A3981 View Datasheet(PDF) - Allegro MicroSystems

Part Name

Description

Manufacturer

A3981

Automotive, Programmable Stepper Driver

Allegro MicroSystems 

A3981

Automotive, Programmable Stepper Driver

starts and the bridge is switched into one of two decay modes,

slow decay or fast decay:

• Slow decay is most effective when the current is rising from

step to step, and it occurs when the phase winding is effectively

shorted by switching-on either both high-side FETs or both low-

side FETs in the full bridge.

• Fast decay is most effective when the current is falling from

step to step, and it occurs when the voltage on the phase is

reversed.

One disadvantage of fast decay is the increased current ripple in

the phase winding. However, this can be reduced while main-

taining good current control, by using a short time of fast decay

followed by slow decay for the remainder of the PWM off-time.

This technique is commonly referred to as mixed decay.

The A3981 provides two methods to determine the PWM

frequency: fixed off-time and fixed frequency. At power-up the

default mode is fixed off-time. Fixed frequency can be selected

through the serial interface. Fixed off-time provides a marginal

improvement in current accuracy over a wide range of current

levels. Fixed frequency provides a fixed fundamental frequency

to allow more precise supply filtering for EMC reduction. In both

cases the PWM off-time will not be present if the peak current

limit is not attained during the PWM on-time.

Phase Current Table

The relative phase currents are defined by the Phase Current table

(table 7). This table contains 64 lines and is addressed by the Step

Angle Number, where Step Angle Number 0 corresponds to 0° or

360°. The Step Angle Number is generated internally by the step

sequencer, which is controlled either by the STEP and DIR inputs

or by the step change value from the serial input. The Step Angle

Number determines the motor position within the 360° electri-

cal cycle and a sequence of Step Angle Numbers determines the

motor movement. Note that there are four full mechanical steps

per 360° electrical cycle.

Each line of the Phase Current table (table 7) has a 6-bit value per

phase to set the DAC level for that phase, plus an additional bit

per phase to determine the current direction for that phase. The

Step Angle Number sets the electrical angle of the stepper motor

in one-sixteenth microsteps, approximately equivalent to electri-

cal steps of 5.625°.

On first power-up or after a VDD power-on reset, the Phase Cur-

rent table values are reset to define a sinusoidal current profile

and the Step Angle Number is set to 8, equivalent to the electri-

cal cycle 45° position. This position is defined as the “home”

position. The maximum current in each phase, IPMAX , is defined

by the sense resistor and the Maximum Current setting (bits

MXI[0..1]) in Configuration Register 0. The phase currents for

each entry in the Phase Current table are expressed as a percent-

age of this maximum phase current.

When using the STEP and DIR inputs to control the stepper

motor, the A3981 automatically increases or decreases the Step

Angle Number according to the step sequence associated with

the selected step mode. The default step mode, reset at power-

up or after a power on reset, is full step. Half-, quarter-, and

sixteenth-step sequences are also available when using the STEP

and DIR inputs, and are selected using the logical OR of the MS0

and MS1 inputs and the MS0 and MS1 bits in Configuration Reg-

ister 0. The eighth-step sequence is shown in the Phase Current

table for reference only.

When using the serial interface to control the stepper motor, a

step change value (6-bit) is input through the serial interface to

increase or decrease the Step Angle Number. The step change

value is a two’s complement (2’sC) number, where a positive

value increases the step angle and a negative value decreases

the step angle. A single step change in the Step Angle Number is

equivalent to a single one-sixteenth microstep. Therefore, for cor-

rect motor movement, the step change value should be restricted

to no greater than 16 steps, positive or negative.

This facility enables full control of the stepper motor at any

microstep resolution up to and including sixteenth-step, plus

the ability to change microstep resolution “on-the-fly” from one

microstep to the next.

In both control input method cases, the resulting Step Angle

Number is used to determine the phase current value and current

direction for each phase, based on the Phase Current table. The

decay mode is determined by the position in the Phase Current

table and the intended direction of rotation of the motor.

Diagnostics

The A3981 integrates a number of diagnostic features to protect

the driver and load as far as possible from fault conditions and

extreme operating environments. At the system level the supply

voltages and the chip temperature are monitored. A number of

these features automatically disable the current drive to protect

the outputs and the load. Others only provide an indication of

the likely fault status, as shown in the Fault table (table 1). A

single diagnostic output pin (DIAG) can be programmed through

the serial interface to provide several different internal signals.

At power-up, or after a power-on-reset the DIAG pin outputs a

simple Fault Output flag which will be low if a fault is present.

The Fault Output flag remains low while the fault is present or if

one of the latched faults (for example, a bridge short circuit) has

been detected and the outputs disabled.

Allegro MicroSystems, LLC

10

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

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