Applications Note AN36
Issue1 - JULY 2002
The buffered type of Hall sensor should be of
the latching or bipolar type. These have an
internal amplifier and thus provide a large
amplitude square wave output. This signal is
single ended and is applied into Pin 3, H+, as
shown in Figure 2. On some buffered Hall
sensors the output is of the Open Collector
type and will therefore require a pull-up
resistor (R1 in Figure 2) to attain the full signal
amplitude. The H- pin will require to be held at
a voltage approximately half the Hall output
swing. The potential divider R2 and R3 in
Figure 2 is provided for this.
Voltage control of the SPD pin
Voltage control is achieved by applying a
voltage of between 1 volt and 2 volts to the
SPD pin. A voltage of 1 volt on this pin will
ensure the drivers are switched on for 100%
of the PWM duty cycle i.e. the motor will run
at full speed.
Progressively increasing the SPD voltage to 2
volts will reduce the percentage of PWM duty
cycle drive to the Phase windings to reduce
the motor speed. With a SPD voltage of 2
volts the outputs will be switched off all the
time i.e. no drive will be present.
Speed Control
In reality a 2-phase DC brushless motor will
have a minimum practical rotational speed
below which the motor will not run. This is
very much dependant upon the power, size
and mechanics of the motor concerned but
could well mean that anything less than 40%
PWM drive, represented by a SPD voltage of
1.6V, is impractical.
Applied power, motor efficiency and
mechanical loading determine maximum
motor speed, the controller allows full speed
adjustment by modifying the applied power
only.
Speed control is attained through an integral
Pulse Width Modulation (PWM) circuit within
the ZXBM200x series. The PWM signal
controls the speed by switching the
respective active Phase output at a much
higher frequency, typically 25kHz, compared
with the commutation1 frequency. The PWM
is used to control the percentage of time the
output driver is turned on for. This PWM
circuit is controlled through two pins, Pin 6,
the CPWM pin, and Pin 4, the SPD input. The
The ability of starting the motor under such
low speed conditions also has to be
considered as in most cases a motor will
rotate at a far lower power input than is
needed to start it. Further discussion will be
given on this subject later.
Note:
1 Commutation is the process of alternate (phase)
switching at the speed of rotation.
C
PWM pin has a capacitor attached of 150pF to
produce a 1V p-p 25kHz triangular waveform.
The circuit has also been used quite
successfully with 100pF to produce a PWM
frequency of 34kHz. The motor speed is
determined by the voltage at the SPD pin.
There are two methods for controlling the
ZXBM200x using the SPD pin. The first is the
direct control of the SPD pin by a voltage from
an external voltage source or system
controller. The second method is to use a
thermistor as a temperature sensor so as the
ZXBM200x is in a feedback loop to control the
fan speed against the temperature
conditions. A third method of control is
available by applying a PWM signal from an
external source. All these will be discussed in
more detail in the following sections.
AN 36 - 3
ZETEX [ ZETEX SEMICONDUCTORS ]
