6

26184.30A

Worcester, Massachusetts 01615-0036 (508) 853-5000

115 Northeast Cutoff, Box 15036

www.allegromicro.com

Allegro MicroSystems, Inc.

A3984

DMOS Microstepping Driver with Translator

Functional Description

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

(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

motor.

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.

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.

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.

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.

controlled by a fixed off-time PWM current control circuit

tially, a diagonal pair of source and sink DMOS outputs are

enabled and current flows through the motor winding and

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-

ductance function is approximated by the maximum value of

is the input voltage on the REF pin (V).

sense comparator in precise steps, such that

It is critical that the maximum rating (0.5 V) on the SENSE1

and SENSE2 pins is not exceeded.

cuitry uses a one-shot circuit to control the duration of time

is determined by the selection of an external resistor con-

nected from the ROSC timing pin to ground. If the ROSC

Functional Description