ADT7476A_16 Datasheet, PDF (44/69 Pages) ON Semiconductor – Remote Thermal Controller and Voltage Monitor

ADT7476A
100
90
80
70
60
50
40
30
20
10
0
0
20
40
60
80
100
TEMPERATURE ABOVE T_MIN
25C
2.55C
3.335C
45C
55C
6.675C
85C
105C
13.35C
165C
205C
26.65C
325C
405C
53.35C
805C
120
100
90
80
70
60
50
40
30
20
10
0
0
20
40
60
80
100
TEMPERATURE ABOVE T_MIN
25C
2.55C
3.335C
45C
55C
6.675C
85C
105C
13.35C
165C
205C
26.65C
325C
405C
53.35C
805C
120
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 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.
100
VRM TEMP.
90
80
70
CPU TEMPERATURE
60
50
AMBIENT TEMPERATURE
40
30
20
10
0
0 10 20 30 40 50 60 70 80 90 100
TEMPERATURE ABOVE T_MIN
100
VRM TEMP.
90
80
70
CPU TEMPERATURE
60
AMBIENT TEMPERATURE
50
40
30
20
10
0
0 10 20 30 40 50 60 70 80 90 100
TEMPERATURE ABOVE T_MIN
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 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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