Preliminary Specification
aMC8500
Speed Control
Motor speed is efficiently controlled with the use of pulse width modulation, PWM. The voltage applied to the Speed
Control Input, Pin 10, provides control of motor speed by varying the drive percent on-time or conduction time of the Phase 1
and Phase 2 outputs during the commutation cycle. The control signal can be in the form of an analog voltage ranging from
1.0 V to 3.0 V, or a variable duty cycle digital pulse train having a low state maximum of 0.98 V, a high state minimum of
3.02 V, with maximum transition times 50 µs. The control signal to PWM transfer slope or Speed Control Input voltage to
drive percent on-time, is controlled by the Slope Select input, Pin 14. When connected to ground, an increase in control
voltage or a digital high state results in an increase in drive output on-time. When unconnected, an increase in control
voltage or a digital high state results in a decrease in drive output on-time.
A Minimum Speed Set Input is made available at Pin 6 and it has a control transfer and slope that is identical to that of
Pin 10. It is designed to be programmed form an analog voltage that ranges from 1.0 V to 3.0 V, which can be derived from
the Reference. The minimum speed programmed at this input will take control of the motor speed if it is greater than the
speed being requested at the Speed Control input. When directly controlling motor speed from a variable duty cycle digital
pulse train, the minimum speed set feature is not available and the comparator input must be disabled. A method for
preserving this feature is shown in Figure 40.
Figure 1 shows the Motor Drives percent on-time versus the Speed Control input voltage with Pin 14 connected to ground
for positive slope control. Notice that there are two defined outcomes when the Speed Control input voltage falls below that
of the Minimum Speed Set. The first is that the motor continuously runs at the programmed minimum speed setting and this
is selected by loading the Reference with 2.0 mA or more to disable auto Power Down. The second outcome is that the
motor turns off after 1.0 second and this is selected by loading the Reference with 1.0 mA or less to enable auto Power
Down. This gives the fan designer a choice between letting the motor run at a minimum speed or to stop running when
Speed Control falls below the programmed minimum speed setting.
Figure 2 shows the Motor Drives percent on-time versus the Speed Control input voltage with Pin 14 unconnected for
negative slope control. The minimum speed operating characteristics are selected in the same manner as above but with the
defined outcomes now occurring when the Speed Control input voltage rises above that of the Minimum Speed Set. The
speed control and minimum speed operations are shown in table form in Figure 14. Compatibility with both analog and digital
control signals combined with the ability to select both the transfer slope and automatic power down, allows this device to
interface into a vast array applications.
Figure 14- Speed Control and Minimum Speed Set Operation
Speed Control Input
Note that if the end application does not require a programmed minimum speed or is to be controlled by a variable duty
cycle digital pulse train, the Minimum Speed Set comparator must be disabled as shown in Figure 28. For applications that
do not require speed control, the device can be configured to provide commutation only, yielding maximum motor speed
without requiring any additional components. This is accomplished by directly grounding Pins 6 and 10, while leaving Pin 14
open.
- 8 -
© Andigilog, Inc. 2006
www.andigilog.com
August 2006 - 70A04018
Pin 10
Signal
(V)
Minimum
Speed Set
Input
Pin 6
(V)
Controlling
Input
Slope
Select
Pin 14
Motor
Drive
(% Low)
Comments
0 to 1.0
0
Zero speed, power down mode if IO(ref) ≤1.0 mA, t > 1.0s.
>1.0 to <3.0
0 to 100
%ton = (VSC - 1.0) / 0.02
3.0 to 5.0
Gnd
(Disabled)
Speed
Control
100
Maximum speed.
0 to <1.7
Minimum
Speed Set
35
Speed control voltage is less than minimum speed set.
>1.7 to 3.0
35 to
100
Speed control voltage is greater than minimum speed set.
Voltage
>3.0 to 5.0
1.7
Speed
Control
100
Maximum speed.
Positive
Pulse
Pulse
<0.98 to >3.02
Gnd
(Disabled)
Speed
Control
Gnd
Duty
Cycle
Positive pulse width duty cycle controlled from Pin 10.
5.0 to 3.0
0
Zero speed, power down mode if IO(ref) ≤1.0 mA, t > 1.0s.
<3.0 to >1.0
0 to 100
%ton = 100 - (VSC - 1.0 / 0.02)
1.0 to 0
Tied to Pin 14
(Disabled)
Speed
Control
100
Maximum speed.
3.0 to >2.3
Minimum
Speed Set
35
Speed control voltage is greater than minimum speed set.
<2.3 to 1.0
35 to
100
Speed control voltage is less than minimum speed set.
Voltage
<1.0 to 0
2.3
Speed
Control
100
Maximum speed.
Negative
Pulse
Pulse
>3.02 to <0.98
Tied to Pin 14
(Disabled)
Speed
Control
Open
Duty
Cycle
Negative pulse width duty cycle controlled from Pin 10.
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