PBL 3774/1
inductance, that still gives the required
torque, and use as high supply voltage
as possible, without exceeding the
maximum recommended 40 V. Check
that the chopping duty cycle does not
exceed 50% at max. current.
General
Phase inputs.
A logic
HIGH
on a Phase
input gives a current flowing from pin M
A
into M
B
. A logic
LOW
gives a current flow
in the opposite direction. A time delay
prevents cross conduction in the H-
bridge when changing the Phase input.
Heat sinking.
Soldering the batwing
ground leads onto a copper ground
plane of 20 cm
2
(approx. 1.8" x 1.8"),
copper foil thickness 35
µm,
permits the
circuit to operate with 650 mA output
current, both channels driving, at
ambient temperatures up to 70°C.
Consult figures 8, 9 and 13 in order to
determine the necessary copper ground
plane area for heat sinking at higher
current levels.
Thermal shutdown.
The circuit is
equipped with a thermal shutdown func-
tion that turns the output off at tempera-
tures above 160°C. Normal operation is
resumed when the temperature has
decreased about 20
°C.
Programming
Figure 14 shows the different input and
output sequences for full-step, half-step
and modified halfstep operations.
Full-step mode.
Both windings are
energized at all the time with the same
current, I
M1
= I
M2
. To make the motor take
one step, the current direction (and the
magnetic field direction) in one phase is
reversed. The next step is then taken
when the other phase current reverses.
The current changes go through a
sequence of four different states which
equal four full steps until the initial state
is reached again.
Half-step mode.
In the half-step mode,
the current in one winding is brought to
zero before a complete current reversal
is made. The motor will then have taken
two half steps equalling one full step in
rotary movement. The cycle is repeated,
but on the other phase. A total of eight
states are sequenced until the initial
state is reached again.
Half-step mode can overcome
potential resonance problems. Resonan-
ces appear as a sudden loss of torque at
one or more distinct stepping rates and
must be avoided so as not to loose
control of the motor´s shaft position.
One disadvantage with the half-step
mode is the reduced torque in the half
step positions, in which current flows
through one winding only. The torque in
this position is approximately 70 % of the
full step position torque.
Modified half-step mode.The
torque
variations in half step mode will be elimi-
nated if the current is increased about
1.4 times in the halfstep position. A
constant torque will further reduce
resonances and mechanical noise,
resulting in better performance, life
Thermal resistance [°C/W]
expectancy and reliability of the mecha-
nical system.
Modifying the current levels must be
done by bringing the reference voltage
up (or down) from its nominal value
correspondingly. This can be done by
using DACs or simple resistor divider
networks, as shown in figure 7.
The PBL 3774/1 is designed to handle
about 1.4 times higher current in one
channel on mode, for example 700 mA
per winding in the full-step position, and
1000 mA in the half-step position.
80
70
60
50
40
30
20
5
10
15
20
25
30
35
PCB copper foil area [cm
2
]
PLCC package
DIP package
Figure 13. Typical thermal resistance vs. PC Board copper area and suggested layout.
Phase
1
Dis
1
Phase
2
Dis
2
V
R1
140%
100%
V
R2
140%
100%
I
MA1
140%
100%
–100%
–140%
I
MA2
140%
100%
–100%
–140%
Full step mode
Half step mode
Modified half step mode
Figure 14. Stepping modes.
7
ERICSSON [ ERICSSON ]
