JPH04217356A - Ic input protection circuit - Google Patents

Abstract

PURPOSE:To definitely absorb any positive or negative electrostatic voltage applied to an IC circuit by connecting a diode between a gate and an earth terminal of an NPN transistor circuit. CONSTITUTION:A collector C of an NPN transistor Q is connected to an input terminal 31, and an emitter E is connected to an earth terminal 32. And a base B of the NPN transistor Q is connected to the earth terminal 32 through a diode D. When an electrostatic positive voltage is applied to the input terminal 31, breakdown occurs between emitter and base through a diffused resistor rc located between collector and base of the transistor Q, thereby preventing thermal breakdown of the junction face of collector and base. In the other hand, when an electrostatic negative voltage is applied to the input terminal, a passage 200 passing through the base and collector of the transistor D is formed through diode D from the earth terminal 32. Voltage of diode D in forward direction holds a constant value relative to the passage 200, and the base-emitter breakdown of the transistor Q is prevented.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an IC input protection circuit, and more particularly to an improvement in an input protection circuit that effectively absorbs an external surge voltage applied to an IC from an input terminal.

0002

2. Description of the Related Art IC circuits, particularly MOS-ICs, have a problem in that they operate extremely sensitively to externally applied surge voltages and are easily destroyed by accumulation of static charges.

In order to improve these weaknesses of IC circuits, it has been proposed to provide a protection circuit on the input side of the IC circuit.

An example of such an input protection circuit is disclosed in Japanese Patent Laid-Open No. 62-69678. In order to protect the gate of an MIS type IC from excessive voltage, a protection element, for example, is installed at the IC gate input. Bipolar transistors, diodes, etc. are provided.

However, this conventional device has a problem in that it cannot perform reliable surge absorption with excellent responsiveness.

Japanese Patent Application Laid-open No. 183184/1984 is known as a conventional improved input protection circuit.

In this conventional device, a protective transistor and its gate resistor are formed on the same substrate as the IC circuit,
This forms a protection circuit integrated with the IC circuit.

FIG. 5 shows the structure of the conventional protection circuit, and FIG. 6 shows its equivalent circuit.

As is clear from the figure, the conventional protection circuit is
The IC circuit 10 consists of an NPN transistor Q and a resistor R.
This protection circuit 20 is integrally formed on the same substrate in the preceding stage.

The NPN transistor Q has its collector C connected to the input terminal 31, and its emitter E connected to the ground terminal 3.
Connected to 2. The base B of this NPN transistor Q is connected to the ground terminal 32 via a resistor R.

Therefore, according to this conventional protection circuit, when an external positive electrostatic voltage is applied to the input terminal 31,
A breakdown occurs between the collector and base of the transistor Q, and the current flowing through the resistor R between the base and ground turns on the transistor Q. As a result, most of the static electricity flowing into the input terminal 31 is transferred to the transistor Q. It is discharged from the collector emitter to the ground terminal 32, and the surge voltage is reliably absorbed.

[0012] Therefore, the surge static charge does not have an adverse effect on the IC circuit, and it is possible to reliably protect the circuit.

As described above, according to the conventional protection circuit,
When a positive surge voltage is applied to the input terminal 31,
Voltage is absorbed by the breakdown of the transistor Q, and furthermore, when the transistor Q breaks down between the collector and base, the presence of the diffused resistance rc between the collector and base prevents the collector-base junction surface of the transistor Q from being thermally destroyed. There is. That is, at the time of the breakdown, a large amount of charge rushes into the collector-base junction surface of the transistor Q, but the voltage at the collector-base junction surface decreases due to the inflow charge at this time and the voltage drop due to the diffusion resistor rc. No thermal damage occurs on the collector-base joint surface.

[0014]

However, the conventional protection circuit has a problem in that thermal breakdown occurs at the base-emitter junction surface of the transistor Q when a negative electrostatic voltage is applied.

FIG. 7 shows the flow of charge when a conventional protection circuit receives a negative electrostatic voltage, and the equivalent circuit in FIG. 8 similarly shows the flow of current when a negative electrostatic voltage is applied. It is shown.

When a negative electrostatic voltage is applied to the input terminal 31, a surge current flows from the ground terminal 32 toward the input terminal 31 through two paths. The first path is a path from the ground terminal 32 to the parasitic diode Ds, the resistor rs, and the collector of the transistor Q.

On the other hand, the second path is from the ground terminal 32, passes through the resistor R, and passes through the base collector of the transistor Q. Both of these routes are shown at 100 in FIGS.
, 200.

When the voltage effect of rs in the first path 100 is large, the current in the second path 200 increases the potential difference between the base and emitter of the transistor Q, and when this potential difference exceeds the base-emitter breakdown voltage, Breakdown occurs between the base and emitter of transistor Q. Therefore, the resistance of the base-emitter junction surface sharply decreases due to the breakdown, and a large amount of charge is concentrated on the base-emitter junction surface as shown by path 300, causing thermal breakdown.

Therefore, in the conventional protection circuit, the input terminal 3
1 has a drawback in that it cannot provide sufficient protection against negative electrostatic voltages applied to it.

In the conventional protection circuit, in order to cope with such a negative electrostatic voltage, it is necessary to reduce the resistance value of the resistor R, thereby suppressing the potential difference between the base and emitter of the transistor Q. However, in this case, the transistor Q
cannot turn on, so electrostatic charge cannot flow through the collector-emitter path of transistor Q.
It was not possible to obtain sufficient protection.

The present invention was made in view of the above-mentioned conventional problems, and its purpose is to reliably apply both positive and negative electrostatic voltages while using an NPN transistor formed on the same substrate as an IC circuit. An object of the present invention is to provide an IC input protection circuit that can provide a protective effect.

[0022]

[Means for Solving the Problems] In order to achieve the above object, the present invention is characterized in that a diode is connected between the gate of the NPN transistor circuit and the ground terminal instead of the conventional resistor R.

That is, the cathode of this diode is connected to the base of the transistor Q, and the anode is connected to the ground terminal.

[0024]

[Operation] Therefore, according to the present invention, when a positive electrostatic voltage is applied, the transistor Q has a diffusion resistance r between the collector and the base.
C is used to break down between the collector and base to absorb electrostatic current, and when a negative electrostatic voltage is applied, breakdown between the base and emitter of transistor Q is prevented by the forward voltage of the diode.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings.

FIGS. 1 and 2 show an IC input protection circuit according to the present invention, and the same members as in FIGS. 5 and 6 described above are designated by the same reference numerals, and their explanation will be omitted.

A feature of the present invention is that a diode D is connected between the base of the NPN transistor Q and the ground terminal 32, similar to the conventional one.

As is clear from the figure, this diode D
has its cathode connected to the base B of transistor Q and its anode connected to ground terminal 32.

Therefore, diode D provides a large resistance to the path flowing from the base of transistor Q through diode D to ground terminal 32, but on the other hand,
A low resistance is provided to the path flowing from the ground terminal 32 to the base B of the transistor Q through the diode D.
At this time, the forward voltage of diode D is approximately constant value VF
give.

FIGS. 1 and 2 show a state in which a positive electrostatic voltage is applied to the input terminal 31. FIG.

At this time, the transistor Q discharges the positive electrostatic charge to the ground terminal 32 through a collector-emitter path. That is, as is clear from FIG. 1, there is a diffused resistor rc between the collector and base of the transistor Q, and a breakdown occurs between the emitter and base through this diffused resistor rc, and as a result, a large amount of charge is transferred to the collector of the transistor Q. When it enters the base junction surface, the voltage on the collector base junction surface decreases due to the inflowing charge and the voltage drop due to the diffused resistance rc, and thermal destruction of the collector base junction surface can be prevented.

On the other hand, when a negative electrostatic voltage is applied to the input terminal 31, the first
A path 100 is formed. At the same time, the ground terminal 32
From there, a second path 200 is formed through diode D and the base collector of transistor Q.

The characteristic feature of the present invention is that this second
In path 200, when charge flows, diode D holds the forward voltage VF between the base and emitter of transistor Q, so the base and emitter of transistor Q does not break down.

Therefore, the transistor Q is not thermally destroyed by such a second path 200, and both the paths 100 and 200 have a sufficient charge absorption effect even for negative electrostatic voltage. is possible.

[0035]

[Effects of the Invention] As explained above, according to the present invention,
The NPN transistor and the diode connected between the base of the NPN transistor and the ground terminal can reliably absorb charge from both positive and negative electrostatic voltages applied from the outside, preventing excessive voltage from being applied to the IC circuit. It has the advantage of not giving away anything.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing a preferred embodiment of an IC input protection circuit according to the present invention, showing a state where a positive electrostatic voltage is applied.

FIG. 2 is an equivalent circuit diagram of FIG. 1;

FIG. 3 is a cross-sectional view of the circuit of FIG. 1 when a negative electrostatic current is applied.

FIG. 4 is an equivalent circuit diagram of FIG. 3;

FIG. 5 is a cross-sectional view of a conventional IC input protection circuit, showing a state in which a positive electrostatic voltage is applied.

FIG. 6 is an equivalent circuit diagram of FIG. 5;

FIG. 7 is a cross-sectional view showing a state in which a negative electrostatic voltage is applied to a conventional circuit.

8 is an equivalent circuit diagram of FIG. 7. FIG.

[Explanation of symbols]

10 IC circuit 20 Input protection circuit 31 Input terminal 32 Ground terminal Q NPN transistor D Diode

Claims (1)

[Claims] 1. An input protection circuit formed in the same substrate as an IC, comprising an NPN transistor whose collector is connected to an input terminal and whose emitter is connected to a ground terminal;
An IC input protection circuit including a diode having a cathode connected to the base of the transistor and an anode connected to a ground terminal.