PBL 3717/2
The sinking transistor then closes and
the motor current starts to increase
again, The cycle is repeated until the
current is turned off via the logic inputs.
By reversing the logic level of the
2
1
200 mA/div
1 ms/div
phase input (pin 8), both active
transistors are turned off and the
0
opposite pair turned on after a slight
delay. When this happens, the current
must first decay to zero before it can
reverse. This current decay is steeper
because the motor current is now forced
to circulate back through the power
supply and the appropriate sinking
transistor protection diode. This causes
higher reverse voltage build-up across
the winding which results in a faster
current decay (see figure 6, arrow 4).
For best speed performance of the
stepper motor at half-step mode opera-
tion, the phase logic level should be
changed at the same time the current-
inhibiting signal is applied (see figure 2).
3
100µs/div
R
S
Figure 5. Motor current (IM ),
Vertical : 200 mA/div, Horizontal: 1 ms/
div, expanded part 100 µs/div.
Motor Current
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.
1
2
3
Fast Current Decay
Slow Current Decay
Time
Heatsinking
Figure 6. Output stage with current paths
for fast and slow current decay.
The junction temperature of the chip
highly effects the lifetime of the circuit. In
high-current applications, the
heatsinking must be carefully conside-
red.
Phase shift here
gives fast
current decay
Phase shift here
gives slow
current decay
The Rthj-a of the PBL 3717/2 can be
reduced by soldering the ground pins to
a suitable copper ground plane on the
printed circuit board (see figure 16) or by
applying an external heatsink type V7 or
V8, see figure 15.
The diagram in figure 14 shows the
maximum permissible power dissipation
versus the ambient temperature in °C,
for heatsinks of the type V7, V8 or a 20
cm2 copper area respectively. Any
external heatsink or printed circuit board
copper must be connected to electrical
ground.
I0A
I1A
PhA
PhB
I0B
I1B
IMA
100%
60%
For motor currents higher than 500
mA, heatsinking is recommended to
assure optimal reliability.
The diagrams in figures 13 and 14 can
be used to determine the required
heatsink of the circuit. In some systems,
forced-air cooling may be available to
reduce the temperature rise of the
circuit.
–20%
–60%
–100%
IMB
100%
60%
20%
–60%
–100%
Full step position
Half step position
Applications Information
Stand by mode
at 20 %
Half step mode at 100 %
Full step mode at 60 %
Motor selection
Some stepper motors are not designed
for continuous operation at maximum
current. As the circuit drives a constant
Figure 7. Principal operating sequence.
6
ERICSSON [ ERICSSON ]
