The Super
STEPPER DRIVER
by Fred Eady
Because I get to put together all
kinds of neat stuff. And, when
I’m done playing with my new
garage-brewed toys, I get to show
them to you and tell you all about
them. So, get your soldering irons
hot and stuff your face into that illuminated desktop magnifier. We are going
to build an intelligent stepper motor
controller from scratch that is based on
the Allegro MicroSystems A3979.
So What??
YIPEE. Another stepper controller.
NOT! This stepper controller is compact
and when properly heatsinked, handles
two ampere motor loads. The Allegro
A3979 is an upgraded version of the
venerable A3977. All of the easy to use
motor control knobs and dials you
were used to on the A3977 are also
present on the A3979 DMOS driver.
We can even use the same external
components in an A3979 design that
we would use in an A3977 design. The
big difference in the two DMOS drivers
is the addition of 16th stepping
capability to the A3979. If you think
you’re about to read about a rehashed
version of the A3977 circuits you can
get from various Internet points, sit
down, shut up, and read on.
Getting Small
If you have ever worked with the
A3977, you probably used the 44-pin
PLCC version of the part. The PLCC is big
and relatively easy to put down on a
printed circuit board (PCB). You can’t
get the A3979 in a 44-pin PLCC
package; it only comes in a 28-pin
TSSOP package. Instead of dissipating
heat though pins as the PLCC-packaged
A3977 does, the A3979 has an exposed
heatsink pad on its belly. A woodpecker
would see that pad on the underside of
the A3979 in the perspective of Photo 1.
The A3979 can be crammed into
a very tiny space if you’re not
driving stepper motors the size of
watermelons. The only design point
that increases the size of the A3979
PHOTO 2. This was a challenging build as I
had to be extra careful not to short the
SMT 0805 components on the ground
plane extensions under some of the SMT
parts. A fine tipped soldering iron and an
illuminated magnifier are must-have tools
for this project.
PHOTO 1. Don’t think that you can’t access this
heatsink pad without special soldering equipment.
A few holes in the printed circuit board under the
belly of the A3979 provides access.
32 SERVO 01.2008
