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A8905CLB View Datasheet(PDF) - Allegro MicroSystems

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A8905CLB Datasheet PDF : 12 Pages
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8905
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
FUNCTIONAL DESCRIPTION
Power Outputs. The power outputs of the
A8905CLB are n-channel DMOS transistors with
a total source plus sink rDS(on) of typically 1.1 ā„¦.
Internal charge pump boost circuitry provides
voltage above supply for driving the high-side
DMOS gates. Intrinsic ground clamp and flyback
diodes provide protection when switching induc-
tive loads and may be used to rectify motor back-
EMF in power-down conditions. An external
Schottky power diode or pass FET is required in
series with the load supply to allow motor back-
EMF rectification in power-down conditions.
Back-EMF Sensing Motor Startup and
Running Algorithm. The A8905CLB provides a
complete self-contained back-EMF sensing startup
and running commutation scheme. The three half-
bridge outputs are controlled by a state machine.
There are six possible combinations. In each state,
one output is high (sourcing current), one low
(sinking current), and one is OFF (high impedance
or ā€˜Zā€™). Motor back EMF is sensed at the OFF
output. The truth table for the output drivers
sequencing is:
Sequencer
State
1
2
3
4
5
6
OUTA
High
Z
Low
Low
Z
High
OUTB
Low
Low
Z
High
High
Z
OUTC
Z
High
High
Z
Low
Low
At startup, the outputs are enabled in one of
the sequencer states shown. 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 moves, the
back-EMF detection circuit waits for the correct
polarity back-EMF zero crossing (output crossing
through centertap). True back-EMF zero cross-
ings are used by the adaptive commutation delay
circuit to advance the state sequencer (commutate)
at the proper time to synchronously run the motor.
Back-EMF zero crossings are indicated by FCOM, an internal signal that
toggles at every zero crossing. FCOM is available at the DATA OUT terminal
via the programmable data out multiplexer.
V OUTA
V OUTB
V OUTC
SOURCE ON
V CTAP
BACK-EMF VOLTAGE
SINK ON
FCOM TOGGLES AT
BACK-EMF ZERO CROSSING
FCOM
Dwg. WP-016-1
Startup Oscillator. If the motor does not move at the initial startup state,
then it is in a null-torque position. In this case, the outputs are commutated
automatically by the startup oscillator after a period set by the external
capacitor at CST.
VCST
VCSTH
VCWD
VCSTL
t CST
Dwg. WP-020
where
tCST =
4(VCSTH - VCSTL) x CST
IST(charge) + IST(discharge)
In the next state, the motor will move, back EMF will be detected, and the
motor will accelerate synchronously. Once normal synchronous back-EMF
commutation occurs, the startup oscillator is defeated by pulses of pulldown
current at CST at each commutation, which prevents CST from reaching its
upper threshold and thus completing a cycle and commutating.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
 

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