AN720 データシートの表示（PDF） - Vishay Semiconductors
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AN720 Datasheet PDF : 2 Pages
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AN720
Vishay Siliconix
INA
Shoot-
Through
Protection
Logic
INB
Voltage
Feedback
VDD
+
VEE
VCC
–
GND
VDD
Motor
INA
INB
Current
Feedback
Shoot-
Through
Protection
Logic
VDD
+
–
GND
VDD
Motor
= Si9986 Terminals
FIGURE 2. Voltage Feedback for PWM Motor Control
= Si9986 Terminals
FIGURE 3. Current Feedback for PWM Motor Control
FEEDBACK TECHNIQUES
The Si9986 uses pulse width modulation (PWM) for
proportional control. Linear control requires additional external
devices and will not be discussed here. The examples in
Figure’s 2 and 3 show how to obtain a voltage and a current
signal for feedback purposes.
Voltage feedback is used where motor speed is the primary
controlled variable. In this case, a low-pass filter is used since
the voltage at the load is a high-frequency rectangular wave.
The baseband component of this voltage is proportional to the
duty cycle of the rectangular wave. The cut-off of the filter is
chosen to stop the PWM frequency but pass the lower
baseband frequencies required for feedback and control. The
rectangular wave switches between the two active states 1
and 2. Switching between these output states tends to give
greater control of the voltage and speed, since the output
impedance is always low.
Current feedback is used where motor torque or acceleration
is the primary controlled variable. A low-pass filter is normally
not required, since the motor inductance acts as the filter. If the
inductance or the PWM frequency is too low, a low-pass filter
may be required. At the output, the rectangular wave switches
between active states (1 or 2) and the high impedance state
(3). This tends to give greater control of the torque and
acceleration, because the output impedance is high when the
PWM cycle is off. Motor braking during these periods is thus
minimized.
With either feedback technique, state 0 (see Truth Table) will
result in motor braking. The analog output of the voltage sense
filter or current sense is fed back to the controller input to close
the loop. This may be a discrete controller, a microcontroller,
DSP, or fuzzy controller. The operation of the controller is
beyond the scope of this application brief.
www.vishay.com S FaxBack 408-970-5600
2
Document Number: 70590
25-Oct-95
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