MC33030
http://onsemi.com
8
OPERATING DESCRIPTION
The MC33030 was designed to drive fractional horsepower
DC motors and sense actuator position by voltage feedback.
A typical servo application and representative internal block
diagram are shown in Figure 17. The system operates by
setting a voltage on the reference input of the Window
Detector (Pin 1) which appears on (Pin 2). A DC motor then
drives a position sensor, usually a potentiometer driven by
a gear box, in a corrective fashion so that a voltage
proportional to position is present at Pin 3. The servo motor
will continue to run until the voltage at Pin 3 falls within the
dead zone, which is centered about the reference voltage.
The Window Detector is composed of two comparators,
A and B, each containing hysteresis. The reference input,
common to both comparators, is pre−biased at 1/2 V
CC
for
simple two position servo systems and can easily be
overridden by an external voltage divider. The feedback
voltage present at Pin 3 is connected to the center of two
resistors that are driven by an equal magnitude current
source and sink. This generates an offset voltage at the input
of each comparator which is centered about Pin 3 that can
float virtually from V
CC
to ground. The sum of the upper and
lower offset voltages is defined as the window detector input
dead zone range.
To increase system flexibility, an on−chip Error Amp is
provided. It can be used to buffer and/or gain−up the actuator
position voltage which has the effect of narrowing the dead
zone range. A PNP differential input stage is provided so that
the input common−mode voltage range will include ground.
The main design goal of the error amp output stage was to
be able to drive the window detector input. It typically can
source 1.8 mA and sink 250 mA. Special design
considerations must be made if it is to be used for other
applications.
The Power H−Switch provides a direct means for motor
drive and braking with a maximum source, sink, and brake
current of 1.0 A continuous. Maximum package power
dissipation limits must be observed. Refer to Figure 15 for
thermal information. For greater drive current requirements,
a method for buffering that maintains all the system features
is shown in Figure 30.
The Overcurrent Monitor is designed to distinguish
between motor startup or locked rotor conditions that can
occur when the actuator has reached its travel limit. A
fraction of the Power H−Switch source current is internally
fed into one of the two inverting inputs of the current
comparator, while the non−inverting input is driven by a
programmable current reference. This reference level is
controlled by the resistance value selected for R
OC
, and must
be greater than the required motor run−current with its
mechanical load over temperature; refer to Figure 8. During
an overcurrent condition, the comparator will turn off and
allow the current source to charge the delay capacitor, C
DLY
.
When C
DLY
charges to a level of 7.5 V, the set input of the
overcurrent latch will go high, disabling the drive and brake
functions of the Power H−Switch. The programmable time
delay is determined by the capacitance value−selected for
C
DLY
.
t
DLY
+
V
ref
C
DLY
I
DLY(source)
+
7.5 C
DLY
5.5 μA
+ 1.36 C
DLY
in μF
This system allows the Power H−Switch to supply motor
startup current for a predetermined amount of time. If the
rotor is locked, the system will time−out and shutdown. This
feature eliminates the need for servo end−of−travel or limit
switches. Care must be taken so as not to select too large of
a capacitance value for C
DLY
. An overcurrent condition for
an excessively long time−out period can cause the integrated
circuit to overheat and eventually fail. Again, the maximum
package power dissipation limits must be observed. The
overcurrent latch is reset upon powerup or by readjusting
V
Pin
2
as to cause V
Pin
3
to enter or pass through the dead
zone. This can be achieved by requesting the motor to
reverse direction.
An Overvoltage Monitor circuit provides protection for
the integrated circuit and motor by disabling the Power
H−Switch functions if V
CC
should exceed 18 V. Resumption
of normal operation will commence when V
CC
falls below
17.4 V.
A timing diagram that depicts the operation of the
Drive/Brake Logic section is shown in Figure 18. The
waveforms grouped in [1] show a reference voltage that was
preset, appearing on Pin 2, which corresponds to the desired
actuator position. The true actuator position is represented
by the voltage on Pin 3. The points V
1
through V
4
represent
the input voltage thresholds of comparators A and B that
cause a change in their respective output state. They are
defined as follows:
V
1
= Comparator B turn−off threshold
V
2
= Comparator A turn−on threshold
V
3
= Comparator A turn−off threshold
V
4
= Comparator B turn−on threshold
V
1
−V
4
= Comparator B input hysteresis voltage
V
2
−V
3
= Comparator A input hysteresis voltage
V
2
−V
4
= Window detector input dead zone range
|(V
2
−V
Pin2
) − (V
Pin2
− V
4
)| = Window detector input voltage
