STARTUP CIRCUIT AND POWER SUPPLY CIRCUIT

摘要

In a startup circuit, an error amplifier receives a target voltage in a startup period that is a terminal voltage with a shape of a slope generated by charging a capacitor for soft starting with a current from a constant current circuit. A detecting circuit monitors variation of the terminal voltage and blocks pulse from a logic circuit until the terminal voltage, which has been zero volts at the moment of startup, reaches a predetermined threshold value. In this period, the detecting circuit disables the function of the error amplifier. It is not until the terminal voltage VCS reaches the predetermined threshold value and the PWM pulse begins to be delivered that the error amplifier is enabled. At this moment, startup control begins based on the difference between the terminal voltage and the feedback signal. Therefore, the output voltage never rises abruptly.

主权项

1. A startup circuit comprising:
an error amplifier that amplifies an error between an output voltage and a target voltage for the output voltage; a PWM comparator that compares an output signal of the error amplifier with a triangular signal and generates a PWM signal; a soft start circuit that charges a capacitor for soft start by a current source and delivers a terminal voltage with a shape of a slope developing across the capacitor as a target voltage in a startup period to the error amplifier; a detecting circuit that delivers a first detection signal until the terminal voltage reaches a predetermined value and delivers a second detection signal after the terminal voltage exceeds the predetermined value; and a logical product circuit that blocks output of the PWM signal in response to the first detection signal and allows output of the PWM signal in response to the second detection signal; wherein the error amplifier disables an error amplification function in response to the first detection signal and enables the error amplification function in response to the second detection signal.