PBL 3770A
voltage across the sensing resistor is
below the comparator threshold level.
The sinking transistor then turns on
and the motor current starts to increase
again, The cycle is repeated until the
current is turned off via the logic inputs.
When both I
1
and I
0
are high, all four
transistors in the output H-bridge are
turned off, which means that inductive
current recirculates through two opposite
free-wheeling diodes (see figure 6, arrow
3). this method of turning off the current
results in a faster current decay than if
only one transistor was turned off and
will therefore improve speed
performance in half-stepping mode.
Heatsinking
The junction temperature of the chip
highly effects the lifetime of the circuit. In
high-current applications, the
heatsinking must be carefully conside-
red.
The Rth
j-a
of the PBL 3770A can be
reduced by soldering the ground pins to
a suitable copper ground plane on the
printed circuit board (see figure 14) or by
applying an external heatsink type V7 or
V8, see figure 14.
The diagram in figure 13 shows the
maximum permissible power dissipation
versus the ambient temperature in
°C,
for heatsinks of the type V7, V8, or a 20
cm
2
copper area respectively. Any
external heatsink or printed circuit board
copper must be connected to electrical
ground.
For motor currents higher than approx
600 mA, some form of heatsinking is
recommended to assure optimal
reliability.
The diagrams in figures 12 and 13 can
be used to determine the required
heatsinking of the circuit. In some
systems, forced-air cooling may be
available to reduce the temperature rise
of the circuit.
200 mA/div
1 ms/div
External recirculation
diodes
2 1
0
100µs/div
3
Figure 5. Motor current (I
M
),
Vertical : 200 mA/div,
Horizontal: 1 ms/div,
expanded part 100
µ
s/div.
R
S
Motor Current
1
2
3
Fast Current Decay
Slow Current Decay
Time
Figure 6. Output stage with current paths
for fast and slow current decay.
I
0A
I
1A
Ph
A
Ph
B
I
0B
I
1B
I
MA
100%
60%
–20%
–60%
–100%
I
MB
100%
60%
20%
–60%
–100%
Full step position
Half step position
Stand by mode
at 20 %
Half step mode at 100 %
Full step mode at 60 %
Applications Information
Motor selection
Some stepper motors are not designed
for continuous operation at maximum
current. As the circuit drives a constant
current through the motor, its tempera-
ture can increase, both at low- and high-
speed operation.
Some stepper motors have such high
core losses that they are not suited for
switched-mode operation.
6
Figure 7. Principal operating sequence.
ERICSSON [ ERICSSON ]
