PBL 377 17/1
Functional Description
| V
– V
|
MA
MB
t
The PBL 377 17/1 is intended to drive a
bipolar constant current through one
motor winding of a 2-phase stepper
motor.
t
on
off
50 %
Current control is achieved through
switched-mode regulation, see figure 5
and 6.
Three different current levels and zero
current can be selected by the input
logic.
t
V
E
t
d
V
The circuit contains the following
functional blocks:
CH
•
•
•
•
Input logic
Current sense
Single-pulse generator
Output stage
t
Input logic
t
on
1
Phase input. The phase input
f =
D =
s
t
+ t
+
t
t
determines the direction of the current in
the motor winding. High input forces the
current from terminal MA to MB and low
input from terminal MB to MA. A Schmitt
trigger provides noise immunity and a
delay circuit eliminates the risk of cross
conduction in the output stage during a
phase shift.
on
off
on
off
Figure 4. Definition of terms.
Current sensor
Overload protection
The current sensor contains a reference
voltage divider and three comparators
for measuring each of the selectable
current levels. The motor current is
sensed as a voltage drop across the
current sensing resistor, RS, and
compared with one of the voltage
references from the divider. When the
two voltages are equal, the comparator
triggers the single-pulse generator. Only
one comparator at a time is activated by
the input logic.
The circuit is equipped with a thermal
shut-down function, which will limit the
junction temperature. The output current
will be reduced if the maximum permis-
sible junction temperature is exceeded.
It should be noted, however, that it is not
short circuit protected.
Half- and full-step operation is possible.
Current level selection. The status of I0
and I1 inputs determines the current level
in the motor winding. Three fixed current
levels can be selected according to the
table below.
Operation
When a voltage VMM is applied across
the motor winding, the current rise
Motor current
High level
I0
I1
L
follows the equation:
im = (VMM / R) • (1 - e-(R
R = Winding resistance
L = Winding inductance
)
• t ) / L
100% L
Medium level 60%
Low level 20%
Zero current 0%
H
L
L
Single-pulse generator
H
H
The pulse generator is a monostable
multivibrator triggered on the positive
edge of the comparator output. The
multivibrator output is high during the
pulse time, toff , which is determined by
the timing components RT and CT.
t
=
time
H
(see figure 6, arrow 1)
The specific values of the different
current levels are determined by the
reference voltage VR together with the
value of the sensing resistor RS.
The motor current appears across the
external sensing resistor, RS, as an
analog voltage. This voltage is fed
through a low-pass filter, RCCC, to the
voltage comparator input (pin 10). At the
moment the sensed voltage rises above
the comparator threshold voltage, the
monostable is triggered and its output
turns off the conducting sink transistor.
The polarity across the motor winding
reverses and the current is forced to
circulate through the appropriate upper
protection diode back through the source
transistor (see figure 6, arrow 2).
toff = 0.69 • RT • CT
The peak motor current can be
calculated as follows:
The single pulse switches off the power
feed to the motor winding, causing the
winding to decrease during toff.If a new
trigger signal should occur during toff , it is
ignored.
im = (VR • 0.083) / RS [A], at 100% level
im = (VR • 0.050) / RS [A], at 60% level
im = (VR • 0.016) / RS [A], at 20% level
The motor current can also be
continuously varied by modulating the
voltage reference input.
Output stage
The output stage contains four
transistors and four diodes, connected in
an H-bridge. The two sinking transistors
are used to switch the power supplied to
the motor winding, thus driving a
constant current through the winding.
See figures 5 and 6.
After the monostable has timed out, the
current has decayed and the analog
voltage across the sensing resistor is
below the comparator threshold level.
5
ERICSSON [ ERICSSON ]
