DMOS Microstepping Driver with Translator

A3979

8

Allegro MicroSystems, Inc.

115 Northeast Cutoff, Box 15036

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

www.allegromicro.com

Functional Description

stepping 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 (all of the N-channel MOSFETs) are regulated

with fixed off-time PMW (pulse width modulated) control

circuitry. At each step, the current for each full-bridge is

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

figures 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.

While stepping is occurring, if the next output level of the

DACs is lower than the immediately preceeding output

level, then the decay mode (Fast, Slow, or Mixed) for the

active full bridge is set by the PFD input. If the next DAC

output level is higher than or equal to the preceeding level,

then the decay mode for that full bridge will be Slow decay.

This automatic current-decay selection improves microstep-

ping performance by reducing the distortion of the current

waveform due to back EMF of the motor.

RESET Input ( ¯

R

¯¯

E

¯¯

S

¯¯

E

¯¯

T

low) sets the translator to a predefined Home state (shown

in figures 2 through 5), and turns off all of the DMOS out-

puts. The HOME output goes low and all STEP inputs are

ignored until the ¯R¯¯E¯¯S¯¯E¯¯T¯ input is set to high.

output indicator of the initial state of the translator. At

power-on, the translator is reset to the Home state (shown in

figures 2 through 5).

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 (see table 1).

terminals MS1 and MS2 selects the microstepping format,

as shown in table 1. Any changes made to these inputs do not

take effect until the next rising edge of a step command signal

on the STEP input.

mines the direction of rotation of the motor. Any changes

made to this input does not take effect until the next rising

edge of a step command signal on the STEP input.

controlled by a fixed–off-time PWM current-control circuit

a diagonal pair of source and sink MOS outputs are enabled

and current flows through the motor winding and the current

DAC output voltage, the current-sense comparator resets the

PWM latch. The latch then turns off either the source MOS-

FETs (when in Slow decay mode) or the sink and source

MOSFETs (when in Fast or Mixed decay mode).

The maximum value of current limiting is set by the selec-

ductance function approximated by:

rent-sense comparator in precise steps (see table 2 for

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

SENSE1 and SENSE2 pins is not exceeded. For full step-

Functional Description