Clamp circuit

Abstract

The clamp circuit clamps an input voltage at prescribed higher and lower clamp voltages which are stabilized under a temperature fluctuation. Transistors Q12 and Q14 are switched on in their linear region. In a lower voltage clamp circuit 18, an Vin detecting circuit 20 outputs Va1 by level-shifting Vin by Q13 and voltage-divides by series resistance circuit 23 the level-shifted Vin, while a reference voltage generating circuit 21 outputs Vr1 by level-shifting 0 V by Q15 and voltage-divides by series resistance circuit 25 the level-shifted voltage. Q11 is switched on, when a comparator 22 determines that Va1 descends and goes across Vr1. Here, Q12 is of the same characteristics as Q14, while Q13 is of the same characteristics as Q15. Further, the resistance of the circuits 23 is the same as that of the circuit 25. The higher voltage clamp circuit 19 is similar to the circuit 18.

Description

[0001] 1. Field of the Invention[0002] The present invention relates to a clamp circuit for clamping a voltage inputted into a signal input terminal in a semiconductor integrated circuit (IC).[0003] 2. Description of the Related Art[0004] Recently, a large scale semiconductor IC (LSI) is manufactured in a smaller scale rule, in order to raise a response speed of the device and to reduce its chip area. Accordingly, for example, a thickness of a gate oxide film is made thinner, thereby causing such a necessity that a gate voltage be lower in order to ensure a sufficient device life and therefore, any overvoltage not be applied to the device in order to prevent a device destruction.[0005] In order to lower the gate voltage, it is effective to employ a step-down circuit mainly used in an internal logic circuit in LSI for lowering a supply voltage. Further, in order to avoid an application of the overvoltage, it is effective to add a clamp circuit to a buffer circuit or interface circuit...

AI Technical Summary

Problems solved by technology

However, if the control IC 3 receives a larger number of input signals, the required number of the Zener diodes become larger, thereby increasing an area of the control substrate 1 and raising a production cost of the clamp circuit.

Accordingly, the low withstand voltage device process can not employed, but a high withstand vol

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