Patent ID: 8610385

Claim:
A motor driving circuit comprising: an inverter circuit which supplies a driving current to a coil of a single phase brushless DC motor; a position detection sensor which detects a magnetic pole position of a magnet rotor of the single phase brushless DC motor and outputs a position detection signal; and a controller which controls the inverter circuit based on the position detection signal and a speed instruction signal which is supplied from an external source for instructing a rotating speed of the single phase brushless DC motor, wherein at a time of startup of the single phase brushless DC motor, the controller makes a pulse width of a PWM signal for controlling the inverter circuit constant in a first time period which starts after the position detection signal zero-crosses and lasts until the position detection signal zero-crosses next time, and narrows the pulse width of the PWM signal as time elapses in a second time period immediately after the first time period, and wherein the controller comprises: a position estimation unit which generates a position signal synchronous with the position detection signal, at the time of startup of the single phase brushless DC motor; a counter unit which starts a count operation from a time when the position signal zero-crosses, at the time of startup of the single phase brushless DC motor; an output waveform generation unit which generates a modulation signal having a voltage amplitude that is constant in the first time period and is decreased as time elapses in the second time period, based on the speed instruction signal and a count value of the counter unit; and a PWM generation unit which generates the PWM signal having the pulse width that is constant in the first time period and is narrowed as time elapses in the second time period, based on the modulation signal.