| 摘要 |
PURPOSE:To raise the reliability for detecting a power failure at the time when an AC power supply voltage has dropped, to improve the power failure detection response time, and also, to reduce the variance, by providing a hysteresis characteristic to a comparator of an input level monitoring circuit part. CONSTITUTION:When a voltage of an AC power source 1 drops, the output of a rectifying element 2 does not exceed a high level of a comparator 3a, and the output of the comparator 3a is held in an H state as long as a voltage drop state continues. As a result, in a timer circuit part 4, as time elapses, a time-up operation is executed, the value of a power source abnormality detecting output PF is varied to L from H, and a power supply voltage drop signal is outputted to an external circuit. In such a case, a hysteresis is supplied to the comparator 3a, therefore, when the AC power supply voltage is normal, a period in which a value of a rectifying output voltage is lower than a value of a detection reference voltage is shortened. In such a way, a variation due to the phase of power failure of the time until a power failure signal is sent out can be suppressed.
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