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© Semiconductor Components Industries, LLC, 2012
April, 2012− Rev. 6
1 Publication Order Number:
AN1046/D
AN1046/D
Two Chip Solution for
Brushless Motor Controller
Design
Motor control designers who wish to take advantage of the
brushless DC motor’s unique attributes are faced with a
difficult task. There are limited control ICs designed to
decode data coming from Hall effect sensors, let alone
perform all the ancillary functions such as forward/reverse
selection, overcurrent shutdown, undervoltage lockout,
overtemperature shutdown, and so forth. Using discrete
components to include these functions is an alternative, but
discretes often consume too much circuit board area,
especially if the control unit is to be placed inside the motor
housing.
Two linear ICs, combine to form a simple design, high
performance system.
THE BRAINS
The system highlighted here is built with
ON Semiconductor’s MC33035 and a support chip, the
MC33039. Automotive grade parts are also available as
NCV33035 and NCV33039. They are examples of ICs that
dramatically simplify the design of brushless motor control
systems and reduce required circuit board area and parts
count. Added benefits are shorter design times and improved
system performance.
The MC33035 is a 24−pin linear IC that can operate as the
control center for a brushless DC motor control system. The
main duty of the MC33035 is to decode the signals from the
Hall effect sensors and generate logic for electronically
commutating the motor. The commutation logic is internally
fed into the six output drivers consisting of three open
collector NPN transistors that drive the upper legs of the
bridge and three totem pole drivers that control the lower
bridge transistors. The open collector outputs can sink 50
mA; with some additional circuitry this allows control of
either N−channel MOSFETs for higher power applications
or P−channel MOSFETs if a simple interface is desired.
Since the three lower totem pole outputs can source and sink
100 mA, they can drive power MOSFETs directly.
Fault Management
The MC33035 can detect and manage several types of
faults. A common method of overcurrent detection is to tie
the sources of the lower three transistors together and return
them to the negative supply rail through a current sense
resistor. The sense voltage, which is proportional to load
current, is fed into a comparator internal to the MC33035.
The comparator then feeds an RS Flip−Flop, which ensures
that once an overcurrent condition is detected, the output
drivers turn off the power transistors for the remainder of the
oscillator cycle. Without the internal flip−flop, the
overcurrent protection loop would rapidly cycle On and Off
about the comparator’s threshold, causing excessive power
transistor heating.
In addition to overcurrent management, the MC33035
provides undervoltage lockouts that terminate the drive to
the output transistors if any of three conditions occur. The
first is insufficient voltage to operate the IC. The second
occurs when there is insufficient voltage to drive the power
MOSFET gates. Finally, the output drivers turn off the
power transistors when the MC33035 does not sustain its on
board 6.25 V reference. An invalid set of Hall effect signals
or excessive temperature will also cause shutdown.
Whenever any fault condition is present, an NPN
transistor capable of sinking 16 mA pulls the Fault Output
pin low. Potential uses of the Fault Output include alerting
a microprocessor of a problem, lighting an LED,
implementing a soft start feature to limit motor start up
currents, or latching the system off at the first sign of a
problem or after a fixed delay.
APPLICATION NOTE
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