FAN8621B

12

Spindle Motor Driver

The spindle motor driver has two operating modes : hard commutation in start-up and acceleration mode and soft commutation

in steady state to reduce acoustic noise.

Start-up and Acceleration

To spin up the motor, open mode start-up clock must be fed to CNTL2 pin in range 20 ~ 200Hz depending on number of plat-

ter and motor RPM. This HIGH and OPEN signal commutates motor current in turn while back-EMF comparator checks

back-EMF level. When the back-EMF detects motor position, then operating mode is changed from start-up mode to accelera-

tion mode. Also internal commutation logic starts sensorless commutation.

Speed Control

Spindle motor speed is defined by system clock and RPMSEL (pin48) . For your reference, see table2

Figure 2. Typical Control Sequence of the FAN8621B

Speed and rotor position of spindle motor is measured by back-EMFs. Back-EMF comparator detects zero crossing point

(ZCP) of unexcited phase back-EMF. The output frequency of comparator multiplied by 3 represents motor speed FG ( pin6).

FG frequency is calculated as follows

Table 2. RPM selection

For example, 8 pole, three phase 5400 rpm motor,

Pin Value

Speed

System clock

(CLK, pin8)

Rpm selection

(RPMSEL, pin48)

5400 rpm

5MHz

L

7200 rpm

H

3600rpm

3.333MHz

L

4800rpm

H

POR

POR

POR

POR

CNTL1

CNTL1

CNTL1

CNTL1

CNTL2

CNTL2

CNTL2

CNTL2

CNTL3

CNTL3

CNTL3

CNTL3

SPEED

SPEED

SPEED

SPEED

INTERNAL

INTERNAL

INTERNAL

INTERNAL

READY

READY

READY

READY

READY

READY

READY

READY

soft

commutation

hard

commutatio

n

user

option

VCM

enable

0.27sec

TARGET

TARGET

TARGET

TARGET

RPM

RPM

RPM

RPM

+0.7%

-0.7%

start-up