ADT7476A
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44
Figure 62. T
RANGE
and % Fan Speed Slopes with
PWM
MIN
= 20%
2
5
C
80
5
C
53.3
5
C
40
5
C
32
5
C
26.6
5
C
20
5
C
16
5
C
13.3
5
C
10
5
C
8
5
C
6.67
5
C
5
5
C
4
5
C
3.33
5
C
2.5
5
C
2
5
C
80
5
C
53.3
5
C
40
5
C
32
5
C
26.6
5
C
20
5
C
16
5
C
13.3
5
C
10
5
C
8
5
C
6.67
5
C
5
5
C
4
5
C
3.33
5
C
2.5
5
C
TEMPERATURE ABOVE T
MIN
0
PWM DUTY CYCLE (%)
0
20
40
60
80
100
120
10
20
30
40
50
60
70
80
90
100
TEMPERATURE ABOVE T
MIN
0
F
AN SPEED (% OF MAX)
0
20
40
60
80
100
120
10
20
30
40
50
60
70
80
90
100
Example: Determining T
RANGE
for Each Temperature
Channel
The following example shows how the different T
MIN
and
T
RANGE
settings can be applied to three different thermal
zones. In this example, the following T
RANGE
values apply:
T
RANGE
= 80
°
C for ambient temperature
T
RANGE
= 53.33
°
C for CPU temperature
T
RANGE
= 40
°
C for VRM temperature
This example uses the mux configuration described in
Step 2 - Configuring the Mux with the ADT7476A
connected as shown in Figure 52. Both CPU temperature
and VRM temperature drive the CPU fan connected to
PWM1. Ambient temperature drives the front chassis fan
and rear chassis fan connected to PWM2 and PWM3. The
front chassis fan is configured to run at PWM
MIN
= 20%.
The rear chassis fan is configured to run at PWM
MIN
= 30%.
The CPU fan is configured to run at PWM
MIN
= 10%.
Note: The control range for 4-wire fans is much wider than
that of 3-wire fans. In many cases, 4-wire fans can start with
a PWM drive of as little as 20% or less. In extreme cases,
some 3-wire fans cannot run unless a PWM drive of 60% or
more is applied.
Figure 63. T
RANGE
and % Fan Speed Slopes for VRM,
Ambient, and CPU Temperature Channels
TEMPERATURE ABOVE T
MIN
0
PWM DUTY CYCLE (%)
0
10
20
30
40
50
60
70
80
90
100
10
20
30
40
50
60
70
80
90
100
VRM TEMP.
CPU TEMPERATURE
AMBIENT TEMPERATURE
TEMPERATURE ABOVE T
MIN
0
F
AN SPEED (% MAX RPM)
0
10
20
30
40
50
60
70
80
90
100
10
20
30
40
50
60
70
80
90
100
VRM TEMP.
CPU TEMPERATURE
AMBIENT TEMPERATURE
Step 7 − T
THERM
for Temperature Channels
T
THERM
is the absolute maximum temperature allowed
on a temperature channel. Above this temperature, a
component such as the CPU or VRM can operate beyond its
safe operating limit. When the temperature measured
exceeds T
THERM
, all fans are driven at 100% PWM duty
cycle (full speed) to provide critical system cooling.
The fans remain running at 100% until the temperature
drops below T
THERM
minus hysteresis, where hysteresis is
the number programmed into the hysteresis registers (0x6D
and 0x6E). The default hysteresis value is 4
°
C.
The T
THERM
limit should be considered the maximum
worst-case operating temperature of the system. Because
exceeding any T
THERM
limit runs all fans at 100%, it has
very negative acoustic effects. Ultimately, this limit should
be set up as a fail-safe, and users should ensure that it is not
exceeded under normal system operating conditions.
Note: T
THERM
limits are nonmaskable and affect the fan
speed no matter how automatic fan control settings are
configured. This allows some flexibility, because a T
RANGE
value can be selected based on its slope, while a hard limit
(such as 70
°
C), can be programmed as T
MAX
(the
temperature at which the fan reaches full speed) by setting
T
THERM
to that limit (for example, 70
°
C).
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