ADM1030
http://onsemi.com
13
temperature channels may be used as the basis for an
automatic fan speed control loop to drive a fan using
Pulsewidth Modulation (PWM).
How Does the Control Loop Work?
The Automatic Fan Speed Control Loop is shown in
Figure 21 below.
Figure 21. Automatic Fan Speed Control
TEMPERATURE
T
MIN
MIN
MAX
T
MAX
= T
MIN
+ T
RANGE
SPIN-UP FOR 2 SECONDS
In order for the fan speed control loop to work, certain
loop parameters need to be programmed into the device.
1. T
MIN
. The temperature at which the fan should
switch on and run at minimum speed. The fan will
only turn on once the temperature being measured
rises above the T
MIN
value programmed. The fan
will spin up for a predetermined time
(default = 2 secs). See Fan Spin-up section for
more details.
2. T
RANGE
. The temperature range over which the
ADM1030 will automatically adjust the fan speed.
As the temperature increases beyond T
MIN
, the
PWM_OUT duty cycle will be increased
accordingly. The T
RANGE
parameter actually
defines the fan speed versus temperature slope of
the control loop.
3. T
MAX
. The temperature at which the fan will be at
its maximum speed. At this temperature, the PWM
duty cycle driving the fan will be 100%. T
MAX
is
given by T
MIN
+ T
RANGE
. Since this parameter is
the sum of the T
MIN
and T
RANGE
parameters, it
does not need to be programmed into a register
on-chip.
4. A hysteresis value of 5癈 is included in the control
loop to prevent the fan continuously switching on
and off if the temperature is close to T
MIN
. The fan
will continue to run until such time as the
temperature drops 5癈 below T
MIN
.
Figure 22 shows the different control slopes determined
by the T
RANGE
value chosen, and programmed into the
ADM1030. T
MIN
was set to 0癈 to start all slopes from the
same point. It can be seen how changing the T
RANGE
value
affects the PWM duty cycle versus temperature slope.
33
40
47
53
60
66
73
80
87
93
100
Figure 22. PWM Duty Cycle vs. Temperature
Slopes (T
RANGE
)
TEMPERATURE (癈)
T
MAX
= T
MIN
+ T
RANGE
T
MIN
0 5 10 20
40
60
80
A
B
C
D
E
A T
RANGE
= 5癈
B T
RANGE
= 10癈
C T
RANGE
= 20癈
D T
RANGE
= 40癈
E T
RANGE
= 80癈
Figure 23 shows how, for a given T
RANGE
, changing the
T
MIN
value affects the loop. Increasing the T
MIN
value will
increase the T
MAX
(temperature at which the fan runs full
speed) value, since T
MAX
= T
MIN
+ T
RANGE
. Note,
however, that the PWM Duty Cycle vs Temperature slope
remains exactly the same. Changing the T
MIN
value merely
shifts the control slope. The T
MIN
may be changed in
increments of 4癈.
33
40
47
53
60
66
73
80
87
93
100
Figure 23. Effect of Increasing T
MIN
Value
on Control Loop
TEMPERATURE (癈)
T
MAX
= T
MIN
+ T
RANGE
T
MIN
0
20
40
60
80
T
RANGE
= 40癈
Fan Spin-up
As was previously mentioned, once the temperature being
measured exceeds the T
MIN
value programmed, the fan will
turn on at minimum speed (default = 33% duty cycle).
However, the problem with fans being driven by PWM is
that 33% duty cycle is not enough to reliably start the fan
spinning. The solution is to spin the fan up for a
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