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A3984 Datasheet(PDF) 6 Page - Allegro MicroSystems

Part No. A3984
Description  DMOS Microstepping Driver with Translator
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Manufacturer  ALLEGRO [Allegro MicroSystems]
Direct Link  http://www.allegromicro.com
Logo ALLEGRO - Allegro MicroSystems

A3984 Datasheet(HTML) 6 Page - Allegro MicroSystems

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Worcester, Massachusetts 01615-0036 (508) 853-5000
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
DMOS Microstepping Driver with Translator
Functional Description
Device Operation. The A3984 is a complete microstep-
ping motor driver with a built-in translator for easy operation
with minimal control lines. It is designed to operate bipolar
stepper motors in full-, half-, quarter-, and sixteenth-step
modes. The currents in each of the two output full-bridges
and all of the N-channel DMOS FETs are regulated with
fixed off-time PMW (pulse width modulated) control cir-
cuitry. At each step, the current for each full-bridge is set by
the value of its external current-sense resistor (RS1 or RS2), a
reference voltage (VREF), and the output voltage of its DAC
(which in turn is controlled by the output of the translator).
At power-on or reset, the translator sets the DACs and the
phase current polarity to the initial Home state (shown in fig-
ures 2 through 5), and the current regulator to Mixed Decay
Mode for both phases. When a step command signal occurs
on the STEP input, the translator automatically sequences the
DACs to the next level and current polarity. (See table 2 for
the current-level sequence.) The microstep resolution is set
by the combined effect of inputs MS1 and MS2, as shown in
table 1.
When stepping, if the new output levels of the DACs are
lower than their previous output levels, then the decay mode
for the active full-bridge is set to Mixed. If the new output
levels of the DACs are higher than or equal to their previous
levels, then the decay mode for the active full-bridge is set
to Slow. This automatic current decay selection improves
microstepping performance by reducing the distortion of
the current waveform that results from the back EMF of the
RESET Input (RESET). The RESET input sets the
translator to a predefined Home state (shown in figures 2
through 5), and turns off all of the DMOS outputs. All STEP
inputs are ignored until the RESET input is set to high.
Step Input (STEP). A low-to-high transition on the STEP
input sequences the translator and advances the motor one
increment. The translator controls the input to the DACs and
the direction of current flow in each winding. The size of
the increment is determined by the combined state of inputs
MS1 and MS2.
Microstep Select (MS1 and MS2). Selects the micro-
stepping format, as shown in table 1. MS2 has a 50 kΩ pull-
down resistance. Any changes made to these inputs do not take
effect until the next STEP rising edge.
Direction Input (DIR). This determines the direction of
rotation of the motor. When low, the direction will be clock-
wise and when high, counterclockwise. Changes to this input
do not take effect until the next STEP rising edge.
Internal PWM Current Control. Each full-bridge is
controlled by a fixed off-time PWM current control circuit
that limits the load current to a desired value, ITRIP. Ini-
tially, a diagonal pair of source and sink DMOS outputs are
enabled and current flows through the motor winding and
the current sense resistor, RSx. When the voltage across RSx
equals the DAC output voltage, the current sense compara-
tor resets the PWM latch. The latch then turns off either the
source DMOS FETs (when in Slow Decay Mode) or the sink
and source DMOS FETs (when in Mixed Decay Mode).
The maximum value of current limiting is set by the selec-
tion of RSx and the voltage at the VREF pin. The transcon-
ductance function is approximated by the maximum value of
current limiting, ITripMAX (A), which is set by
ITripMAX = VREF /(8
S )
where RS is the resistance of the sense resistor (Ω) and VREF
is the input voltage on the REF pin (V).
The DAC output reduces the VREF output to the current
sense comparator in precise steps, such that
Itrip = (%ITripMAX /100)
× ITripMAX
(See table 2 for %ITripMAX at each step.)
It is critical that the maximum rating (0.5 V) on the SENSE1
and SENSE2 pins is not exceeded.
Fixed Off-Time. The internal PWM current control cir-
cuitry uses a one-shot circuit to control the duration of time
that the DMOS FETs remain off. The one shot off-time, tOFF,
is determined by the selection of an external resistor con-
nected from the ROSC timing pin to ground. If the ROSC
Functional Description

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