ADT7476
Rev. B | Page 48 of 72
TEMPERATURE ABOVE T
MIN
0
20
40
60
80
100
120
0
P
W
M
D
UT
Y
CY
CL
E
(
%
)
10
20
30
40
50
60
70
80
90
100
TEMPERATURE ABOVE T
MIN
(A)
(B)
0
20
40
60
80
100
120
0
F
A
N
SP
EED
(
%
O
F
MA
X
)
10
20
30
40
50
60
70
80
90
100
05
38
2-
0
63
2°C
80°C
53.3°C
40°C
32°C
26.6°C
20°C
16°C
13.3°C
10°C
8°C
6.67°C
5°C
4°C
3.33°C
2.5°C
2°C
80°C
53.3°C
40°C
32°C
26.6°C
20°C
16°C
13.3°C
10°C
8°C
6.67°C
5°C
4°C
3.33°C
2.5°C
Figure 62. T
RANGE
and % Fan Speed Slopes with PWM
MIN
= 20%
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 ADT7476 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 that the control range for 4-wire fans is much wider than
for 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.
TEMPERATURE ABOVE T
MIN
0
10
20
30
40
100
50
60
70
80
90
0
P
W
M DUTY
CY
CLE
(%)
10
20
30
40
50
60
70
80
90
100
TEMPERATURE ABOVE T
MIN
0
FA
N
SPEED
(
%
M
A
X R
P
M
)
10
20
30
40
50
60
70
80
90
100
0
10
20
30
40
100
50
60
70
80
90
05382-064
VRM TEMPERATURE
AMBIENT TEMPERATURE
CPU TEMPERATURE
VRM TEMPERATURE
CPU TEMPERATURE
AMBIENT TEMPERATURE
Figure 63. T
RANGE
and % Fan Speed Slopes for VRM, Ambient, and
CPU Temperature Channels
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 may be operating 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.
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