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A3989

Bipolar Stepper and High Current DC Motor Driver

Allegro MicroSystems 

A3989

Bipolar Stepper and High Current DC Motor Driver

Functional Description

Device Operation The A3989 is designed to operate one

dc motor and one bipolar stepper motor. The currents in each of

the full bridges, all N-channel DMOS, are regulated with fixed

off-time pulse width modulated (PWM) control circuitry. The

peak current in each full bridge is set by the value of an external

current sense resistor, RSx , and a reference voltage, VREFx .

If the logic inputs are pulled up to VDD, it is good practice to use

a high value pullup resistor in order to limit current to the logic

inputs should an overvoltage event occur. Logic inputs include:

PHASEx, I0x, I1x, ENABLE, and MODE.

Internal PWM Current Control Each full-bridge is

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

limits the load current to a user-specified value, ITRIP . Initially,

a diagonal pair of source and sink DMOS outputs are enabled

and current flows through the motor winding and RSx. When the

voltage across the current sense resistor equals the voltage on the

VREFx pin, the current sense comparator resets the PWM latch,

which turns off the source driver.

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

RS and the voltage at the VREF input with a transconductance

function approximated by:

ITripMax = VREF / (3×RS)

The stepper motor outputs will define each current step as a

percentage of the maximum current, ITripMax. The actual current at

each step ITrip is approximated by:

ITrip = (% ITripMax / 100) ITripMax

where % ITripMax is given in the Step Sequencing table.

Note: It is critical to ensure that the maximum rating of ±500 mV

on each SENSEx pin is not exceeded.

Fixed Off-Time The internal PWM current control circuitry

uses a one shot circuit to control the time the drivers remain off.

The one shot off-time, toff , is internally set to 30 µs.

Blanking This function blanks the output of the current sense

comparator when the outputs are switched by the internal current

control circuitry. The comparator output is blanked to prevent

false detections of overcurrent conditions, due to reverse recovery

currents of the clamp diodes, or to switching transients related to

the capacitance of the load. Dc motors require more blank time

than stepper motors. The stepper driver blank time, tBLANKst ,

is approximately 1 μs. The dc driver blank time, tBLANKdc , is

approximately 3 μs.

Control Logic Stepper motor communication is implemented

via industry standard I1, I0, and PHASE interface. This commu-

nication logic allows for full, half, and quarter step modes. Each

bridge also has an independent VREF input so higher resolution step

modes can be programmed by dynamically changing the voltage

on the corresponding VREFx pin. The dc motor is controlled using

standard PHASE, ENABLE communication. Fast or slow current

decay during the off-time is selected via the MODE pin.

Charge Pump (CP1 and CP2) The charge pump is used to

generate a gate supply greater than the VBB in order to drive the

source-side DMOS gates. A 0.1 μF ceramic capacitor should be

connected between CP1 and CP2 for pumping purposes. A 0.1 μF

ceramic capacitor is required between VCP and VBBx to act as a

reservoir to operate the high-side DMOS devices.

Shutdown In the event of a fault (excessive junction tem-

perature, or low voltage on VCP), the outputs of the device are

disabled until the fault condition is removed. At power-up, the

undervoltage lockout (UVLO) circuit disables the drivers.

Synchronous Rectification When a PWM-off cycle is

triggered by an internal fixed off-time cycle, load current will

recirculate. The A3989 synchronous rectification feature will

turn on the appropriate MOSFETs during the current decay. This

effectively shorts the body diode with the low RDS(on) driver. This

significantly lowers power dissipation. When a zero current level

Allegro MicroSystems, Inc.



115 Northeast Cutoff, Box 15036

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

www.allegromicro.com

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