A8904SLB View Datasheet(PDF) - Allegro MicroSystems
Part Name
Description
MFG CO.
'A8904SLB' PDF : 19 Pages View PDF
8904
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
Functional Description (cont’d)
Speed and current control
Sector mode. An external tachometer signal, such as
sector or index pulses, can be used to create the TACH signal,
rather than the internally generated once-around scheme. The
external signal is applied to the SECTOR DATA terminal and
the serial port bit (D19 = 1) must be programmed to enable this
feature.
In applications where both internal and external TACH
signals are used, it is important to only switch between modes
when the SYNC signal on DATA OUT is low. This ensures the
speed control information that is being processed during the
transition, is not corrupted. SYNC is accessed through the
DATA OUT multiplexer, which is controlled by D22 & D23.
DATA OUT. The DATA OUT terminal is the output of a
2-bit input multiplexer controlled by D22 & D23 of the serial
port. Data available are TACH signal (internally or externally
generated), SYNC signal, FCOM signal, and thermal shutdown
(LOW = A8904 operating within thermal limits, HIGH =
thermal shutdown has occurred).
Speed loop initialization (YANK). To ensure rapid
transition from start-up to the normal operating condition, the
FILTER terminal is pulled up to the filter threshold voltage,
VFILTERTH, by the internal YANK command and the initial
output current will be set to the maximum selected current limit.
This condition is maintained until the motor reaches the correct
speed and the first ERROR FAST signal is produced which
removes the YANK and allows linear current control.
The YANK feature is also activated when an external speed
control scheme is used (D24 = 1). To ensure the YANK is
released at start-up by the internal speed control, it is important
to ensure the speed reference is set at a lower speed than what
the motor is designed to run at. Note that when the serial port is
programmed to run initially, the default condition for the speed
is set for the slowest condition so this will guarantee the YANK
to be released. It is important when using external speed control
that, as a minimum, the number of poles, speed control mode,
and speed reference are programmed in the serial port.
Forward/reverse. Directional control is managed
through D25 in the serial port.
Serial port. Control features and diagnostic data selection
are communicated to the A8904 through the 29-bit serial port.
See serial port timing diagrams on page 6. When CHIP SE-
LECT is low, data is written to the serial port on the positive
edge of the clock with the MSB (D28) fed in first. At the end of
115 Northeast Cutoff, Box 15036
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Worcester, Massachusetts 01615-0036 (508) 853-5000
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