3967 datasheet(5/16 Pages) ALLEGRO

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3967

MICROSTEPPING DRIVER

WITH TRANSLATOR

www.allegromicro.com

Device Operation. The A3967 is a complete

microstepping 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 H-bridges is regulated with fixed off time pulse-

width modulated (PWM) control circuitry. The H-bridge

current at each step is set by the value of an external

current sense resistor (RS), a reference voltage (VREF), and

the DAC’s 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

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 H-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 H-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 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 H-bridge is

controlled by a fixed off time PWM current-control circuit

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 com-

parator 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

current-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

Functional Description

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