JPH04260366A - input protection circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

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 constructed by combining a diode and a transistor.

0002

2. Description of the Related Art FIG. 2(a) shows a conventional input protection circuit constructed by combining diodes. Figure 2(a)
, 1 is an input terminal of a semiconductor integrated circuit protected by an input protection circuit, 2 and 3 are diodes, 4 and 5 are
is the P that constitutes the CMOS semiconductor integrated circuit to be protected.
-MOS transistor and N-MOS transistor, and only the first stage thereof is representatively shown here. Further, 6 is a power supply for the semiconductor integrated circuit, and 7 is a ground for the semiconductor integrated circuit.

FIG. 2 shows an input protection circuit connected to the input gate of a CMOS inverter. The input gate of a CMOS inverter has a high impedance, and is therefore vulnerable to surges and has a property that the gate oxide film is easily destroyed.

In order to overcome these drawbacks of CMOS inverters, a circuit configuration that absorbs surges is created by connecting low impedance diodes 2 and 3 shown in FIG. 2 in front of the input gate of the inverter. . That is, when a surge with a positive voltage is applied to the input terminal 1 with respect to the ground 7 and the power supply terminal 6, the diode 2 is forward biased and turned on, absorbing the positive surge. Further, when a surge with a negative voltage is applied, the diode 3 is forward biased and turned on, absorbing the negative surge.

[0005]

[Problem to be Solved by the Invention] Incidentally, in the case of the conventional input protection circuit shown above, the power supply (+V
When a voltage VI such that VI −(+VCC)≧0.7V is applied to the input terminal 1, which has undergone the above protection measures, before the rising of the diode 2 in Fig. 2(a).
There is a problem in that a forward bias is applied to the circuit, causing an overcurrent to flow, and that a sufficient input voltage is not applied to the input terminal 1, resulting in unstable circuit operation.

The present invention was made to solve the above-mentioned problems, and it is possible to stably operate a protected circuit without being affected by the order of voltage application.
Another object of the present invention is to provide an input protection circuit that can absorb surges in the same manner as conventional ones.

[0007]

[Means for Solving the Problems] The input protection circuit according to the present invention is an input protection circuit in which diodes are connected in series, and a transistor whose collector and base are short-circuited is further connected between the power supply terminal and the input terminal. .

[0008]

[Operation] Since the present invention is configured as described above, when voltage is preferentially applied to the input terminal,
The diode on the power supply side is forward biased, but the transistor is reverse biased, which prevents current from flowing from the input terminal to the power supply. Further, when a positive surge is applied to the input terminal, the transistor breaks down and becomes low in resistance, so the provision of this transistor does not impair the input protection function.

[0009]

[Embodiment] FIG. 1 shows an input protection circuit according to an embodiment of the present invention. In FIG. 1(a), 1 is an input terminal, 2 and 3 are diodes, 4 is a P-MOS transistor, and 5 is an N-M
An OS transistor, 6 is a power supply for the semiconductor integrated circuit, 7 is a ground for the semiconductor integrated circuit, 8 is a transistor whose collector and base are short-circuited, and is connected between the power supply 6 and the cathode of the diode 2 so as to be forward biased. There is.

The operation of this embodiment is explained using the power supply voltage (+Vcc).
will be explained by taking as an example an input protection circuit that operates under +5V conditions. In Figure 1, when voltage is applied preferentially to input terminal 1, diode 2 becomes forward biased, but transistor 8 becomes reverse biased, which prevents current from flowing from the input terminal to the power supply. . Also,
The advantage of a transistor with the collector and base shorted is that the reverse breakdown voltage VR is low, and the reverse breakdown voltage VR is approximately 5.5.
As long as the power supply is used at +5V, stable operation is possible without breakdown.

The transistor 8 is also effective against surges. That is, when a positive surge is applied between the input terminal 1 and the power supply 6, the transistor 8 is reverse biased, but the transistor 8 breaks down at a voltage exceeding the reverse breakdown voltage VR ≧5.5V. Since it has a low impedance, it can absorb surges without destroying the junction. Incidentally, with respect to a surge applied between the input terminal 1 and the ground 7, the diode 3 absorbs the surge by the same operation as in the conventional case.

0012

[Effects of the Invention] As described above, according to the input protection circuit according to the present invention, a transistor whose collector and base are short-circuited is further connected between the power supply terminal and the input terminal of the input protection circuit in which diodes are connected in series. Therefore, the power supply voltage can be raised regardless of the order in which voltages are applied, and this function can be realized without impairing the input protection function.

[Brief explanation of the drawing]

1 is a diagram showing an input protection circuit according to an embodiment of the present invention, FIG. 1(a) is the circuit diagram thereof, and FIG. 1(b) is the circuit diagram of FIG.
It is a sectional view of the input protection circuit of (a).

FIG. 2 is a diagram showing a conventional input protection circuit, and FIG. 2(a)
is its circuit diagram, and FIG. 2(b) is a sectional view of the input protection circuit of FIG. 2(a).

[Explanation of symbols]

1 Input terminal 2 Diode 3 Diode 4 P-MOS transistor 5 N-MOS transistor 6 Power supply 7 Grounding 8 Transistor

Claims (1)

[Claims] Claim 1. An input protection circuit comprising two diodes connected in series between a power supply and a ground, and a connection point between the diodes connected to an input terminal and an input of a protected circuit. An input protection circuit comprising a transistor whose base and collector are short-circuited between a power supply and a diode on the power supply side.