PBL3776-1 Datasheet, PDF (7/8 Pages) Ericsson – Dual Controller IC for High Current Stepper Motor Applications

PBL 3776/1
connected together close to the
main filtering capacitor at the power
supply.
• Decouple the supply voltages close to
the PBL 3776/1 circuit. Use a ceramic
capacitor in parallel with an electrolytic
type for both V_CC and V_BB. Route the
power supply lines close together.
• Do not place sensitive circuits close to
the driver. Avoid physical current loops,
and place the driver close to both the
motor and the power supply connector.
The motor leads could preferably be
twisted or shielded.
Motor selection
The PBL 3776/1 is designed for two-
phase bipolar stepper motors, i.e.
motors that have only one winding per
phase.
The chopping principle of the PBL
3776/1 is based on a constant
frequency and a varying duty cycle. This
scheme imposes certain restrictions on
motor selection. Unstable chopping can
occur if the chopping duty cycle exceeds
approximately 50%. See figure 3 for
definitions. To avoid this, it is necessary
to select a motor with a low winding
resistance and inductance, i.e. windings
with fewer turns.
It is not possible to use a motor that is
rated for the same voltage as the actual
supply voltage. Only rated current needs
to be considered. Typical motors to be
used together with the PBL 3776/1 in a
high current application, have a voltage
rating of 0.5 to 6 V, while the supply
voltage usually ranges from 12 to 40 V.
Low inductance, especially in
combination with a high supply voltage,
enables high stepping rates. However, to
give the same torque capability at low
speed, the reduced number of turns in
the winding in the low resistive, low
inductive motor must be compensated
To achieve the best utilization of the
motor driver combination it is important
to find the correct operation conditions in
terms of motor voltage, winding current
and stepping mode to fit the motor type
and the motor winding.
To find the correct operation conditions
for a certain application the following
procedure can be used.
1. If low noise and low resonance’s or
high resolution is required, use half step
or even better modified half step, quarter
step, etc. In order to implement modified
half step or modes with better resolution
an external sequence generator must be
used. See the testboard manual for
TB 313i testboard for more information.
If the required stepping rate is high or if
low cost is more important than low
noise use full step mode.
2. Set the motor supply voltage and the
winding currents to their maximum
values (limited by the motor or the
driver). Run the motor in the application
at the lowest frequency with maximum
load.
3. Decrease the current, by decreasing
the Vref voltage, until the motor phases
out, then raise the current with the
V_{CE Sat} (V)
0.6
0.4
0.2
0
0
0.20
0.40
I _M (A)
Figure 8. Typical lower transistor
saturation voltage vs. output current.
V_{CE Sat} (V)
1.2
1.0
0.8
0.6
0.4
0.2
0
0
0.20
0.40
I _M (A)
Figure 9. Typical upper transistor satura-
tion voltage vs. output current.
selected torque margin, 25 to 50% as a
guideline. This sets a first approximation
of the suitable current level.
4. Run the motor at the highest
frequency with maximum load. Decrease
the motor voltage until the motor phases
out. Increase the motor voltage with 15
to 30% as a guideline to find a first
estimation of the required motor voltage.
To get an even better estimation
continue to adjust the current in the low
frequency range and the voltage in the
high frequency range. This is a very
simplified method for finding the correct
operating conditions for the motor but it
will be helpful in most cases. If the motor
fails to run in the high frequency range at
maximum voltage a motor with lower
winding resistance should be selected. If
the problems occur in the low frequency
range a larger motor or a gearbox will
have to be used.
by a higher current. A compromise has
to be made. Select a motor with the
lowest possible winding resistance and
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 maximum current.
Thermal shutdown
The circuit is equipped with a thermal
shutdown function that turns the outputs
off at a chip (junction) temperature
above 160°C. Normal operation is
resumed when the temperature has
decreased about 20°C.
Programming
Figure 10 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
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