Therm - Analog Devices dBCool ADT7473 Manual

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%.
100
90
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70
60
50
40
30
20
10
0
0 10 20 30 40
TEMPERATURE ABOVE T
100
90
80
70
60
50
40
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0
0 10 20 30 40
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 70 80 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 (Register 0x6D and
Register 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, an
one should ensure it is not exceeded
operating conditions.
Note that the T
speed no matter how the 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
Register 0x6A, Remote 1 THERM Temperature Limit = 0xA4
(100°C default)
Register 0x6B, Local THERM Temperature Limit = 0xA4
(100°C default)
Register 0x6C, Remote 2 THERM Temperature Limit = 0xA4
(100°C default)
Hysteresis Registers
Register 0x6D, Remote 1 Local Temperature Hysteresis Register
Bits [7:4] Remote 1 temperature hysteresis (4°C default)
Bits [3:0] Local temperature hysteresis (4°C default)
Register 0x6E, Remote 2 Temperature Hysteresis Register
Bits [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 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.
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under normal system
limits are nonmaskable and affect the fan
THERM
(the temperature at which
MAX
THERM
d
value
RANGE
to that limit (for