Applications Note AN36
Issue 1 - JULY 2002
ZXBM200x SERIES OF VARIABLE SPEED 2-PHASE
FAN MOTOR CONTROLLER
PURPOSE
This Applications document is intended to aid
users in their development of the controller
electronics for fan and blower motors using
the ZXBM200x series of variable speed,
2-phase, DC brushless motor controllers.
The document will not discuss mechanical
details of motor design including such
aspects as the position of commutation in
relationship to windings etc, for which it is
assumed the user already has prior
knowledge.
ZXBM200x DESCRIPTION
The ZXBM200x is a series of 2-phase, DC
brushless motor pre-drivers with variable
speed control suitable for fan and blower
motors. A full description is to be found in the
device Datasheet available by logging on to
www.zetex.com/zxbm.
There are three variants available for the
controller and all are identical in every aspect
except the Lock and FG output functions on
Pin 6. The output on the ZXBM2003 is
rotational frequency FG, on the ZXBM2002
the pin indicates when the controller in the
Lock state whilst in the ZXBM2001 the output
is a combination of both the Lock and FG
signals.
For the purpose of aiding clarity in this
document the device Block Diagram and
Pinout Diagram.
Block Diagram
Pinout Diagram
The Pinning is shown for the ZXBM2001 as this is the variant
discussed and illustrated in this document. Applications for
the ZXBM2002 and ZXBM2003 will be identical with the only
difference being the function of Pin 6 as discussed above.
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Applications Note AN36
Issue1 - JULY 2002
APPLICATION REQUIREMENTS
Introduction
A typical electronics application circuit for a
2-phase, DC brushless motor will consist of,
but not be restricted to, three main building
blocks. First will be the controller itself
containing the functions as described in the
datasheet. Attached to this will be a Hall
device as a means of monitoring and
controlling the commutation of the motor
and finally there will be the power driver
devices used to switch the two Phase
windings.
On top of these three building blocks will be
other items such as the Speed Control
stimulus circuit, e.g. a thermistor. Further
functions may be required dependant upon
application, perhaps as a means of ‘value
adding’ to the overall final product. Some
ideas of this nature will also be discussed in a
later section.
The first part of the Applications Note,
however, discusses the various interface
aspects of the external component
requirements to the ZXBM200x series. To aid
clarity in this first section any circuits given
will show only the areas of interest to that
section of discussion. This is followed by a
section of practical examples that will include
circuits in their entirety.
The unbuffered type have a differential low
level signal output with the shape of this
signal being a direct representation of the
magnetic field from the permanent magnet
Rotor. These sensors are connected to the
ZXBM using the differential inputs H+ and H-
on Pins 2 and 3 respectively. See Figure 1 for
connection details. R1 in Figure 1 should be
chosen to suit the Hall sensor type and is
provided to bias to the Hall device.
Figure 1
Connection of unbuffered (naked) Hall
The Hall Sensor
Two types of Hall sensor can be used in
conjunction with the ZXBM200x controllers,
those having an unbuffered output stage,
sometimes known as a ‘naked’ Hall, and
those with a buffered output. Figures 1 and 2
show the two respective types and their
connection to the ZXBM2001.
Figure 2
Connection of buffered Hall
AN 36 - 2
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.
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.
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.
Speed Control
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 commutation
1
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 C
PWM
pin, and Pin 4, the SPD input. The
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
Applications Note AN36
Issue1 - JULY 2002
NTC Thermistor
The ZXBM200x series are designed to be
used in conjunction with an NTC type
thermistor with a value of 100k at 25 C. The
thermistor is connected directly between the
SPD pin and Ground. The thermistor can
either be local i.e. part of the Fan or Blower
itself and positioned where it can easily
detect the air temperature, or it can be remote
to the device, perhaps positioned on a
motherboard or in an air duct.
An NTC thermistor has a non-linear
characteristic whereby there is a larger
change in resistance between 25 C and 30 C
than there is between 55 C and 60 C. See
Figure 3. However from Ohms Law when two
resistors are in parallel and one changes in a
linear fashion it will produce a resultant
non-linear resistance characteristic but with
the opposite curve to that of the thermistor.
See Figure 4. Now when these two effects are
put together the resultant characteristic seen
in Figure 3 is achieved.
Figure 4
The SPD pin on the ZXBM200x series
contains an internal resistor network of
19.5k to an internal 2V rail and 52. 5k to
Ground. The purpose of this is to set the
speed range for a 100k NTC thermistor and
to help cancel out the thermistor’s
non-linearity against temperature.
The manner in which the non-linearity is
removed can be seen in Figure 14 where the
SPD voltage is linear with any slight non-
linearity in the speed response more likely
due to other motor characteristics.
It is also possible to use the ZXBM200x series
in conjunction with other lower values of NTC
thermistor, for example one with a value of
10k at 25 C. In this case an external resistive
divider as shown in Figure 5 below is used to
set the control speed range. The ratio of R1
and R2 in this figure should be chosen to give
the desired speed range against temperature.
With careful selection of the R1 and R2, it is
possible for a linear response within ±1% to
be achievable.
If needed R1 and R2 can also be added when
using a 100k thermistor in order to adjust
the speed range to match the particular
thermistor’s characteristics.
Figure 3
AN 36 - 4
Applications Note AN36
Issue1 - JULY 2002
When driving the SPD pin from a signal or
thermistor remote from the fan or controller
there may be a need to provide noise
protection at the SPD pin. In order to maintain
a smooth speed control characteristic this
will take the form of a capacitor of between
100nF and 1 F connected between the SPD
and Gnd pins somewhere close to the device.
This should suffice for most purposes and
can be determined for each particular
application.
A capacitor on this pin also has an effect on
the start-up characteristic of the motor as the
charging of the capacitor will lag behind the
rise of the supply voltage. As a low voltage on
the SPD pin represents a higher speed the
effect at start-up, if the SPD value is not set for
full speed, will be for the motor speed to
increase past its allotted point before settling
back to that required. Whilst for the capacitor
values stated above this effect may not be
noticeable, for values greater than 1 F it can
be used to advantage. In situations where the
motor will be regularly started at a speed
lower than its practical start value it can help
to get motors started, in effect giving the
motor a ‘kick’ to start it.
Table
1
Thermistor Supply
Value
Voltage
5V
4.7k
12V
24V
5V
10k
12V
24V
5V
47k
12V
24V
5V
100k
12V
24V
MOSFET
R1
3.3k
6.8k
15k
5.6k
13k
36k
24k
56k
130k
*
*
*
R2
2.2k
1.2k
1.3k
3.3k
2.2k
3.3k
12k
9.1k
10k
*
*
*
Bipolar
R1
3.3k
6.8k
15k
5.6k
13k
33k
22k
56k
130k
*
*
*
R2
1.5k
1.0k
1.0k
2.4k
1.8k
2.2k
8.6k
7.5k
8.6k
*
*
*
Figure 5
showing resistive divider used with <100k
thermistors
One problem with using an external potential
divider (R1 & R2 in Figure 5) is that the motor
becomes susceptible to supply voltage
changes. Say the supply voltage falls,
normally the speed would drop due to less
voltage across the windings, however as the
potential divider is across the supply, as the
voltage falls so the voltage at the SPD pin will
fall. As a lower SPD voltage represents a
higher speed so a lowering of the supply
therefore causes the fan to speed up. To
overcome this it is probably best to use a
Zener regulated supply to the ZXBM200x.
The following Table 1 gives an indication of
the values required for R1 and R2 against
different thermistor values. It should be
noted that these are only for guidance as the
final value will depend upon the thermistor
and motor characteristics and so will need
fine adjustment by experimentation. If the
ZXBM200x device is to be run from a Zener
regulated supply to control the speed verses
supply voltage variation so R1 and R2 will
need allowances made for the voltage value
of Zener chosen.
As the Bipolar drivers have a slower turn-off
characteristics when driven from the
ZXCB200x series slightly different values of
R2 are required over those for MOSFETs.
* Note: Not normally required - dependent upon
thermistor and speed range needed.
AN 36 - 5
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