Low Voltage Stepper and Single/Dual DC Motor Driver

A3906

8

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Parallel Operation

The A3906 can be paralleled for applica-

tions that require higher output currents. In paralleled mode the

driver can source 2 A continuous. The A3906 has two completely

independent bridges with separate overcurrent latches. This

allows the device to supply two separate loads, and as a result,

when paralleled it is imperative that the internal current control is

disabled by shorting the sense pins to ground.

Because the overcurrent trip threshold is internally fixed at 0.2 V,

the trace resistance must be kept small so the internal current

latch is not triggered prematurely. With acceptable margin, the

voltage drop across the trace resistance should be under 0.1 V. At

a peak current of 2.5 A, the trace resistance should be kept below

40 mΩ to prevent false tripping of the overcurrent latch.

Each bridge has some variation in propagation delay. During this

time it is possible that one bridge will have to support the full

load current for a very short period of time. Propagation delays

are characterized and guard banded to protect the driver from

damage during these events.

Layout

The printed circuit board should use a heavy ground-

plane. For optimum electrical and thermal performance, the

A3906 must be soldered directly onto the board. On the under-

side of the A3906 package is an exposed pad, which provides a

path for enhanced thermal dissipation. The thermal pad should be

soldered directly to an exposed surface on the PCB. Thermal vias

are used to transfer heat to other layers of the PCB.

Grounding

In order to minimize the effects of ground bounce

and offset issues, it is important to have a low impedance single-

point ground, known as a star ground, located very close to the

device. By making the connection between the exposed thermal

pad and the ground plane directly under the A3906, that area

DC Motor Operation (Parallel Bridge)

IN1/IN3

IN2/IN4

OUT1A/2A

OUT1B/2B

Function

0

0

OFF

OFF

Disabled

1

0

H

L

FOR

0

1

L

H

REV

1

1

L

L

BRAKE

OUT1A

OUT1B

OUT2A

OUT2B

VBB

GND

IN1

IN2

IN3

10 μF

10 V

IN4

SENSE2

SENSE1

FL1

FL2

0.1 μF

0.1 μF

A3906

0.1 μF

SLEEP