MC33035, NCV33035
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26
Brush Motor Control
Though the MC33035 was designed to control brushless
DC motors, it may also be used to control DC brush type
motors. Figure 46 shows an application of the MC33035
driving a MOSFET H−bridge affording minimal parts count
to operate a brush−type motor. Key to the operation is the
input sensor code [100] which produces a top−left (Q
1
) and
a bottom−right (Q
3
) drive when the controller’s
forward/reverse pin is at logic [1]; top−right (Q
4
), bottom−left
(Q
2
) drive is realized when the Forward/Reverse pin is at
logic [0]. This code supports the requirements necessary for
H−bridge drive accomplishing both direction and speed
control.
The controller functions in a normal manner with a pulse
width modulated frequency of approximately 25 kHz.
Motor speed is controlled by adjusting the voltage presented
to the noninverting input of the error amplifier establishing
the PWM’s slice or reference level. Cycle−by−cycle current
limiting of the motor current is accomplished by sensing the
voltage (100 mV) across the R
S
resistor to ground of the
H−bridge motor current. The over current sense circuit
makes it possible to reverse the direction of the motor, using
the normal forward/reverse switch, on the fly and not have
to completely stop before reversing.
LAYOUT CONSIDERATIONS
Do not attempt to construct any of the brushless motor
control circuits on wire−wrap or plug−in prototype
boards. High frequency printed circuit layout techniques
are imperative to prevent pulse jitter. This is usually caused
by excessive noise pick−up imposed on the current sense or
error amp inputs. The printed circuit layout should contain
a ground plane with low current signal and high drive and
output buffer grounds returning on separate paths back to the
power supply input filter capacitor V
M
. Ceramic bypass
capacitors (0.1 μF) connected close to the integrated circuit
at V
CC
, V
C
, V
ref
and the error amp noninverting input may
be required depending upon circuit layout. This provides a
low impedance path for filtering any high frequency noise.
All high current loops should be kept as short as possible
using heavy copper runs to minimize radiated EMI.
