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ADT7473
Note on 4-Wire Fans
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%.
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0
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TEMPERATURE ABOVE T
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TEMPERATURE ABOVE T
Figure 58. T
and % Fan Speed Slopes for VRM, Ambient, and
RANGE
CPU Temperature Channels
STEP 7: T
FOR TEMPERATURE CHANNELS
THERM
T
is the absolute maximum temperature allowed on a
THERM
temperature channel. When operating above this temperature, a
component such as the CPU or VRM might be beyond its safe
operating limit. When the temperature measured exceeds
T
, all fans are driven at 100% PWM duty cycle (full speed)
THERM
to provide critical system cooling.
50
60
70
80
90
100
MIN
50
60
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90
100
MIN
The fans remain running at 100% until the temperature drops
below T
− hysteresis, where hysteresis is the number
THERM
programmed into the hysteresis registers (0x6D and 0x6E). The
default hysteresis value is 4°C.
The T
limit should be considered the maximum worst-case
THERM
operating temperature of the system. Because exceeding any
T
limit runs all fans at 100%, it has very negative acoustic
THERM
effects. Ultimately, this limit should be set up as a fail-safe, and
one should ensure it is not exceeded under normal system
operating conditions.
Note that the T
speed no matter how automatic fan control settings are
configured. This allows some flexibility, because a T
can be selected based on its slope, while a hard limit (such as
70°C), can be programmed as T
the fan reaches full speed) by setting T
example, 70°C).
THERM Registers
Reg. 0x6A, Remote 1 THERM limit = 0xA4 (100°C default)
Reg. 0x6B, Local THERM limit = 0xA4 (100°C default)
Reg. 0x6C, Remote 2 THERM limit = 0xA4 (100°C default)
Hysteresis Registers
Reg. 0x6D, Remote 1, Local Hysteresis Register
<7:4>, Remote 1 temperature hysteresis (4°C default).
<3:0>, Local temperature hysteresis (4°C default).
Reg. 0x6E, Remote 2 Temperature Hysteresis Register
<7:4>, Remote 2 temperature hysteresis (4°C default).
Because each hysteresis setting is four bits, hysteresis values are
programmable from 1°C to 15°C. It is not recommended that
hysteresis values ever be programmed to 0°C, because this
disables hysteresis. In effect, this would cause the fans to cycle
between normal speed and 100% speed, creating unsettling
acoustic noise.
Rev. 0 | Page 42 of 76
limits are nonmaskable and affect the fan
THERM
(the temperature at which
MAX
THERM
value
RANGE
to that limit (for
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