SLFS040

– DECEMBER 1993

POST OFFICE BOX 655303

DALLAS, TEXAS 75265

14

PRINCIPLES OF OPERATION

back-EMF mode (continued)

The transfer function between COMDLY2 and VPHASE is :

Vcomdly2 =

s

(Rcom2

Ccom1)

VPHASE

[1+ s

(Rcom1

Ccom1)]

[1 + s

(Rcom2

Ccom2)]

where:

s is a Laplace operator

The component design procedures include the following:

Choose Rcom2 with a value as large as possible to permit small capacitor values. The typical value for

Rcom2 is 1 M

Calculate Ccom2 so that there is at least 45 of phase shift at the phase frequency where the TLS2205 is

switched into the internally commutated mode. The approximate value is 10% of the target frequency.

Calculate Rcom1 based on the desired COMDLY2 signal swing at the target frequency. The

recommended signal swing is 1.25 V peak. Near the target frequency the integrator function,

VCOMDLY2/VPHASE, becomes:

VCOMDLY2[Mag] =

VPHASE

1

(Rcom1

Ccom2)

(3

num motor poles/2)

Wmotor

VCOMDLY2[Phase] =

–90

VPHASE

VCOMDLY2 =

0.433

Kb

Wmotor

sin(Wmotor

t)

(Rcom1

Ccom2)

(3

num motor poles/2)

Wmotor

VCOMDLY2 =

0.433

Kb

sin(Wmotor

t)

(Rcom1

Ccom2)

(3

num motor poles/2)

The peak value of this function is independent of the frequency.

Rcom1 =

0.433

Kb

1.25

Ccom2

(3

Num Motor Poles/2)

Choose Ccom1 such that the dc-blocking pole, 1/(Rcom1

Ccom1), occurs at a frequency below the

integration pole, 1/(Rcom2 Ccom2). Typically the dc-blocking pole is placed at 1/2 the integration pole.

Where:

Wmotor = Mechanical frequency of the motor, r/s

Kb = Back-EMF constant, V/(r/s)

The internal speed-regulation feedback loop then takes control of spindle commutation, and further

microprocessor

intervention

is

not

required.

Figure

5

shows

the

timing

relationships

for

the

spindle-motor-control sequencing in both Hall and back-EMF modes. Internal logic and analog filtering provide

the commutation function in the commonly used back-EMF mode. In this mode, VPHASE (the difference

between the undriven phase and the center tap voltage) is filtered to generate a signal that determines

commutation timing (NHphase).