potential of the external input terminal 3, that is, the drain potential of the JFET 6 goes up, the channel closes, and the source of the JFET 6, that is, an internal circuit 1 is protected from high voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an input protection circuit for an overvoltage.

[0002]

2. Description of the Related Art As an input protection circuit for a semiconductor integrated circuit such as a general OP amplifier (operational amplifier), FIG.
There is something like 2. In the figure, 1 is an internal circuit, 2 is its input electrode, 3 is an external input terminal, 4 is a protection resistor, and 5 is a constant voltage diode. The resistance drawn inside the internal circuit 1 indicates the input impedance, and generally has a very high value. Such an overvoltage protection circuit is generally used.

The operation will be described. For example, if the breakdown voltage of the constant voltage diode 5 is 5 V, 15
Even if V is applied, no voltage of 5 V or more is applied to the internal circuit 1. In principle, the resistance value R of the protection resistor 4 may be zero. However, actually, when a breakdown voltage flows through the constant voltage diode 5, heat is generated, and when the amount of current is excessive, the diode burns out. Therefore, it is necessary to appropriately set the resistance value R in consideration of a possible overvoltage value.

[0004] Such a protection circuit is effective when the input potential is relatively low or high, but is instantaneous. However, when it is necessary to protect the internal circuit from a situation where a very high voltage is continuously applied for a relatively long time, there are the following problems.

The lower limit of the resistance value R of the protection resistor 4 depends on the maximum voltage applied to the external input terminal 3 and the allowable heat amount of the constant voltage diode 5, that is, the allowable current amount at the time of breakdown. Assuming that the allowable heat quantity of the constant voltage diode 5 is Wz [W] and the breakdown voltage is Vz [V], the allowable current value Iz is determined by the following equation (1).

Wz = Iz · Vz (Equation 1) Then, assuming that the difference between the maximum voltage applied to the external input terminal 3 and Vz is Vr, the resistance value R of the protection resistor 4 is represented by the following (Equation 2) Must be satisfied.

Vr / R ≦ Iz (Equation 2) The resistance value R is preferably large in order to suppress the amount of heat generation, but is preferably as small as possible in consideration of the influence on the internal circuit 1. The internal circuit 1 is, for example, an input terminal of an OP amplifier, and generally has an extremely high input impedance. For example, if the input impedance is 10 MΩ and the effect on the input signal of 1% is acceptable, the above-mentioned resistance value R is 1
Up to 00 kΩ can be tolerated. However, when the voltage (Vr + Vz) applied to the external input terminal 3 is by far larger than the power supply voltage used in a normal IC circuit, for example, about 300 V, and is applied for a relatively long time, (Iz) to satisfy the expression (2)
Must be increased, and a diode having a large area must be used. Calculating the above example, when 300 V is applied to the external input terminal 3, the value of the current flowing through the protection resistor 4 is about 30 mA. In general, a discrete diode does not have good heat dissipation, so the area of a chip through which such a current can flow becomes large.

Further, even if a diode having a large area can be used, the following problem occurs. When the constant voltage diode 5 clamps the voltage, a depletion layer is formed inside the diode, and charges are accumulated in the depletion layer. When the potential of the external input terminal 3 drops rapidly, this charge is discharged through the protection resistor 4. In this process, the diode equivalently functions as a capacitor, but the accumulated charge is large and the equivalent capacitance is large. That is, the potential at the connection point between the constant voltage diode 5 and the internal circuit 1 does not follow the potential change at the external input terminal 3 and only changes slowly with a large CR time constant.

This will be described with an example. The constant voltage diode 5 that allows this breakdown current is, for example,
Assuming a millimeter disk shape, the amount of charge stored in the diode by a reverse bias of 5 V is about 7 nC. Therefore, the equivalent capacitance is 1.4 nF from the relationship of Q = CV.
And the CR product is 140 μs. That is, even if the potential of the external input terminal 3 changes from 5V to 0V, the input electrode 2
It takes a lot of 300 μs for the potential of 0 V to reach 0V. This CR product depends on the applied voltage and the input impedance of the internal circuit. If the resistance R is increased, the area of the diode can be reduced, but the CR product is approximately constant and does not decrease.

[0010]

As described above, if the conventional means tries to protect the internal circuit from a very high voltage, the problem is that the protection circuit itself inhibits transmission of external signals to the internal circuit. There was a point. The present invention has been made in view of the above-described problems, and has as its object to provide an input protection circuit that protects an internal circuit from a high voltage while avoiding an influence on a signal delay and a signal value.

[0011]

In order to achieve the above object, the present invention has the following arrangement. That is,
According to the first aspect of the present invention, a voltage control transistor (for example, a depletion type n
One main electrode (eg, a drain electrode) of the channel JFET) is connected, and the other main electrode (eg, a source electrode) of the transistor is connected to an input electrode of an internal circuit. Further, the control electrode (the gate electrode in this case) of the transistor is held at a predetermined fixed potential (for example, the ground potential in FIG. 1 and + V in FIG. 5). This corresponds to FIGS. 1 and 5 described later.

The operation of such a configuration will be described. Following the above example, the voltage-controlled transistor is an n-channel depletion-type JFET. It is also assumed that the threshold value is -5 V and the gate electrode is grounded. First, when the potential of the external input terminal, that is, the drain of the transistor connected to the external input terminal is grounded or a relatively low positive potential, the channel of the transistor is sufficiently open and the potential of the source electrode is the same as the potential of the drain electrode. become. For example, if the drain potential is +2 V, the source potential is also +2 V, and therefore, the potential difference between the gate and the source is -2 V. Since this is equal to or larger than the threshold value, the gate is open.

When the potential of the external input terminal, that is, the potential of the drain electrode approaches +5 V, the potential of the source electrode also approaches 5 V, the potential difference between the gate and the source approaches -5 V, and the resistance between the drain and the source increases. When the potential of the drain electrode exceeds +5 V, the state between the drain and the source is cut off, so that the potential of the source electrode, that is, the input electrode of the internal circuit does not fluctuate, and is maintained at +5 V in principle. When the potential of the external input terminal falls below +5 V again, the drain-source state of the transistor becomes conductive again, and the potential of the input electrode immediately follows the potential of the external input terminal.

Next, in the invention according to claim 2,
When a potential having at least a first polarity is applied between the external input terminal and the input electrode of the internal circuit in both directions (for example, a current flows from the external input terminal to the input electrode). In the flowing direction), a constant current diode having a property of limiting the flowing current to a certain value or less is connected. Further, even if a voltage of a predetermined value or more of a first polarity is applied to the external input terminal, a constant voltage diode is applied to the input electrode so that a potential of the predetermined value or more is not applied to the input electrode. (Eg, anode terminal) is connected, and the second terminal (here, cathode terminal) of the constant voltage diode is held at a predetermined fixed potential (eg, ground potential). Note that this corresponds to FIG. 7 described later.

The operation of such a configuration will be described.
For example, when the clamp voltage of the constant voltage diode is 5 V
If the potential of the external input terminal varies from 0 to 5 V when the potential of the external input terminal varies, the potential of the input electrode also varies. At this time, since the input electrode has high impedance, no appreciable current flows through the constant current diode. When the potential of the external input terminal exceeds 5 V, the potential of the input electrode does not rise to 5 V or more because the constant voltage diode bypasses the current.
At this time, the current flowing through the constant voltage diode is limited by the action of the constant current diode. When the potential of the external input terminal falls below 5 V again, the electric charge accumulated in the constant voltage diode is quickly discharged through the constant current diode, so that the potential of the input electrode follows without delay.

[0016] According to the third aspect of the present invention,
In addition to the configuration of claim 1 or claim 2, by limiting the current not to exceed a certain value even when a second polarity potential (for example, a negative potential) is applied to the external input terminal. In order to protect the circuit, between the transistor or the constant current diode and the external input terminal,
The configuration is such that a second constant current diode is inserted. In addition,
This corresponds to FIGS. 9 and 10 described later.

The operation of such a configuration will be described.
According to the above example, the input electrode of the internal circuit is 0
It is protected to be in the range of up to 5V. However, when the potential of the external input terminal becomes a negative potential, the pn junction in the protection circuit is forward-biased in both the configuration of claim 1 and the configuration of claim 2 and a large current flows in the protection circuit itself. Will flow. Therefore, if the second constant current diode is inserted in this way, although the pn junction of the transistor or the constant voltage diode is forward-biased, a large current does not flow and the circuit is not broken by heat generation.

Next, in the invention according to claim 4,
The external input terminal is connected to one end of a pair of constant current diodes connected in series with each other connecting anodes or cathodes, and the other end is connected to the input electrode of an internal circuit. Further, one end of a pair of constant-voltage diodes connected in series, in which anodes or cathodes are connected to each other, is connected to the input electrode, and the other end is maintained at a predetermined fixed potential (for example, ground potential). . This is described later,
This corresponds to FIG.

The operation of such a configuration will be described.
With this configuration, the internal circuit is simultaneously protected from the high voltage of the second polarity, and the external input terminal is connected to the high voltage of the second polarity. When the potential is within a predetermined range, a voltage signal is transmitted to the input electrode without any trouble.

[0020]

As described above, according to the present invention, the input electrode of a delicate circuit such as a normal IC can be protected from a very high voltage, and furthermore, the protection circuit itself operates for input operation. Has little effect on

Further, according to the first aspect of the present invention,
Little energy is lost during protection. further,
According to the second aspect of the present invention, although some energy is lost during protection, the purpose can be realized with a simple element configuration. Further, according to the third aspect of the present invention, the internal circuit can be protected against a voltage opposite to the normal polarity of the input signal. Furthermore, according to the invention described in claim 4, even in a circuit in which an input signal has both positive and negative polarities, the internal circuit can be protected from any positive or negative overvoltage.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to embodiments. FIG. 1 shows a first embodiment of the present invention, and corresponds to claim 1 described above. FIGS. 2 to 4 are diagrams schematically showing states inside the transistor in the operation state of the circuit of FIG.

The configuration will be described. In the figure, 1 is an internal circuit, 2
Is an input electrode, 3 is an external input terminal, and 6 is a voltage-controlled transistor, which is a depletion-type n-channel JFET here. The drain electrode of the transistor 6 is connected to the external input terminal 3, and the source electrode is connected to the input electrode 2 of the internal circuit. Here, the gate electrode is grounded.

The operation will be described. It is assumed that the threshold value of the transistor 6, that is, the potential difference between the gate and the source is -5V. First, when the external input terminal 3 is 5 V or less, for example, 2 V, the channel of the transistor is sufficiently open, so that there is almost no potential difference or delay between the drain electrode and the source electrode. FIG. 2 schematically shows the internal state of the transistor 6 in this state. In FIG. 2, 10 is an n-type source region, 11 is an n-type drain region, 12 is a p-type gate region, and 13 is an n-type drift region. The above-mentioned source electrode has ohmic contact with the source region 10 and the drain electrode has ohmic contact with the drain region 11, but is not shown. Note that the broken line in the figure indicates the end face of the depletion layer. The drift region 13 sandwiched between the two p-type gate regions 12 is a channel. In this state, the channel is not covered by the depletion layer but is open.

When the potential of the drain electrode fluctuates, it is actually affected by the fluctuation of the source electrode potential to charge / discharge the depletion layer capacitance of the pn junction in the figure. Therefore, the equivalent capacitance of the depletion layer at the channel pn junction of JFET 6 in this state is roughly estimated. Withstand voltage of JFET is 50
If the semiconductor is silicon at 0 V, the drift region 13
Required impurity concentration is 5.4 × 10 ¹⁴ / cm ² and about 8Ω
Cm and the thickness required to ensure the pressure resistance is about 30 μm. Therefore, the resistance of the main current path is 0.0
It becomes 26 Ω · cm ² . Since the area of the pn junction of the gate is on the order of several times the active area of the transistor, if the thickness of the depletion layer near the typical p-type gate region in the ON state is 1 μm, the capacitance is 10 nF / cm ² . It is about several times. Therefore, the order of the CR product is at most nanosecond.

When the drain potential rises and approaches 5 V, the source potential also approaches 5 V. Then, the gate-source potential difference also approaches -5 V, and the channel shifts to the cutoff state. FIG. 3 shows that the drain potential is 5 V
7 schematically shows the internal state of the transistor when the state becomes. The numbers in FIG. 3 indicate the same as those in FIG. In FIG. 3, the depletion layer fills the drift region 13 to block the channel. Even if the drain potential further rises, the source potential does not fluctuate in principle because the channel is in the cutoff state, and the source potential does not exceed the threshold value in principle. For example, if the withstand voltage of the transistor 6 is 5
If the voltage is 00 V or more, the input electrode 2 such as an IC gate electrode can be protected from a high voltage even if the external input terminal 3 becomes 500 V.

If the source potential becomes 5 V or more, the channel is more firmly cut off, so that no new charge is supplied. Furthermore, since a leakage current always exists in the reverse-biased pn junction between the gate and the source, and the impedance of the internal circuit is not completely infinite,
The potential is decreasing. When the source potential falls slightly below 5 V, the channel is slightly opened to supply electric charge, and the source potential rises again. Therefore, the source potential is settled to the threshold value of 5V.

When the drain potential drops from this state and falls below 5 V again, the potential difference between the gate and the drain falls below the threshold value. FIG. 4 schematically shows the internal state of the transistor at this moment. The numbers in FIG.
Indicates the same as The channel is open at the portion of the channel depletion layer facing the drain side, and remains closed at the portion facing the source. That is, since this is the pinch-off state of the reverse transistor, the charge can move from the drain to the source, and the source potential decreases soon to reduce the potential in FIG.
State. The delay time is determined by the impedance of the internal circuit 1.

The above configuration can be similarly applied to a case where a negative potential is used as an input signal by using a depletion-type p-channel JFET.

FIG. 5 shows an example using an enhancement type n-channel MOSFET. In the figure, 7 is MOSF
At ET, the gate is held at a potential (+ V) higher than the threshold. Reference numeral 51 denotes a constant voltage diode, and the same reference numerals as those in FIG. 1 denote the same components.

The operation will be described. The threshold value of the MOSFET 7 is +5 V between the gate and the source. Now the gate is +
It is assumed that the voltage is clamped at 10V. Drain potential is 5
When the voltage is less than V, the potential difference between the gate and the source of the MOSFET 7 becomes 5 V or more, so that the channel is opened and the source and the drain can have the same potential. As the drain potential rises, the source potential also rises. However, when doing so, the potential difference between the gate and the source is reduced and the channel is closed. Therefore, as in the case of FIG. 1, the source potential does not exceed 5 V. This state is the same even when the drain potential becomes several hundred volts.

Next, FIG. 6 shows the operation of the MOSFET 7 in order to explain the operation in which the drain potential drops again and the channel opens.
Is a schematic view of the structure of FIG. 14 in the figure
Is a p-type base region, and 15 is an insulated gate electrode. The periphery of the insulated gate electrode 15 is covered with an insulating film 16. Other reference numerals indicate the same as those in FIG. The base region 14 is kept at the same potential as the source region 10. In the drawing, a broken line between the p-type base region 14 and the insulated gate electrode 15 indicates the presence of a channel during conduction. When the channel was shut off, the n-type source region was at 5V. If the drain voltage drops again to 2 V, the pn junction between the p-type base region 14 and the n-type drift region 13 is forward-biased, which immediately lowers the source potential and again causes the channel to turn off. open.

Although not essential to the present invention, when the drain potential rises sharply, it is fully conceivable that the insulation of the insulated gate 15 is broken due to the difference in capacitance. Then p
By connecting the constant voltage diode 51 for clamping the voltage of the mold base region as shown in FIG. 5, the potential of the floating p-type drift region can be kept from rising.

Note that the above-described structure can be applied to an enhancement-type p-channel MOSF even when a negative potential is used as an input signal.
The same can be applied by using ET.

Next, FIG. 7 is a circuit diagram illustrating a second embodiment of the present invention, and corresponds to claim 2 described above. In the figure, 1
Is an internal circuit, 2 is its input electrode, 3 is an external input terminal,
5 is a constant voltage diode and 9 is a constant current diode.
The constant current diode has a current-voltage characteristic as shown in FIG. 8, and is, for example, a depletion-type n-channel FET in which the gate and the source are short-circuited.

The operation will be described. The withstand voltage of the constant voltage diode 5 is set to 5V. When the potential of the external input terminal 3 fluctuates between 0 and 5 V, the impedance of the input electrode 2 of the internal circuit is very high, so that the current flowing through the constant current diode 9 is instantaneous and extremely small. Therefore, the relationship between the current and voltage of the constant current diode 9 is only around the origin in FIG. That is, in this region, the constant current diode 9 is equivalent to a resistor having a low resistance value. When the potential of the external input terminal 3 exceeds 5 V, the potential of the input electrode 2 is clamped at 5 V, and the difference is applied to both ends of the constant current diode 9.
However, since the current value is maintained at a constant value, it can be set to such a degree that the constant voltage diode 5 is not broken by overheating. When the potential of the external input terminal 3 drops below 5 V again, the constant voltage diode 5 in the conventional example of FIG.
It takes time to release the electric charge stored inside the device. However, since the constant current diode 9 is a forward-biased pn junction with respect to the reverse current, it discharges it almost instantaneously.

The above configuration can be similarly applied to a case where a negative potential is used as an input signal by connecting all the diodes in reverse.

Next, FIG. 9 is a circuit diagram for explaining a third embodiment of the present invention, and corresponds to claim 3 described above. FIG.
In this circuit, a constant current diode 91 newly connected to the opposite polarity to the constant current diode 9 is added between the external input terminal 3 and the constant current diode 9 in FIG.
In the circuit of FIG. 7, the internal circuit can be effectively protected when the potential of the external input terminal 3 is in a positive range. However, when the potential of the external input terminal 3 becomes negative, the input electrode 2 becomes -0.0.
Although the voltage does not fall below 7 V, the pn junction of both the constant voltage diode 5 and the constant current diode 9 is in a forward bias state,
Both can be destroyed by overheating. Therefore, FIG.
This can be prevented by adding a constant current diode 91 as shown in FIG. Further, since the heat generated by the forward current of the constant voltage diode 5 can be more allowed than at the time of the breakdown current, the upper limit of the forward current of the newly added constant current diode 91 can be several times larger than that of the constant current diode 9. Can be designed so as not to hinder normal operation.

By the way, in this configuration, even if the voltage of the external input terminal 3 suddenly drops, the current in the reverse direction is also limited, so that the potential of the input electrode 2 instantaneously drops as in the circuit of FIG. It doesn't seem like that.
Therefore, the time required for the potential of the input electrode 2 to decrease will be roughly estimated with reference to the example of FIG.

In the example shown in FIG. 1, when the constant voltage diode 5 is in a breakdown state, the amount of charge stored inside the constant voltage diode 5 is about 7 nC. This is 10
If it were to be released in mAs, the time required would be roughly 0.7 μs. Further, the potential of the external input terminal 3 changes in the range of 0 V to 5 V, and the constant voltage diode 5
If no breakdown occurs, no significant current flows in the system of FIG. 9, so that the operating points of the two constant current diodes 9 and 91 change near the origin of FIG. Therefore, the effective resistance is 0.
Staying on the order of 1Ω, the CR product is on the order of a few ns. Thus, even if the constant current diode 91 is inserted, almost no adverse effect occurs.

Similarly, FIG. 10 is obtained by adding the same constant current diode 91 to the circuit of FIG. 1 and has the same effect on the same problem.

FIG. 11 is a circuit diagram for explaining a fourth embodiment of the present invention, and corresponds to claim 4. The protection circuit described so far is effective only when the voltage signal applied to the external input terminal 3 is positive or negative. However, as shown in FIG.
And 91 and the constant voltage diodes 5 and 52 as a pair, it is possible to protect the internal circuit for receiving an input signal having both positive and negative polarities from a high voltage of either positive or negative polarity.

Note that FIG. 7, FIG. 9, FIG.
In FIG. 2, one end of the constant voltage diode 5 is illustrated as being grounded (fixed to 0 V), but this is not necessarily required to be 0 V. The essential meaning of this constant voltage diode is
The purpose is to prevent an excessive voltage from being applied to the input section of the subsequent internal circuit 1. Therefore, the other end of the input section shown as a resistor in the internal circuit 1 in the drawing is not grounded (to 0 V) as shown in the drawing, but is fixed to a predetermined positive potential, for example. If this is the case, the other end of the constant voltage diode 5 that protects it is also fixed to the same potential or a potential close thereto. Further, even when the other end of the input section of the internal circuit 1 is grounded as shown in the figure, the other end of the constant voltage diode is not grounded (0 V) if the above-mentioned purpose of input protection can be achieved. May be fixed to another predetermined potential.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention corresponding to claim 1;

FIG. 2 is a schematic diagram illustrating an internal state of the transistor in FIG.

FIG. 3 is a schematic diagram illustrating another internal state of the transistor in FIG.

FIG. 4 is a schematic diagram illustrating still another internal state of the transistor in FIG. 1;

FIG. 5 is a circuit diagram showing another embodiment of the present invention corresponding to claim 1;

FIG. 6 is a schematic diagram illustrating an internal state of the transistor in FIG.

FIG. 7 is a circuit diagram showing an embodiment of the present invention corresponding to claim 2;

FIG. 8 is a characteristic diagram showing voltage-current characteristics of a constant current diode.

FIG. 9 is a circuit diagram showing an embodiment of the present invention corresponding to claim 3;

FIG. 10 is a circuit diagram showing another embodiment of the present invention corresponding to claim 3;

FIG. 11 is a circuit diagram showing an embodiment of the present invention corresponding to claim 4;

FIG. 12 is a circuit diagram showing an example of a conventional circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Internal circuit 2 ... Input electrode 3 ... External input terminal 4 ... Protective resistor 5, 51 ... Constant voltage diode 6 ... Depletion type n channel JFET 7 ... Enhancement type n-channel MOSFET 8 constant-voltage diode 9, 91 constant-current diode 10 n-type source region 11 n-type drain region 12 p-type gate region 13 n Drift region 14 ... p-type base region 15 ... insulating gate 16 ... insulating film

Claims (4)

[Claims] An external input terminal for receiving an external signal is connected to one main electrode of a voltage-controlled transistor, and an input electrode of a subsequent internal circuit is connected to the other main electrode of the voltage-controlled transistor. When a voltage less than a predetermined value of the first polarity is applied to the external input terminal, the two main electrodes of the voltage-controlled transistor become conductive, and the external input terminal has the first polarity. When a voltage equal to or higher than the predetermined value is applied, the control electrode of the transistor is held at a predetermined fixed potential so that the two main electrodes of the voltage-controlled transistor are cut off.
An input protection circuit characterized in that: 2. A constant current diode having conductivity in both directions between an external input terminal for receiving an external signal and an input electrode of a subsequent internal circuit, but restricting a current flowing in at least one direction to a predetermined value or less. Is connected, and even when a voltage of a predetermined value or more of the first polarity is applied to the external input terminal, a voltage of the predetermined value or more is not applied to the input electrode. An input protection circuit, wherein one terminal of a constant voltage diode is connected, and the other terminal of the constant voltage diode is held at a predetermined fixed potential. 3. When the potential of the second polarity is applied to the external input terminal, the voltage control type transistor or the constant current diode is provided to protect a circuit by preventing a current of a predetermined value or more from flowing. Between the external input terminal, a second constant current diode is inserted with a polarity that limits the current flowing when the potential of the second polarity is applied to both ends thereof to a certain value or less, The input protection circuit according to claim 1 or 2, wherein 4. An external input terminal for receiving an external signal is connected to one end of a pair of series-connected constant current diodes having anodes or cathodes connected to each other, and the other end of the set of constant current diodes is connected to the other end. One end of a pair of series-connected constant-voltage diodes in which an anode or a cathode is connected is connected to the input electrode of the subsequent internal circuit, and the input electrode is further connected to the input electrode. An input protection circuit, wherein the other end of the voltage diode is held at a predetermined fixed potential.