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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： | 8/18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
8
Functional Description
Overview of operation.
Each electrical revolution
contains six states that control the three half-bridge outputs.
Optimized switching from state to state is achieved through the
adaptive commutation circuitry. During any state, one output is
high, one is low and the other is high impedance. The back-
EMF at the high-impedance output is sensed and compared to
the voltage of the centertap and when the two signals are
equivalent, the FCOM signal toggles. A controlled delay is then
introduced before the sequencer commutates into the next state.
Linear current-mode control is employed to provide
precision control of the motor speed while maintaining ex-
tremely low electrical noise emissions. The speed control is
realized through a frequency-locked loop that processes the
sensed back-EMF signals from the stator phases to eventually
produce a TACH signal. The TACH signal is then compared to
the desired programmed speed, to produce an error. The error
signal is then used to linearly control the current through the
low-side DMOS power devices to obtain the correct speed.
Alternative control schemes can be introduced, giving the
user maximum flexibility and optimization for each application.
An external tachometer signal applied to the SECTOR DATA
input, along with the internal speed reference can be used for
high-precision speed control. As another alternative, the user can
introduce external speed control by driving the FILTER terminal
directly.
Start-up routines are inherent in the solution to guarantee
reliable start-up. During start-up, a YANK feature allows rapid
transition to the nominal operating condition on the FILTER
terminal. This feature is also available when the external speed
control is used.
Dynamic braking can be introduced by either the external
BRAKE terminal or through the brake bit in the serial port.
A serial port allows the user to program various features and
modes of operation, such as motor speed, internal or external
speed control, internal or external speed reference, current limit,
sleep mode, direction, charge current (for blanking pulse), motor
poles, transconductance gain, and various diagnostic outputs.
Full device protection is incorporated, including program-
mable overcurrent limit, thermal shutdown, and undervoltage
shutdown on the logic supply.
Power outputs.
The power outputs of the A8904 are n-
channel DMOS transistors with a total source plus sink r
DS(on)
of
typically 1
. An internal charge pump provides a voltage rail
above the load supply for driving the high-side DMOS gates.
Intrinsic ground clamp and flyback diodes provide protection
when switching inductive loads. These diodes will also rectify
the motor back-EMF during power-down conditions. If neces-
sary, a transient voltage supply can be provided, by connecting
an external Schottky power diode or pass FET in series, between
the power source and the load supply (V
BB
). This FET or diode
effectively isolates the low impedance path through the power
source. A filter capacitor is also required to ‘hold up’ the
rectified signal, and is connected between the load supply and
ground.
Back-EMF sensing motor startup and running
algorithm.
The A8904 provides a complete self-contained
back-EMF sensing, startup and running commutation scheme. A
state machine with six states, (shown in the tables below for
both forward and reverse direction) controls the three half-
bridge outputs. In each state, one output is high (sourcing
current), one low (sinking current), and one is OFF (high
impedance or ‘Z’). Motor back-EMF is sensed at the output that
is OFF.
Sequencer State
(forward direction)
OUT
A
1
High
2
High
3
Z
4
Low
5
Low
6
Z
OUT
B
Z
Low
Low
Z
High
High
OUT
C
Low
Z
High
High
Z
Low
Sequencer State
(reverse direction)
1
6
5
4
3
2
OUT
A
High
Z
Low
Low
Z
High
OUT
B
Z
High
High
Z
Low
Low
OUT
C
Low
Low
Z
High
High
Z
At start-up, the outputs are always enabled in state 1. The
back-EMF is examined at the OFF output by comparing the
output voltage to the motor centertap voltage at CENTERTAP.
The motor will then either step forward, step backward or
remain stationary (if in a null-torque position).
If the motor does not move during the initial start-up state,
the outputs are commutated automatically by the start-up
oscillator. When suitable back-EMF signals are detected, the
start-up oscillator is overridden and the corresponding timing
clock is generated, providing synchronous back-EMF commuta-
tion. The start-up oscillator period is determined by
t
CST
= (V
CSTH
- V
CSTL
) x C
ST
/ I
ST(charge)
where C
ST
is the start-up capacitor.
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