AN42

Application Note 42
Application note for the ZXBM1004 and
ZXBM2004 variable speed motor controllers
- thermal control from an external PWM source
Purpose
This applications document provides details of
thermally controlling the speed of both
single-phase and 2-phase fan and blower
motors from an external PWM source using the
ZXBM1004 and ZXBM2004 motor pre-drivers
from Zetex.
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.
As far as the features to be described in this
document are concerned they will be common
to both devices and it is only the driving of the
winding that will vary between the ZXBM1004
and ZXBM2004.
Block diagrams and pinouts of both these
devices are included, however please refer to
the ZXBM1004 and ZXBM2004 datasheets
when using of this application note.
This applications note is one of a series with
the others dealing with other aspects of using
the ZXBM1004 and ZXBM2004 devices. Also
available are AN41 - Speed control using a
thermistor signal and AN43 - Interfacing to the
motor windings.
ZXBM1004 and ZXBM2004 descriptions
The ZXBM1004 and ZXBM2004 devices are
both variable speed fan motor pre-drivers. The
ZXBM1004 is for use with single-phase motors
and the ZXBM2004 is for use with 2-phase
motors. Full details and datasheets for both
devices are available by logging on to
www.zetex.com/zxbm.
Both of these devices have the same
operational and control features and in
essence are identical with the exception of the
output stage. Where the ZXBM2004 has two
phase outputs capable of driving the two
external power devices for the two phase
windings, the ZXBM1004 has 4 outputs
capable of driving an H-bridge power device
arrangement for driving a single Phase
winding.
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Application Note 42
ZXBM2004 Block diagram and pinning
ZXBM1004 block diagram and pinning
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Application Note 42
PWM thermal control requirements
Introduction
A popular way of controlling DC fans situated
in PCs and instrumentation is to use a
temperature sensing diode situated in the
various processor and controller ICs within the
system. This diode is interfaced to a thermal
management circuit of some sort that will itself
provide a signal to a DC fan to control it's
speed. This control normally takes the form of
a digital Pulse Width Modulation (PWM)
signal.
The ZXBM1004 and ZXBM2004 motor
pre-drivers are capable of taking this signal
and controlling the speed of the fan through
their external driver power transistors.
The methodology of driving the ZXBM1004
and ZXBM2004 with an external PWM signal is
illustrated in Figure 1.
Other aspects of using the ZXBM1004 and
ZXBM2004 are discussed in their datasheet or
applications notes AN41 and AN43.
External PWM control
The basic method of driving the ZXBM1004
and ZXBM2004 with a PWM signal derived
from an external source is as shown in Figure
2. The PWM signal can be a standard 3.3V or 5V
compatible digital signal and is applied
through a diode onto the C PWM pin. The
purpose of the diode is to ensure the low level
of the signal does not go below 200mV on the
CPWM pin and thus out of the operational range
of the input.
Figure 2 - External PWM drive
Figure 1 - methodology of driving the
ZXBMn004 series by external PWM
In order for the device to sense this signal
correctly the SPD pin requires a voltage to be
set on it. For 3.3V and 5V signals this would
normally be set to 1.5V so 2 equal value
resistors are attached as a potential divider
between Gnd and ThRef. Two 10k⍀ resistors
will be ideal.
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Application Note 42
If a signal of a smaller amplitude is needed to
be sensed then R1 and R2 will need to be
adjusted to provide a suitable threshold
voltage for that signal.
The signal provided to the ZXBM device will be
used to directly control the output drive
transistors as shown in the waveforms in
Figure 3. The top trace is the signal being
applied to the CPWM pin whist the bottom
trace is that to be found on either of the phase
windings of the ZXBM2004.
If it is found the opposite phase is required then
the circuit in Figure 4 is recommended. In this
case the external PWM is applied into the SPD
pin whilst the CPWM pin is used to define the
threshold with the potential divider R1 and R2.
The resultant waveforms are shown in Figure
5. In this waveform it can be seen that a
negative going PWM pulse onto the SPD pin
controls the speed. A continuous high level
being 100% drive or full speed and a
continuous high being 0% drive or a stopped
fan.
Figure 3 - PWM waveforms - normal input
The PWM signal in this circuit drives the fan
with a positive pulse onto the CPWM pin. This
means that a continuous high level on CPWM
represents 100% PWM drive or full speed.
Reducing the positive pulse width down to a
continuous low on CPWM represents a slowing
of the fan until 0% drive is reached and the fan
is stopped.
Figure 4 - External inverted PWM drive
Figure 5 - PWM waveforms - inverted input
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Application Note 42
When driving with external PWM drive the
response characteristics are affected by the
PWM frequency and the nature of the
windings.
A response graph for various frequencies is
shown in Figure 6. It can be seen that at the
normal 25kHz PWM frequency, whilst the
response is a straight line, the fan has dropped
from 2200rpm to 500 which is about 20% of the
speed for 50% PWM drive. The fan would have
stopped with 40% drive. Every fan will be
different as it is influenced by the windings.
Figure 8 - External PWM drive circuit for
linear response
In the circuit in Figure 8, R1 together with R2
in parallel with R3 set the full speed SPD pin
voltage. The ideal values therefore would be:
R1 = 2 x R2//R3
Figure 6 - Speed response vs PWM frequency
In order to overcome this and achieve the ideal
response in Figure 7, the following circuit in
Figure 8 can be considered. In this circuit the
external PWM signal is first integrated into a
voltage using Q1, R1, R2 and R3. This voltage is
then used to control the fan using the PWM
generator internal to the ZXBM device in the
normal manner. R1, R2 and R3 are used to set
the required response.
To derive 1V from the ThRef 3V.
At 0% PWM Q1 will be off all the time so the
SPD voltage required for zero speed is set by
the potential divider of R1 and R3.
There will be a slight voltage error due to the
saturation voltage of Q1 but this is usually
insignificant. The value of C3 is PWM
frequency dependant but for a 25kHz - 30kHz
signal 100nF is sufficient. Lower 100Hz PWM
signals will need 1␮F or more.
Figure 7 - Ideal speed response
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Application Note 42
Minimum speed setting
One of the major problems with remote speed
control of a fan is that it could be asked to run at
a speed lower than it is capable of sustaining or
at a speed at which it might fail to start at
switch-on.
To overcome this both the ZXBM1004 and
ZXBM2004 have a minimum speed setting pin,
SMIN, that allows the user to set a voltage, that
determines the minimum speed.
Values between 5k⍀ to 33k⍀ are
recommended. Care should be taken so as the
total load taken from the ThRef pin by the
Thermistor and Minimum Speed networks
does not exceed 1mA. It is suggested that R6 is
set to 10k⍀ and R5 calculated using the
following equation.
⎛ 3V x R6 ⎞
R5 = ⎜
⎟ − R6
⎝ V SMIN ⎠
The minimum speed function can only be used
when the ZXBM device is being controlled by an
applied voltage on the SPD pin. It cannot be
used when controlling the speed directly by a
PWM waveform as illustrated in Figures 2 and 4.
Removal of supply variation on
minimum speed setting
When using the minimum speed function it will
be apparent that the minimum speed varies
with supply voltage. This is due to the variation
in voltage across the motor windings. It is
possible with the addition of an extra resistor
to remove that variation so that the same
minimum speed is maintained across the
operational supply voltage.
This is achieved by applying a resistor from
the Vcc pin to the SMIN pin as shown by R7 in
Figure 10.
Figure 9 - Minimum speed setting
This is achieved by attaching a potential
divider to the SMIN pin, as shown in Figure 9.
To set up the minimum speed run the fan with
SMIN open circuit, supply an adjustable voltage
onto the SPD pin. This can be as a PWM signal
as described for Figure 8 or as a direct voltage
onto the SPD pin. Adjust the voltage on the
SPD pin until the desired minimum speed is
attained. Note the voltage on the SPD pin. This
same voltage is now set up on the SMIN pin with
the potential divider R5 and R6 using the
following equation.
⎛ 3V ⎞
V SMIN = ⎜
⎟ X R6
⎝ R5 + R6 ⎠
Figure 10 - Circuit for minimum speed setting
with immunity from supply voltage variation
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Application Note 42
Kick start
Complete solution
If it is intended to run the fan's minimum speed
close to it's lowest practical starting speed add
a capacitor to the minimum speed network
from SMIN to Gnd. This ensures that the fan is
given a boost to start it as the charging time of
the capacitor ensures a faster minimum speed
is applied at power-up. 1␮F should suffice in
most applications although it is left to the user
to experiment with other values.
The circuit in Figure 11 illustrates a complete
solution for a 12 volt single-phase fan to be
controlled by an external PWM signal. The
resultant PWM response graph is shown in
Figure 12.
D2
12V
D1
1N4004
1N4148
R14
2kΩ
R7#2
100Ω
VCC
V+OP
H+
Ph1Lo
H-
Ph2Lo
100 Ω
R9#3
Ph2Hi
100 Ω
R10
Hall
R1#1
ThRef
15k Ω
R2#1
16k Ω
SPD
R5#2
33k Ω
R4
10k Ω
ZXBM1004
SMI N
C4
Q5
0.1 µF
CPW M
R3#1
R15 #4
15k Ω
15kΩ
R16 #4
15k Ω
FG
C2
C LCK
#2
R6
33k Ω
100pF
R11
1kΩ
Q1-4
ZXMHC
3A01T8
W1
C1
1µF
PWM
in
R13
1kΩ
R12
C5
R8#3
100 Ω
Ph1Hi
FG
RD
Gnd
C3
1µF
RD
0V
Figure 11 - Typical applications circuit incorporating PWM characteristic correction circuit
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Application Note 42
Selection of R1/R2 R3 must be selected on test
iteratively.
As this will not always be possible to achieve
an alternative integrator is suggested as in
Figure 13. This uses an extra PNP device
together with an extra resistor.
Figure 12 - External PWM drive characteristics
In this graph is shown the response of the motor
to a 25kHz signal applied directly to the
ZXBM1004 as described in Figure 2. In this
situation the motor speed drops off quite steeply
as the PWM is backed off from 100% drive.
With direct PWM control set at 75% PWM the
motor is running at a speed lower than it is
capable of starting at. However, with the PWM
integrator buffer included it is possible to make
full use of the PWM control range such that
1000rpm is reached at around 15% to 20%
PWM drive. The response flattens off at this
point as the minimum speed feature is
included in the circuit by the inclusion of R5, R6
and R7. In this case the minimum speed was
set so that anything less than 20% PWM drive
results in 1000rpm.
Figure 13
In this circuit to attain a perfectly linear
response R3 and R4 should be the same value.
Conditions for min and max speed are:
Max. speed 100% PWM = VSPD =
Min. speed 0% PWM= VSPD =
V TH REF R2 / / R4
R2 / / R4 + R1
V TH REF R2
R1 / / R3 + R2
With reference to Figure 11, as previously
described, the external PWM signal is
converted into a voltage using Q5, R1, R2 and
R3. This voltage is applied to the SPD pin of the
ZXBM1004 and controls the speed using the
device's internal PWM generator. Using this
method results in a slight curve to the response
due to the values of the resistors chosen. For a
perfect straight line it needs to be ensured that
at the voltage generated on the SPD pin for
50% PWM drive. The following condition must
be met:
(IR1 - IR3) = (IR2 + IR3 - IR1)
Q5 off
Q5 on
This ensures the charge and discharge of C4 is
balanced.
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Application Note 42
Layout considerations
Whilst it is understandable that the circuit
layout is likely to be severely compromised in
the restricted environment of small
single-phase or 2-phase brushless fan and
blowers a number of points are worth
mentioning.
The decoupling capacitor (C1 in all the figures)
needs to be as close to the device as possible.
Also the capacitors for CLCK and CPWM (not
discussed here) need to be positioned as close
to the device as possible with the latter being
the more important.
The power rails to the device and to the
windings should be kept separate where
possible. Where the power comes onto the
PCB it should go in one direction to the
windings and in the other direction to the
controller and its associated components, in
effect to form a star connection.
As much area as possible should be kept as
copper for the tracks associated with the
output stage with the technique of laying out
the gaps rather than laying out the tracks being
preferred. Allotting as much copper to the tab
of the winding driver transistors is beneficial
when using surface mount packages as they
rely upon the copper of the PCB to dissipate as
much of the heat as possible with the PCB itself
in effect becoming the heatsink.
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Application Note 42
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© Zetex Semiconductors plc 2004
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SEMICONDUCTORS
AN42 - 10
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