8904

3-PHASE BRUSHLESS DC

MOTOR CONTROLLER/DRIVER

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

10

unbalance in the commutation points.

It is recommended to select the value of C

application circuit with the A8904 put into step mode. C

ST

should be reselected (only for this test), to be between 4.7 µF

and 10 µF, so that the motor comes to rest between steps and the

maximum diode conduction time can be measured. The value of

C

C

where t

start-up, and V

t BLANK

Dwg. W P-022

BLANK

CWD

V

V

TL

t

WD

V

TL

V

TH

Dwg. W P-021

BLANK

CWD

V

After the watchdog capacitor C

and if the correct polarity of back-EMF signal is detected, the

back-EMF detection circuit discharges C

waveform above) and the circuit is ready to detect the next back-

EMF zero crossing.

If the correct polarity of back-EMF is not detected between

the blanking period, t

the back-EMF detection circuit does not allow the watchdog

capacitor C

commutates the outputs to the next sequencer state (see wave-

form above). This mode of operation continues until a suitable

back-EMF signal is detected. This function is useful in prevent-

ing excessive reverse rotation, and helps in resynchronising (or

starting) with a moving spindle.

The duration of the watchdog-triggered commutation is

determined by:

t

where I

Speed control. The actual speed of the motor is mea-

sured by either internally sensing the back-EMFs or by an

external scheme via the SECTOR DATA terminal. A TACH

signal is produced from these signals, which is then compared

against the desired speed, which is programmed into a 14-bit

counter (see diagram and waveforms below - assumes internal

scheme used). The resulting error signal, ERROR, is then used

to charge or discharge the FILTER terminal capacitor depending

on whether the motor is running too slow or too fast. The

FILTER terminal voltage is used to linearly drive the low-side

MOSFETs to match the desired speed.

Each back-EMF signal detected causes the state of the

FCOM signal to change. The number of FCOM transitions per

mechanical revolution is equal to the number of poles times 3.

For example, with a 4-pole motor (as shown on next page), the

number of FCOM transitions will equal 12 per mechanical

revolution. The number of poles are programmed via serial port

bits D20 and D21. There are six electrical states per electrical

revolution, therefore, for this example, there are 12 commuta-

tions or two electrical revolutions per mechanical revolution.

The TACH signal changes state once per mechanical

revolution and as well as providing information on the actual

motor speed is also used to trigger the REF counter which

contains the information on the desired motor speed. Alterna-

tively an external TACH signal can be used, an explanation of

which is presented in the Sector Mode Section.

The duration of REF is set by programming the counter to

count the desired number of OSCILLATOR cycles, according to

the following:

total count = 60 x f

where the total count (number of oscillator cycles) is equal to the

sum of the count numbers selected through bits D5 to D18 in the

serial port and f

quency.

Functional Description (cont’d)

Normal commutation

Watchdog-triggered commutation