A8905CLB データシートの表示（PDF） - Allegro MicroSystems
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A8905CLB Datasheet PDF : 12 Pages
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8905
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
The watchdog timing function allows time to detect correct motor position
by checking the back-EMF polarity after each commutation. If the correct
polarity is not observed between tBLANK and tWD, then the watchdog timer
commutates the outputs to the next state to synchronize the motor. This
function is useful in preventing excessive reverse rotation, and helps in
resynchronizing (or starting) with a moving spindle.
Current Control. The A8905CLB provides linear current control via the
FILTER terminal, an analog voltage input. Maximum current limit is also
provided, and is controlled in four steps via the serial port. Output current is
sensed via an internal sense resistor (RS). The voltage across the sense resistor
is compared to one-tenth the voltage at the FILTER terminal less two diode
drops, or to the maximum current limit reference, whichever is lower. This
transconductance function is IOUT = (VFILTER -2VD) / 10RS, where RS is
nominally 0.3 Ω and VD is approximately 0.7 V.
POWER UP
YANK
SQ
ERROR FAST R
FROM FLL
V DD
SPEED-CONTROL
INITIALIZATION
ERROR SLOW
V DD
FROM FLL
Ic
FILTER
C F1
Id
x1
RF1
C F2
1.85 V
CHARGE
ERROR FAST
SEQUENTIAL
LOGIC
C RES
BOOST
CHARGE
PUMP
MUX
÷10
V BB
OUT
–
+
FROM
SERIAL PORT
REGISTER
D3 AND D4
LINEAR
RS
CURRENT CONTROL
Dwg. EP-046
Speed Control. The A8905CLB includes a frequency-locked loop speed
control system. This system monitors motor speed via internal or external
digital tachometer signals, generates a precision speed reference, determines
the digital speed error, and corrects the motor current via an internal charge
pump and external filtering components on the FILTER terminal.
FCOM
SECTOR
COUNT
(3 x MOTOR POLES)
D20 &
D21
ONCE-AROUND
PULSE
MUX
D19
SERIAL PORT
REGISTER
D5–D18
4-BIT
14-BIT
OSC
FIXED
PROGRAMMABLE
COUNTER
COUNTER
REF
÷2
TACH
REF
TACH
ERROR
SLOW
REF
TACH
ERROR
FAST
Dwg. EP-045
A once-per-revolution TACH signal can be
generated by counting cycles of FCOM (the
number of motor poles must be selected via the
serial port). TACH is then a jitter-free signal that
toggles once per motor revolution. The rising
edge of TACH triggers REF, a precision speed
reference derived by a programmable counter.
The duration of REF is set by programming the
counter to count the desired number of OSC
cycles
desired =
60 x fOSC
total count
desired motor speed (rpm)
where the total count (number of oscillator cycles)
is equal to the sum of the selected (programmed
low) count numbers corresponding to bits D5
through D18.
The speed error is detected as the difference
in falling edges of TACH and REF. The speed
error signals control the error-correcting charge
pump on the FILTER terminal, which drive the
external loop compensation components to correct
the motor current.
Index. An external tachometer signal may be
used to create the TACH signal, rather than the
internally derived once around. To use this mode,
the signal is input to the INDEX terminal, and the
index mode must be enabled via the serial port.
When Switching from the once-around mode to
index mode, it is important to monitor the SYNC
signal on DATA OUT, and switch modes only
when SYNC is low. This ensures making the
transition without disturbing the speed control
loop. The speed reference counter should be
reprogrammed at the same time.
Speed Loop Initialization (YANK). To
improve the acquire time of the speed control
loop, there is an automatic feature controlled by an
internal YANK signal. The motor is started at the
maximized programmed current by bypassing the
FILTER terminal. The FILTER terminal is
clamped to two diodes above ground, initializing it
near the closed loop operating point. YANK is
enabled at startup and stays high until the desired
speed is reached. Once the first error-fast occurs,
indicating the motor crossed through the desired
speed, YANK goes low. This releases the clamp
on the FILTER terminal and current control is
returned to FILTER. This feature optimizes speed
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
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