UC3717A
FUNCTIONAL DESCRIPTION (cont.)
PHASE POLARITY INPUT
ture to a maximum of 180C by reducing the winding cur-
rent.
The UC3717A phase polarity input controls current direc-
tion in the motor winding. Built-in hysteresis insures im-
munity to noise, something frequently present in
switched drive environments. A low level phase polarity
input enables Q2 and Q3 as shown in Figure 7. During
phase reversal, the active transistors are both turned off
while winding current delays through the commutating di-
odes shown. As winding current decays to zero, the inac-
tive transistors Q1 and Q4 turn on and charge the
winding with current of the reverse direction. This delay
insures noise immunity and freedom from power supply
current spikes caused by overlapping drive signals.
PERFORMANCE CONSIDERATIONS
In order to achieve optimum performance from the
UC3717A careful attention should be given to the follow-
ing items.
External Components: The UC3717A requires a mini-
mal number of external components to form a complete
control and switch drive unit. However, proper selection
of external components is necessary for optimum per-
formance. The timing pin, (pin 2) is normally connected
to an RC network which sets the off-time for the sink
power transistor during switched mode. As shown in Fig-
ure 8, prior to switched mode, the winding current in-
creases exponentially to a peak value. Once peak
current is attained the monostable is triggered which
turns off the lower sink drivers for a fixed off-time. During
off-time winding current decays through the appropriate
diode and source transistor. The moment off-time times
out, the motor current again rises exponentially produc-
ing the ripple waveform shown. The magnitude of wind-
ing ripple is a direct function of off-time. For a given
off-time TOFF, the values of RT and CT can be calculated
from the expression:
PHASE INPUT
LOW
Q1, Q4
OFF
Q2, Q3
ON
HIGH
ON
OFF
CURRENT CONTROL
The voltage divider, comparators, monostable, and two-
bit D/A provide a means to sense winding peak current,
select winding peak current, and disable the winding sink
transistors.
The UC3717A switched driver accomplishes current con-
trol using an algorithm referred to as "fixed off-time."
When a voltage is applied across the motor winding, the
current through the winding increases exponentially. The
current can be sensed across an external resistor as an
analog voltage proportional to instantaneous current.
This voltage is normally filtered with a simple RC low-
pass network to remove high frequency transients, and
then compared to one of the three selectable thresholds.
The two bit D/A input signal determines which one of the
three thresholds is selected, corresponding to a desired
winding peak current level. At the moment the sense volt-
age rises above the selected threshold, the UC3717A’s
monostable is triggered and disables both output sink
drivers for a fixed off-time. The winding current then cir-
culates through the source transistor and appropriate di-
ode. The reference terminal of the UC3717A provides a
means of continuously adjusting the current threshold to
allow microstepping. Table 1 presents the relationship
between the two-bit D/A input signal and selectable cur-
rent level.
TOFF = 0.69RTCT
with the restriction that RT should be in the range of 10-
100k. As shown in Figure 5, the switch frequency FS is a
function of TOFF and TON. Since TON is a function of the
reference voltage, sense resistor, motor supply, and
winding electrical characteristics, it generally varies dur-
ing different modes of operation. Thus, FS may be ap-
proximated nominally as:
1
FS = ⁄
(TOFF).
1.5
Normally, Switch Frequency Is Selected Greater than
TABLE 1
IO
0
I1
0
0
1
1
CURRENT LEVEL
100%
1
60%
Figure 8. A typical winding current waveform. Wind-
ing current rises exponentially to a selected peak
value. The peak value is limited by switched mode
operation producing a ripple in winding current. A
phase polarity reversal command is given and wind-
ing current decays to zero, then increases exponen-
tially.
0
19%
1
Current Inhibit
OVERLOAD PROTECTION
The UC3717A is equipped with a new, more reliable ther-
mal shutdown circuit which limits the junction tempera-
5
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