Datasheet

Low-pass filter components R
C
C
C
should be selected so
that all switching transients from the power transistors
and commutating diodes are well smoothed, but the pri-
mary signal, which can be in the range of 1/T
OFF
or
higher must be passed. Figure 5A shows the waveform
which must be smoothed, Figure 5B presents the desired
waveform that just smoothes out overshoot without radi-
cal distortion.
The sense resistor should be chosen as small as practi-
cal to allow as much of the winding supply voltage to be
used as overdrive to the motor winding. V
RS
, the voltage
across the sense resistor, should not exceed 1.5V.
Voltage Overdrive:
In many applications, maximum
speed or step rate is a desirable performance charac-
teristic. Maximum step rate is a direct function of the time
necessary to reverse winding current with each step. In
response to a constant motor supply voltage, the winding
current changes exponentially with time, whose shape is
determined by the winding time constant and expressed
as:
I
m
=
V
m
R
[
1
−
EXP
(
−
R
T
⁄
L
)
]
as presented in Figure 9. With rated voltage applied, the
time required to reach rated current is excessive when
compared with the time required with over-voltage ap-
plied, even though the time constant L/R remains con-
stant. With over-voltage however, the final value of
current is excessive and must be prevented. This is ac-
complished with switch drive by repetitively switching the
sink drivers on and off, so as to maintain an average
value of current equal to the rated value. This results in a
small amount of ripple in the controlled current, but the
increase in step rate and performance may be consider-
able.
Interference:
Electrical noise generated by the chopping
action can cause interference problems, particularly in
the vicinity of magnetic storage media. With this in mind,
printed circuit layouts, wire runs and decoupling must be
considered. 0.01 to 0.1µF ceramic capacitors for high fre-
quency bypass located near the drive package across
V+ and ground might be very helpful. The connection
and ground leads of the current sensing components
should be kept as short as possible.
Half-Stepping:
In half step sequence the power input to
the motor alternates between one or two phases being
energized. In a two phase motor the electrical phase shift
between the windings is 90°. The torque developed is the
vector sum of the two windings energized. Therefore
when only one winding is energized the torque of the mo-
tor is reduced by approximately 30%. This causes a
torque ripple and if it is necessary to compensate for this,
the V
R
input can be used to boost the current of the sin-
gle energized winding.
Figure 9. With rated voltage applied, winding current does not exceed rated value, but takes L/R seconds to
reach 63% of its final value - probably too long. Increased performance requires an increase in applied volt-
age, of overdrive, and therefore a means to limit current. The UC3717A motor driver performs this task effi-
ciently.
UC3717A
FUNCTIONAL DESCRIPTION (cont.)
6