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参数资料
| 型号： | 8904 |
| 厂商： | Allegro MicroSystems, Inc. |
| 英文描述： | 3-PHASE BRUSHLESS DC MOTOR CONTROLLER/DRIVER WITH BACK-EMF SENSING |
| 中文描述： | 3相无刷直流电动机控制器/驱动器，反电动势遥感 |
| 文件页数： | 10/18页 |
| 文件大小： | 744K |
| 代理商： | 8904 |
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
WD
in the actual
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
WD
can be determined as:
C
WD
= I
CWD
x t
d
/ V
TL
where t
d
= measured diode conduction, I
CWD
= charge current at
start-up, and V
TL
= 250 mV.
t
BLANK
Dwg. WP-022
BLANK
CWD
V
V
TL
t
BLANK
t
WD
V
TL
V
TH
Dwg. WP-021
BLANK
CWD
V
After the watchdog capacitor C
WD
charges to the V
TL
threshold,
and if the correct polarity of back-EMF signal is detected, the
back-EMF detection circuit discharges C
WD
to zero volts (see
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
BLANK,
and the watchdog period, t
WD
, then
the back-EMF detection circuit does not allow the watchdog
capacitor C
WD
to be discharged and the watchdog circuit
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
WD
= V
TH
x C
WD
/ I
CWD
where I
CWD
= normal charge current.
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
OSC
/ desired motor speed (rpm)
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
OSC
corresponds to the OSCILLATOR fre-
quency.
Functional Description (cont’d)
Normal commutation
Watchdog-triggered commutation
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