A3967SLBTR-T Просмотр технического описания (PDF) - Allegro MicroSystems

Номер в каталоге

Компоненты Описание

производитель

A3967SLBTR-T

Microstepping Driver with Translator

Allegro MicroSystems 

A3967

Microstepping Driver with Translator

Functional Description

Device Operation. The A3967 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

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 resis-

tor (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 regulator for

both phases to mixed-decay mode. When a step command

signal occurs on the STEP input the translator automati-

cally sequences the DACs to the next level (see table 2 for

the current level sequence and current polarity). The mic-

rostep resolution is set by inputs MS1 and MS2 as shown in

table 1. If the new DAC output level is lower than the pre-

vious 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 microstep-

ping 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 outputs.

STEP inputs are ignored until the RESET input goes 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 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 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

Fixed Off-Time. The internal PWM current-control

circuitry uses a one shot to control the time the driver(s)

remain(s) off. The one shot off-time, toff, is determined by

the selection of an external resistor (RT) and capacitor (CT)

connected from the RC timing terminal to ground. The off

time, over a range of values of CT = 470 pF to 1500 pF and

RT = 12 kΩ to 100 kΩ is approximated by:

toff = RTCT

Allegro MicroSystems, Inc.

5

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Share Link: