A power controller makes use of a line-voltage detection method to perform brown-out protection and brown-in mechanism. The power controller has a high-voltage node connected via a current-limiting resistor to a line voltage, and a high-voltage startup transistor connected to the high-voltage node. The input voltage at the high-voltage node is divided to provide a fraction result. An offset current flowing through the high-voltage startup transistor and the current-limiting resistor is provided in response to the fraction result. The offset current is stopped in response to the fraction result when the offset current flows through the high-voltage startup transistor and the current-limiting resistor.