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Application Brief
Brushless DC Motor Commutation Using
Hall-Effect Sensors

Carolus Andrews, Manny Soltero, Mekre Mesganaw

To actualize motion in a system, various motor types

can be used, such as brushed direct current (DC)
motors, brushless DC motors (BLDC), alternating
current (AC) motors, universal motors, stepper
motors, or servo motors. From these options, BLDC
motors specifically have the following advantages:
• High efficiency
• Reliability and long lifetime
• Compact size
• Low noise
• Support for a wide range of motor speeds
• No generation of sparks
The many advantages of BLDC motors are why this
type of motor is used for a variety of applications,
which include: cordless power tools, air conditioner
output units, and automated door and gates in
building security systems. BLDC motors are also used
in various places within a car, such as sliding door
Figure 1. BLDC Motor Model, Commutation Step 1
modules, window modules, roof motor modules, wiper
modules, seat position and comfort modules, engine As an example, if the magnet is in the position shown
fans, and pumps. in Figure 1 and it is desired to cause the magnet to
BLDC Commutation move in the clockwise direction, inject coil U with a
current to act like the south pole of a magnet and
Going into the details of how a BLDC motor works, inject coil V with a current to act like the north pole
Figure 1 shows a simplified model of a BLDC motor, of a magnet. The south pole generated by coil U
which uses two magnet poles (one north and one repulses the south pole of the permanent magnet
south) and three coils. In this model, a permanent and the north pole generated by coil V attracts the
magnet on the rotor (the rotating part of the motor) is south pole of the permanent magnet, thereby causing
surrounded by coils on the stator (the stationary part the permanent magnet and rotor to rotate clockwise
of the motor). The movement of the magnet is what until it reaches the magnet position shown in Figure
causes the rotor to move. The coils will change their 2. From the magnet position of Figure 2, injecting
magnetic poles (either north or south) depending on current into coil W to act like the north pole of a
the direction of the current that is injected into them. magnet and injecting current to coil U to continue to
The attraction of the electromagnet to the opposite act like the south pole of a magnet, causes the rotor
pole of the permanent magnet and the repulsion of to rotate clockwise again. To keep the magnet and
the electromagnet to the same pole of the permanent rotor continuously moving along the circle, current has
magnet causes the permanent magnet and rotor to to be successively injected into the different coils in
move, thereby producing torque. a very specific sequence. The process of switching
which coils have current injected in them to cause
rotor motion is called commutation.

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to their relatively low cost. From the different types of

Hall position sensors, Hall latches are used to provide
a simple six step method for commutation. With these
Hall latches, alternating north and south poles are
required to toggle the output of a device, as illustrated
in Figure 3.

Figure 2. BLDC Motor Model, Commutation Step 2

Figure 3. Output of a Hall Latch
As Figure 1 and Figure 2 show, the appropriate
coils to inject current into and the polarity of these For the latch shown in Figure 3, the output is only
currents depends on the current position of the asserted low when the device detects the south
magnet. BLDC commutation schemes work by first pole of a magnet and the magnitude of the sensed
determining where the rotor is and then using this magnetic flux density is greater than the magnetic
rotor position information to apply a magnetic field flux density operating point, which is referred to as
to move the rotor in the desired direction. There are BOP in the figure. The output would stay low until
two methods to determine the rotor position. The the device detects the north pole of the magnet and
first approach uses a position sensor. The second the magnitude of the sensed magnetic flux density is
approach is a sensorless method that determines greater than the magnetic flux density release point of
position based on back electromotive force (EMF), the latch, which is referred to as BRP in the figure. In
which is a voltage that is generated on the motor the absence of magnetic input, the last state of the
while it is spinning. The amplitude of the generated latch remains active.
back-EMF waveform on a motor is proportional to the
speed of the motor. Figure 4 shows the output waveform of a sensored
six-step motor commutation control scheme using
The advantages of sensored commutation include the three Hall latches. In this scheme, only two phases
following: are driven at a time, where the third phase is in a
• Since the amplitude of the generated back-EMF high-Z state. In the figure, the numbers represent the
waveform on a motor is proportional to the speed commutation step. Step 1 specifically corresponds to
of the motor, a sensorless commutation scheme the state shown in Figure 1 and step 2 corresponds
is not operable at low speeds because the back- to the state shown in Figure 2. The Hall A, Hall
EMF would be too small to measure to determine B, and Hall C waveforms in Figure 4 correspond
the rotor position. If a system requires significant to the output of the Hall latches during the different
torque at zero speed, sensored commutation is commutation steps. The Phase U, Phase V, and
necessary. Phase W waveforms represent the waveforms to
• At very high motor speeds, it is difficult to apply to the phase in order for the magnet to move
differentiate the different transitions when back- to its next commutation step. In Figure 4, a “+”
EMF is used. Use sensored commutation if the corresponds to injecting a current so that a south
motor moves at very high speeds to alleviate this pole is applied to the phase, a “-“ corresponds to
issue. injecting an opposite current so that a north pole is
• Sensored implementations are relatively simple to applied to the phase, and “Z” refers to the phase
execute and do not require complex calculations that is in a high-Z state. This figure illustrates that
like sensorless implementations. there are six independent Hall states, where each
state corresponds to a different option for driving the
For sensored commutation, Hall position sensors, phases to keep the motor spinning. Therefore, the
encoders, or resolvers can be used. Among these Hall states can be used to provide information on how
options, Hall position sensors are most common due to drive the phases to keep the motor spinning, where

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www.ti.com BLDC Motor System Architecture

the state of the Hall sensors can be used as indexes BLDC Motor System Architecture
into a software lookup table to get information on how
Figure 5 shows a system implementation for BLDC
to drive the different phases based on the current
motor control. Three half-bridge circuits are used
position of the rotor.
to connect the motor phases to VCC or GND,
thereby injecting current in the coils to create the
necessary magnetic fields for the different phases.
In this system, a motor controller is used to dictate
commutation. The controller can be a microcontroller,
FPGA, DSP, digital state machine, or a pure analog
implementation. The motor driver is used to allow
the control block to interface with the half-bridge
circuits, thereby allowing the motor driver to dictate
commutation through the half-bridge circuits. The Hall
position sensors provide information on the location
of the rotor to the motor controller, which the motor
controller uses to determine how the half-bridge
circuits should be driven. As an alternative to the
Figure 4. Six-Step Motor Commutation Control system architecture shown in Figure 5, some motor
Scheme drivers have integrated half-bridge circuits so that the
external half-bridge circuits shown in the figure are not
The previous examples use a simplified model for necessary.
a BLDC motor. Typically, BLDC motors have more
magnet poles and coils than displayed in the figures. +
±

The more coils and rotor magnetic poles that are

used, the finer the control of the magnet. The W

magnets can be implemented as multiple bar magnets Motor Motor

U
M

adjacent to each other, where adjacent magnets Controller Driver

V

have opposite polarity from each other. Using more

magnets increases the number of state transitions
seen by the Hall sensor in a given time, thereby Hall, U

decreasing how much the rotor has to rotate for the Hall, V
Hall, W

Hall sensors to cycle through all their possible states.

Figure 5. Closed-Loop System for BLDC Motor
The three Hall position sensors should be placed so
Control
that the angle differences between their respective
outputs are 120 ° offset from each other. This angle Selecting the Right Hall Latch for BLDC Motor
is referred to as the electrical angle, which may Commutation
differ from the actual angle at which the devices are
mechanically placed from each other. From the center The appropriate Hall latch to use for BLDC
of the motor axis, the number of degrees to space commutation is often selected based on the following
each sensor (the mechanical angle) can be set to 2 / specifications:
[number of poles] × 120° to create the necessary 120 • Sensitivity: The sensitivity of the latch refers to
electrical degrees. the BOP and BRP specifications that cause the
In the simplified two-pole examples in Figure 1 output of the latch to switch states when exposed
and Figure 2, the mechanical and electrical angles to a magnet. The magnetic flux density seen by
are equal. However, for systems that have more a latch is dependent on the size of the magnet,
magnet poles, the electrical angle and mechanical its magnetic material, and the distance from the
angle are not equal due to the increased number of magnet to the sensor. High-sensitivity latches
magnet poles decreasing the time to cycle through the have low BOP and BRP specifications, which can
different Hall output state combinations. To illustrate enable the motor to use smaller magnets to
this, suppose there are 12 magnet poles (6 north create a more compact motor design. For a given
poles and 6 south poles) in a system. To obtain 120 magnet, high sensitivity latches also enable a
electrical degree separation between the Hall latches larger allowable magnet to Hall sensor distance
in this 12-pole system, the Hall position sensors can than lower sensitivity latches.
be placed so that they are mechanically ±20 degrees
from each other.

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• Current consumption: If the application is battery not impede the development of even smaller motor
powered, the current consumption of the Hall designs.
position sensor should be low to maximize the
lifetime of the battery.
• Operating voltage range: Since various systems
have different available supply voltages, choose a
Hall latch that operates within the voltage range
of the system. If the available supply voltages of
a system are all outside of the operating voltage
range of the Hall position sensor, an additional
voltage regulator device is needed to generate a
voltage rail for powering the Hall position sensor. Figure 6. Rotor Position Using TO-92 Hall Sensor
• Open-drain vs push-pull output: Open-drain
outputs are selected when it is desired to have
the logic-high output voltage to be at a different
voltage level than the VCC voltage of the Hall
position sensor. Compared to open drain outputs,
push-pull outputs do not require a pullup resistor,
thereby reducing solution size.
• Frequency bandwidth: The device frequency
bandwidth dictates the fastest changing magnetic
field that can be detected and translated to the
output, thereby determining how fast a motor can
spin and still be detected by the Hall latch.
• Jitter: The jitter of a Hall latch is the variation seen
in the output pulse width if the motor is spinning at Figure 7. Rotor Position Using Surface Mount Hall
a constant speed. Jitter introduces angle error in Sensor
the measurement of the rotor position.
• Temperature: Hall sensors need to operate at The DRV5011 is a digital Hall-effect latch designed
high temperatures if they are implemented in high- specifically for motors and other rotary systems.
temperature motor applications. The device has an efficient, low-voltage architecture
• Size: The physical size of the device is important that operates from 2.5 V to 5.5 V. The device
when designing compact motors. Battery-powered, is offered in the standard SOT-23, as well as low-
handheld drills used in the medical field and profile X2SON and DSBGA (WCSP) packages. The
in dentistry are examples of systems where a DSBGA package represents a 58% reduction in size
compact motor is beneficial. In this scenario, when compared to the X2SON. Table 1 shows a
smaller motors reduce the overall weight of the comparison of the different packages.
system, reducing fatigue and stress on the end
Table 1. Package Comparisons
user while maintaining the same torque and
Package Body Size
battery life. For this reason, smaller Hall position
SOT-23 (3-pin) 2.92 mm × 1.3 mm
sensors become extremely advantageous, as
these can be placed strategically inside the motor X2SON (4-pin) 1.1 mm × 1.4 mm
casing without impacting the overall diameter of DSBGA (WCSP) (4-pin) 0.8 mm × 0.8 mm
the motor design. To illustrate this point, consider
the following package types — TO-92 and X2SON. The digital output of the device is a push-pull
The traditional TO-92 package (Figure 6) is most driver that requires no external pullup resistor, which
logically placed around the circumference of the enables even more compact systems.
rotor. For inner-rotor motor designs, this is typically
not an issue, because the stator has enough
room for the Hall sensors (stator not shown).
The drawback with this approach is that it can
lead to larger-diameter motor designs. In slotless
BLDC motors, space is not available within the
motor windings. Hall position sensors are optimally
placed directly above the motor, on a PCB
centered axially on the rotor shaft (Figure 7). This
configuration shows that this package type does

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www.ti.com Alternative Device Recommendations

Alternative Device Recommendations

Choose a Hall position sensor with specifications that
meet the performance and functionality requirements
of the system. For applications where space is
not as constrained, the DRV5015 offers a good,
high-bandwidth solution available in SOT-23. The
DRV5015 also provides the highest sensitivity solution
in TI’s product portfolio, and enables the use of
smaller permanent motor magnets. Another popular
product is the DRV5013, available in SOT-23 and
TO92. The DRV5013 is a high-voltage solution that
can operate up to 38 V VCC. Table 2 has links that
provide more details on the specifications of these
alternate devices:
Table 2. Alternate Device Recommendations
Device Characteristic
DRV5013 High voltage
DRV5015 High sensitivity, low voltage

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