A8904SLB Allegro Microsystems Inc, A8904SLB Datasheet - Page 12

• Manufacturer Part Number: A8904SLB
• Description: IC CTRLR/DRIVER DC MOTOR 24SOIC
• Manufacturer: Allegro Microsystems Inc

Specifications of A8904SLB

• Applications: DC Motor Driver, Brushless (BLDC), 3 Phase
• Number Of Outputs: 1
• Current - Output: ±1.4A
• Voltage - Load: 4 V ~ 14 V
• Voltage - Supply: 4.5 V ~ 5.5 V
• Operating Temperature: -20°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-SOIC (7.5mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

the blanking period, t
then the back-EMF detection circuit does not allow the watch-
dog 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.
waveform above) and the circuit is ready to detect the next back-
After the watchdog capacitor C
A8904
and if the correct polarity of back-EMF signal is detected, the
back-EMF detection circuit discharges C
If the correct polarity of back-EMF is not detected between
Watchdog-triggered commutation
WD
to be discharged and the watchdog circuit
BLANK,
EMF zero crossing.
and the watchdog period, t
WD
charges to the V
WD
3-Phase Brushless DC Motor Controller/Driver
Functional Description (cont’d)
to zero volts (see
TL
threshold,
WD
,
determined by:
where I
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.
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 revo-
lution. The number of poles are programmed via serial port bits
D20 and D21. There are six electrical states per electrical revolu-
tion, therefore, for this example, there are 12 commutations or
two electrical revolutions per mechanical revolution.
tion 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. Alternatively an
external TACH signal can be used, an explanation of which is
presented in the Sector Mode Section.
count the desired number of OSCILLATOR cycles, according to
the following:
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
The duration of the watchdog-triggered commutation is
Speed control. The actual speed of the motor is measured
Each back-EMF signal detected causes the state of the
The TACH signal changes state once per mechanical revolu-
The duration of REF is set by programming the counter to
total count = 60 x f
CWD
= normal charge current.
OSC
with Back EMF Sensing
corresponds to the OSCILLATOR frequency.
t
WD
= V
OSC
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
TH
/ desired motor speed (rpm)
x C
WD
/ I
CWD
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