Field of the Invention
The present invention relates to an input circuit having hysteresis characteristics in a threshold detecting an input signal.
Background Art
FIG. 5 is a circuit diagram illustrating one example of a related art input circuit having hysteresis characteristics. The related art input circuit 500 is equipped with an N type transistor 501, P type transistors 502 and 503, and an inverter 504.
The transistor 501 and the transistor 502 respectively have drains connected to each other and gates connected in common, and are inputted with an input signal IN at the gates thereof. The drains of the transistor 501 and the transistor 502 are respectively connected to the input of the inverter 504 and a drain of the transistor 503. The output of the inverter 504 becomes an output signal OUT of the input circuit 500. Further, the output signal OUT is inputted to a gate of the transistor 503.
The input circuit 500 is operated at a positive power supply voltage (VDD) and a negative power supply voltage (VSS).
Since the transistor 502 is turned ON and the transistor 501 is turned OFF when the input signal IN is at an L level, the drains of the transistor 501 and the transistor 502 respectively become H in level. Accordingly, the output signal OUT of the inverter 504 becomes L in level and the gate of the transistor 503 becomes L in level. Thus, since the gate-source of the transistor 503 is ON-controlled by the magnitude of VDD, the transistor 503 indicates an ON state. When the input signal IN is increased from the L level, the output signal OUT of the inverter 504 is transited from the L to H level, based on a magnitude relationship between a current driving force of the transistor 501 and net current driving forces of the transistors 502 and 503 which are indicative of the ON state. The threshold of the input circuit 500 at the time that the output signal OUT is transited from the L to H level is determined in the way described above.
Since the transistor 501 is turned ON and the transistor 502 is turned OFF when the input signal IN is at an H level, the drains of the transistors 501 and 502 respectively become L in level. Accordingly, the output signal OUT of the inverter 504 becomes H in level and the gate of the transistor 503 becomes L in level. Thus, since the gate-source of the transistor 503 is OFF-controlled, the transistor 503 indicates an OFF state. When the input signal IN is lowered from the H level, the output signal OUT of the inverter 504 is transited from the H to L level, based on a magnitude relationship between the current driving force of the transistor 501 and the current driving force of the transistor 502. The threshold of the input circuit 500 at the time that the output signal OUT is transited from the H to L level is determined in the way described above.
With the current driving force of the transistor 503 ON-controlled by the magnitude of VDD, the threshold of the input circuit 500 at the time that the output signal OUT is transited from the L level to the H level is higher than the threshold of the input circuit 500 at the time that the output signal OUT is transited from the H level to the L level. In the related art input circuit 500, the hysteresis characteristics are obtained in this way. With the hysteresis characteristics, an unstable operation of a post-stage circuit for the input circuit due to chattering related to the intrusion of external noise is avoided (refer to, for example, Patent Document 1).
[Patent Document 1] Japanese Patent Application Laid-Open No. Hei 9 (1997)-270678