A4979 데이터 시트보기 (PDF) - Allegro MicroSystems

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A4979

Microstepping Programmable Stepper Motor Driver With Stall Detect and Short Circuit Protection

Allegro MicroSystems 

A4979

Microstepping Programmable Stepper Motor Driver

With Stall Detect and Short Circuit Protection

For example:

• if RS = 180 mΩ and VREF = 2 V, then ISMAX = 694 mA

• if MXI1= 1 and MXI0 = 0, then IPMAX = 520 mA

The actual current delivered to each phase at each Step Angle

Number is determined by the value of IPMAX and the contents

of the Phase Current table. For each phase, the value in the table

is passed to the DAC, which uses IPMAX as the reference 100%

level (code 63) and reduces the current target depending on the

DAC code. The output from the DAC is used as the input to the

current comparators.

The current comparison is ignored at the start of the PWM

on-time for a duration referred to as the blank time. The blank

time is necessary to prevent any capacitive switching currents

from causing a peak current detection.

The PWM on-time starts at the beginning of each PWM period.

The current rises in the phase winding until the sense voltage

reaches the required current level. At this point the PWM off-time

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 A4979 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 A4979 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.

Allegro MicroSystems, LLC

11

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

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