A feedback signal sensing method includes the steps of: providing a pulse
width modulation (PWM) signal having a period; charging a capacitor by a
current source during a pulse duration of the period, so as to form an
equivalent slope compensation ramp signal; conducting an inductor current
flowing from a boost inductor to flow through an equivalent resistor
during the pulse duration of the period, so as to form an equivalent
inductor current signal; and using a coupling characteristic of the
capacitor together with the equivalent slope compensation ramp signal and
the equivalent inductor current signal to form a feedback signal.