The present invention relates to a drive control technique for a brushless motor (also called permanent-magnet synchronous motor), and a technique effective for application to the PWM (Pulse Width Modulation) drive control of a three-phase dc motor, for example, a technique effective for use in a drive control device for a spindle motor that drives to rotate a disk-type storage medium such as a hard disk.
A hard disk drive uses a brushless three-phase dc motor being generally called the spindle motor in order for rotating a magnetic disk. While rotating the magnetic disk with the spindle motor at a high speed, and bringing a magnetic head for reading/writing close to the surface of the rotating magnetic disk to travel the magnetic head radially, the hard disk drive performs to write and read information.
With regard to the drive control for the brushless motor, it has been a general trend to adopt the PWM drive control system that controls the magnitude of currents flowing into the coils of the motor by varying the pulse width of the control signal, in order to achieve high efficiency and low power consumption.
The PWM drive control system requires the detection of the dc currents flowing into the coils in order for the current control and excessive surge current protection. In the dc motor control circuit of the conventional PWM drive control system, the detection of the dc currents flowing into the coils has generally been implemented with resistive elements called the shunt resistors connected in series to switching transistors that drive the coils (refer to the Japanese Unexamined Patent Publication No. 2001-275387).