Pre-drive circuit for brushless DC single-phase motor

A pre-drive circuit for a brushless DC single-phase motor for controlling a motor rotational speed by changing a duty factor of an ON/OFF control voltage to switching elements, the pre-drive circuit driving a brushless DC single-phase motor drive circuit comprising a pair of series connections of switching elements being connected between a power supply and ground, each series connection formed of two switching elements, and a motor coil connected between the two nodes, each node of the two switching elements in each series connection, wherein the motor coil is controlled with a current fed therethrough at any timing in any direction in an ON/OFF manner by turning on and off the switching elements, and a control voltage exceeding the voltage of the power supply is needed to turn on two power-supply-side switching elements, the pre-drive circuit comprising:

• a voltage stepup circuit for stepping up the power supply voltage to a predetermined voltage;
• a logic circuit for generating and then outputting pulse signals for controlling the switching elements, based on a motor rotary position signal and a duty factor setting signal for controlling the motor rotational speed,
• a pair of operational amplifiers which are respectively connected to output terminals of the logic circuit for the pulse signals for controlling the two power-supply-side switching elements, are supplied with the stepped up voltage from the voltage stepup circuit a power source, amplify the pulse signal for controlling the two power-supply-side switching elements to a predetermined voltage level above the power supply voltage, and respectively feed the amplified pulse signals to control input terminals of the two power-supply-side switching elements, and
• a pair of ground-side switching element driving signal input circuits which are respectively connected to pulse signal output terminals on the logic circuit for two ground-side switching elements, and feed, to control input terminals of the two ground-side switching elements, the pulse signals from the pulse signal output terminals directly or via resistors.