A4979 View Datasheet(PDF) - Allegro MicroSystems

Part Name

Description

Manufacturer

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

SCK Serial interface clock. Data is latched in from SDI on the

rising edge of the SCK clock signal. There must be 16 rising

edges per write and SCK must be held high when STRn changes.

STRn Serial data strobe and serial access enable. When STRn

is high any activity on SCK or SDI is ignored, and SDO is high

impedance allowing multiple SDI slaves to have common SDI,

SCK, and SDO connections.

DIAG Diagnostic output. Function selected via the serial inter-

face, setting Configuration Register 1. Default is Fault output.

OSC With bit 13 in Configuration Register 1 set to 0, either con-

nect this pin to AGND to use the internal oscillator running at the

default frequency of 4 MHz, or connect a resistor to VDD to set

the internal oscillator frequency. ( The approximate frequency is

calculated from:

fOSC = 10 000 / (48 ROSC – 20)

where fOSC is the internal oscillator frequency in MHz, and ROSC

is the value, in kΩ of the resistor between OSC and VDD.)

If bit 13 in Configuration Register 1 is set to 1, then OSC is the

input for an external system clock, which must have a frequency

between 3 and 5 MHz. In this mode a watchdog is provided to

detect loss of the system clock. If the OSC pin remains high or

low for more than the watchdog time, tWD , 1 μs typical, then the

Fault Register flag (bit 15 in the diagnostic registers) is set and

the outputs are disabled until the clock restarts.

Driving a Stepper Motor

A two-phase stepper motor is made to rotate by sequencing the

relative currents in each phase. In its simplest form, each phase

is simply fully energized in turn by applying a voltage to the

winding. For more precise control of the motor torque through

temperature and voltage ranges, current control is required. For

efficiency this is usually accomplished using pulse width modula-

tion (PWM) techniques. In addition current control also allows

the relative current in each phase to be controlled, providing more

precise control over the motor movement and hence improve-

ments in torque ripple and mechanical noise. Further details of

stepper motor control are provided in Appendix A.

For bipolar stepper motors the current direction is significant,

so the voltage applied to each phase must be reversible. This

requires the use of a full bridge (also known as an H-bridge)

which can switch each phase connection to supply or to ground.

Phase Current Control

In the A4979, current to each phase of the two-phase bipolar

stepper motor is controlled through a low impedance N-channel

DMOS full bridge. This allows efficient and precise control of

the phase current using PWM switching. The full-bridge con-

figuration provides full control over the current direction during

the PWM on-time, and over the current decay mode during the

PWM off-time. Due to the flexibility of the A4979 these control

techniques can be completely transparent to the user or can be

partially- or fully-programmed through the serial interface.

Each leg (high-side, low-side pair) of a bridge is protected from

shoot-through by a fixed dead time. This is the time between

switching off one FET and switching on the complementary FET.

Cross-conduction is prevented by lock-out logic in each driver pair.

The phase currents and in particular the relative phase currents

are defined in the Phase Current table (table 7). This table defines

the two phase currents at each microstep position. For each of the

two phases, the currents are measured using a sense resistor, RS,

with voltage feedback to the respective SENSx pin. The target

current level is defined by the voltage from the digital-to-analog

converter (DAC) for that phase. The sense voltage is amplified by

a fixed gain and compared to the output of the DAC.

There are two types of maximum current: the absolute maximum,

ISMAX , the maximum possible current defined by the sense resis-

tor and the reference input; and the phase maximum, IPMAX , the

maximum current delivered to a motor phase.

The absolute maximum current, ISMAX, is defined as:

ISMAX = VREF / (16 × RS )

where VREF is the voltage at the REF pin, and RS is the sense

resistor value.

The phase maximum, IPMAX , is the 100% reference level for the

phase current table and may be a fraction of the absolute maxi-

mum current, ISMAX , depending on the value of the MXI0 and

MXI1 bits in Configuration Register 0.

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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