| 摘要 |
PURPOSE:To prevent a comparative reference voltage from varying owing to a drop in AC source voltage by suppressing variation in the comparative reference voltage by a Zener diode and canceling the negative temperature characteristic of said Zener diode by a diode. CONSTITUTION:A voltage to be detected which is a DC voltage obtained by rectifying and smoothing variation in the AC source voltage of system equipment, etc., is inputted to the inverted input terminal 1a of a comparator 1. When this voltage to be detected drops below the comparative reference voltage set by a variable resistance 2, a power-down detection signal is outputted from the output terminal 1c of the comparator 1. When the AC source voltage drops, the source voltage Vcc of a source voltage abnormality detecting circuit also drops slightly. The comparative reference voltage, however, is held at the set value by the circuit of the Zener diode 5 without being affected by variation in the AC source voltage. Further, the negative temperature characteristic of the Zener diode 5 is canceled by the diode 4 connected in the opposite direction from the Zener diode 5.
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