ADM1030
AUTOMATIC FAN SPEED CONTROL
ANGE
= 1
0C
PWM DUTY CYCLE – %
RA
NG
E
=2
0
The ADM1030 has a local temperature channel and a remote
temperature channel, which may be connected to an on-chip
diode-connected transistor on a CPU. These two 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?
93
87
80
73
66
60
53
T
RANG
= 5 C
E
100
C
T
R
The Automatic Fan Speed Control Loop is shown in Fig-
ure 6 below.
SPIN UP FOR 2 SECONDS
MAX
T
T
RA
NG
E
=
40
C
T
RA
E
NG
=8
0
C
47
40
33
0
T
MIN
5
10
20
40
TEMPERATURE – C
60
80
T
MAX
= T
MIN
+ T
RANGE
FAN
SPEED
Figure 7. PWM Duty Cycle vs. Temperature Slopes (T
RANGE
)
MIN
T
MIN
TEMPERATURE
T
MAX
= T
MIN
+ T
RANGE
Figure 8 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
°
C.
100
93
PWM DUTY CYCLE – %
Figure 6. Automatic Fan Speed Control
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 maxi-
mum 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
°
C is included in the control loop to
prevent the fan continuously switching on and off if the tem-
perature is close to T
MIN
. The fan will continue to run until
such time as the temperature drops 5
°
C below T
MIN
.
Figure 7 shows the different control slopes determined by the
T
RANGE
value chosen, and programmed into the ADM1030.
T
MIN
was set to 0
°
C 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.
87
80
C
C
40
40
=
=
RA
NG
E
RA
NG
E
60
53
47
40
33
0
T
MIN
T
T
20
40
TEMPERATURE – C
60
80
T
MAX
= T
MIN
+ T
RANGE
Figure 8. Effect of Increasing T
MIN
Value on Control Loop
FAN SPIN-UP
As was previously mentioned, once the temperature being mea-
sured 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 predetermined time, and once the fan
has spun up, its running speed may be reduced in line with the
temperature being measured.
The ADM1030 allows fan spin-up times between 200 ms and
8 seconds. Bits <2:0> of Fan Characteristics Register 1 (Register
0x20) program the fan spin-up time.
–12–
T
RA
NG
E
66
=
40
73
C
REV. 0
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