Microstepping DMOS Driver with Translator

A5976

7

Allegro MicroSystems, LLC

115 Northeast Cutoff, Box 15036

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

www.allegromicro.com

FUNCTIONAL DESCRIPTION

Device Operation

The A5976 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 sixteenth-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

output voltage of its DAC (which in turn is controlled by the

output of the translator).

At power-up, or reset, the translator sets the DACs and phase

current polarity to the 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 automatically sequences the DACs to

the next level (see Table 2 for the current level sequence and cur-

rent 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.

The DECAY input determines how the decay mode is selected

when stepping the motor. If the DECAY input is high, when step-

ping, if the new output levels of the DACs are higher than or equal

to their previous levels, then the decay mode for that full-bridge is

set to slow. If the DECAY input is high and the new output levels

of the DACs are lower than their previous output levels, then the

decay mode for that full-bridge is set by the state of the PFD input

(see PFD input description). 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.

If the DECAY input is low, then the decay mode is always set by

the state of the PFD input (see PFD input description). See Figure 6

on page 13 and Figure 7 on page 14 for decay mode detail.

Internal PWM Current Control

Each full-bridge is controlled by a fixed off-time PWM current-

control circuit that limits the load current to an appropriate

outputs are enabled, and current flows through the motor wind-

resets the PWM latch, which turns off the source driver (in slow-

decay mode) or the sink and source drivers (in fast- or mixed-

decay mode).

The maximum level of current limiting is set by the selection of

function approximated by:

The DAC output reduces the VREF output to the current-sense

step).

It is critical to ensure that the maximum rating on the SENSE

exceed 4 V.

Fixed Off-Time

The internal PWM current-control circuitry uses a one-shot to

control the time that the drivers remain off. The one-shot off-

by: