JPH0414523B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an input protection circuit for a semiconductor circuit such as a semiconductor integrated circuit, and more particularly to a circuit for preventing electrostatic damage to an input circuit.

Figure 1 is a circuit diagram showing an example of a conventional input protection circuit, in which 1 and 2 are npn transistors whose emitters are commonly connected to form a differential amplifier, and 3 is an input terminal connected to the base of transistor 1. , 4 is a reference voltage supply terminal connected to the base of transistor 2, 5 is a high potential side power supply voltage supply terminal, 6 is a low potential side power supply voltage supply terminal (referred to as a ground terminal),
7 has a cathode connected to the base of transistor 1,
A diode whose anode is connected to the ground terminal 6; 8 is a resistance element connected between the commonly connected emitters of transistors 1 and 2 and the ground terminal 6 and serves as an emitter load resistance; 9 and 10 are transistors 1 and 10, respectively;
A load element 11 serving as a collector load resistance connected between the collector of No. 2 and the power supply voltage supply terminal 15;
is an output terminal drawn out from the collector of transistor 1.

A differential amplifier connection circuit including transistors 1 and 2 is widely used as a common mode logic (CML) circuit, and diode 7 constitutes an input protection circuit.

Next, the operation of this circuit will be explained. First, if a predetermined voltage is supplied to the power supply voltage supply terminal 5 and the ground terminal 6 on the high potential side, and a voltage higher than the reference voltage (H level) is applied to the input terminal 3, an L level output is obtained at the output terminal 11. , if a voltage lower than the reference voltage (L level) is applied to the input terminal 3, an H level output is obtained at the output terminal 11.

Now, let us consider a case where a large positive voltage such as a surge is applied to the input terminal 3. At this time, the base-collector junction of transistor 1 and the base-collector junction
The emitter junction conducts in the forward direction. If this circuit is composed of a solid-state integrated circuit, as shown by the broken lines in FIG. The electrostatic charge flows from the power supply terminal 5 to the power supply line. The current at this time causes the base-emitter junction, base-collector junction, and parasitic diodes 12, 1 of transistor 1 to
3 is not destroyed, the circuit of FIG. 1 will operate normally even after the surge voltage has passed.

Next, consider a case where a large negative surge voltage is applied to the input terminal 3. When this surge voltage becomes sufficiently lower than the low-potential side power supply voltage, the diode 7 becomes conductive in the forward direction, and the applied negative electrostatic charge flows from the ground terminal 6 to the power supply line. If the diode 7 is not destroyed by the current at this time, the first
The circuit shown in the figure operates normally even after the surge voltage has passed.
That is, diode 7 operates as an input protection diode.

However, when a large negative surge is applied to the input terminal 3, the diode 7 becomes conductive and a large current flows through the diode 7. In this way, when a large current flows through the diode 7 based on a large negative surge, the voltage across the diode 7 instantaneously increases in response to this large current. In other words, the large current flowing through diode 7 and the diode 7
A voltage corresponding to the product of the impedance of the diode 7 in the conductive state appears across the diode 7. As a result, a fairly large negative voltage remains between the input terminal 3 and the ground terminal 6, which is applied to the base of the transistor 1 and biases the base-collector and base-emitter junctions of the transistor 1 in the opposite direction. This negative voltage gradually attenuates as the electrostatic charge discharges through the diode 7, but both junctions of the transistor 1 may be destroyed before this attenuates. In particular, since the base-emitter junction has a low breakdown voltage, it often breaks down before the base-collector junction.

This invention was made in view of the above points, and by using a transistor as an input protection element, when a surge voltage that applies a reverse voltage to the junction of the protected transistor is applied to the input terminal, It is an object of the present invention to provide an input protection circuit that conducts to short-circuit the surge voltage and also short-circuit the junction of the protected transistor, thereby preventing the protected transistor from being destroyed.

FIG. 2 is a circuit diagram showing a first embodiment of the present invention, and in each of the embodiments hereinafter, parts equivalent to those of the conventional example and the previous embodiment are designated by the same reference numerals. In the figure, 14
is an npn type input protection transistor whose emitter is connected to the input terminal 3, whose base is connected to the ground terminal 6, and whose collector is connected to the emitter of the protected transistor 1.

In this first embodiment circuit, the operation when a positive surge voltage is applied to the input terminal 3 is exactly the same as the conventional circuit.

Next, consider a case where a large negative surge voltage is applied to the input terminal 3. Input protection transistor 1
When the emitter potential of the input protection transistor 14 becomes more negative than the potential of the ground terminal 6 by a forward voltage between the base and emitter of the input protection transistor 14, the input protection transistor 14 becomes conductive, and the base current of the input protection transistor 14 is amplified. A large current flows from the collector of the input protection transistor 14 toward the emitter, discharging the charge accumulated at the emitter point of the transistor 1. At this time, the potential of the emitter of the input protection transistor 14 is lower than the potential of the ground terminal 6, that is, the base of the input protection transistor 14.
4, the input protection transistor 14 becomes conductive, and a large current flows between the emitter and collector of the input protection transistor 14, which is amplified with respect to the base current of the input protection transistor 14. . At this time, the voltage between the base and emitter of the input protection transistor 14 is slightly higher than the forward voltage between the base and emitter. A large current flows between the input protection transistor 14 and the collector, making the emitter and collector potentials of the input protection transistor 14 almost the same potential. Therefore, as a large current flows between the emitter and the collector of the input protection transistor 14, the potential of the emitter of the transistor 1 decreases while being maintained at approximately the same potential as the base of the transistor 1 connected to the input terminal 3. . Therefore, since almost no reverse voltage is applied to the base-emitter junction of transistor 1, this junction can be prevented from being destroyed.

FIG. 3 is a circuit diagram showing a second embodiment of the present invention, in which a second input protection transistor 15 is provided whose base and emitter are commonly connected to the input protection transistor 14 in the first embodiment, and whose collector is connected to the input protection transistor 14 in the first embodiment. It is connected to the collector of transistor 1. In this second embodiment, just as the first input protection transistor 14 protects the base-emitter junction of transistor 1, the second input protection transistor 15 protects the base-collector junction of transistor 1.

Note that in the first and second embodiments, the emitter of the input protection transistor is connected to the input terminal, and the collector is connected to the emitter or collector of the protected transistor, but even if the emitter and collector of the input protection transistor are swapped, Often, this provides almost the same protection against negative surge voltages, and even more protection against positive surge voltages.
Since the reverse breakdown voltage of the base-collector junction is larger than that of the emitter junction, the input protection transistor itself is less likely to be destroyed.

FIG. 4 is a circuit diagram showing a third embodiment of the invention. In this third embodiment, a multi-collector input protection transistor 16 is used, the emitter and base are connected to the input terminal 3 and the ground terminal 6, respectively, as in the first and second embodiments, and the first collector is connected to the transistor 1 and the second collector is connected to the emitter of transistor 1. It will be easily understood that the operation of this third embodiment is substantially the same as that of the second embodiment shown in FIG. Here, the input protection transistor 16 is a multi-emitter transistor which can be configured as a transistor of a normal integrated circuit structure and is operated in the reverse direction and used as a multi-collector transistor. Therefore, this corresponds to the case where the collector of the input protection transistor is connected to the input terminal described above, and there is an advantage that the input protection transistor 16 itself is not easily destroyed by positive surge input.

In addition, in each of the above embodiments, the input transistor is
Although the case of an npn transistor is shown and therefore an npn transistor is used as the protection transistor, it is natural to use a pnp transistor as the protection transistor when the input transistor is a pnp type.

As described above, in this invention, a transistor of the same conductivity type as the input transistor is used as an input protection element, and the base and emitter of the input transistor are short-circuited in response to an input surge voltage having a polarity that cuts off the input transistor. This makes it possible to prevent the junction of the input transistor from being destroyed by reverse voltage.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an example of a conventional input protection circuit, and FIGS. 2, 3, and 4 are circuit diagrams showing first, second, and third embodiments of the present invention, respectively. In the figure, 1 is an input transistor, 3 is an input terminal, 5 is a high potential side power supply voltage supply terminal, 6 is a low potential side power supply voltage supply terminal, 14 and 15 are protection transistors, and 16 is a multi-collector protection transistor. be. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Claims] 1. The base is connected to the input terminal, the emitter is connected to the first power supply voltage supply terminal via the resistance element, and the collector is connected to the second power supply voltage supply terminal via the load element.
A circuit for protecting an input transistor constituting an input circuit connected to a power supply voltage supply terminal of the first power supply voltage supply terminal from input overvoltage, the base of which is connected to the first power supply voltage supply terminal, and one electrode of the emitter or collector connected to the first power supply voltage supply terminal of the first power supply voltage supply terminal. It is connected to the input terminal and
An input protection circuit characterized in that the other electrode of the emitter or collector is connected to the emitter of the input transistor, and a protection transistor having the same conductivity type as the input transistor is provided on the same semiconductor substrate as the input circuit. 2 The protection transistor has two collectors,
The input protection circuit according to claim 1, wherein the emitter is connected to the input terminal, one collector is connected to the emitter of the input transistor, and the other collector is connected to the collector of the input transistor. .