A3977(2013) 查看數據表（PDF） - Allegro MicroSystems
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A3977 Datasheet PDF : 19 Pages
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A3977
Microstepping DMOS Driver with Translator
Functional Description
Device Operation. The A3977 is a complete microstep-
ping motor driver with built in translator for easy operation
with minimal control lines. It is designed to operate bipolar
stepper motors in full-, half-, quarter- and eighth-step
modes. The current in each of the two output full-bridges,
all N-channel DMOS, is regulated with fixed off-time
pulse-width modulated (PWM) control circuitry. The full-
bridge current at each step is set by the value of an external
current sense resistor (RS), a reference voltage (VREF), and
the DACs output voltage controlled by the output of the
translator.
At power up, or reset, the translator sets the DACs
and phase current polarity to initial home state (see figures
for home-state conditions), and sets the current regula-
tor for both phases to mixed-decay mode. When a step
command signal occurs on the STEP input the translator
automatically sequences the DACs to the next level (see
table 2 for the current level sequence and current polarity).
The microstep resolution is set by inputs MS1 and MS2
as shown in table 1. If the new DAC output level is lower
than the previous level the decay mode for that full-bridge
will be set by the PFD input (fast, slow, or mixed decay).
If the new DAC level is higher or equal to the previous
level then the decay mode for that full-bridge will be slow
decay. This automatic current-decay selection will improve
microstepping performance by reducing the distortion of
the current waveform due to the motor BEMF.
Reset Input (RESET). The RESET input (active low)
sets the translator to a predefined home state (see figures
for home state conditions) and turns off all of the DMOS
outputs. The HOME output goes low and all STEP inputs
are ignored until the RESET input goes high.
Home Output (HOME). The HOME output is a logic
output indicator of the initial state of the translator. At
power up the translator is reset to the home state (see fig-
ures for home state conditions).
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 wind-
ing. The size of the increment is determined by the state of
inputs MS1 and MS2 (see table 1).
Microstep Select (MS1 and MS2). Input terminals
MS1 and MS2 select the microstepping format per
table 1. Changes to these inputs do not take effect until the
STEP command (see figure).
Direction Input (DIR). The state of the DIRECTION
input will determine the direction of rotation of the motor.
Internal PWM Current Control. Each full-bridge is
controlled by a fixed off-time PWM current-control cir-
cuit that limits the load current to a desired value (ITRIP).
Initially, a diagonal pair of source and sink DMOS outputs
are enabled and current flows through the motor winding
and RS. When the voltage across the current-sense resistor
equals the DAC output voltage, the current-sense compara-
tor resets the PWM latch, which turns off the source driver
(slow-decay mode) or the sink and source drivers (fast- or
mixed-decay modes).
The maximum value of current limiting is set by the
selection of RS and the voltage at the VREF input with a
transconductance function approximated by:
ITRIPmax = VREF/8RS
The DAC output reduces the VREF output to the cur-
rent-sense comparator in precise steps (see table 2 for %
ITRIPmax at each step).
ITRIP = (% ITRIPmax/100) x ITRIPmax
It is critical to ensure that the maximum rating (0.5 V)
on the SENSE terminal is not exceeded. For full-step
mode, VREF can be applied up to the maximum rating of
VDD, because the peak sense value is 0.707 x VREF/8. In all
other modes VREF should not exceed 4 V.
Allegro MicroSystems, LLC
7
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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