REV. A
34
ADM1027
Step 6
Determine T
THERM
for Each Temperature Channel
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 measured temperature 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
hysteresis. The hysteresis
value is 4rC.
The T
THERM
limit should be considered the maximum worst-
case operating temperature of the system. Since 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
the user should ensure that it is not exceeded under normal
system operating conditions.
Note that the T
THERM
limits are nonmaskable and affect the fan
speed no matter what automatic fan control settings are config-
ured. This allows some flexibility since a T
RANGE
value can be
selected based on its slope, while a hard limit, e.g., 70rC, can be
programmed as T
MAX
(the temperature at which the fan reaches
full speed) by setting T
THERM
to 70rC.
THERM hysteresis is 4rC.
THERM Registers
Reg. 0x6A Remote 1 THERM Limit = 0x64 (100rC default)
Reg. 0x6B Local Temperature THERM Limit = 0x64
(100rC default)
Reg. 0x6C Remote 2 THERM Limit = 0x64 (100rC default)
T
MIN
0%
100%
T
THERM
TACHOMETER 1
MEASUREMENT
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
TACHOMETER 2
MEASUREMENT
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
TACHOMETERS 3
AND 4
MEASUREMENT
PWM
GENERATOR
PWM
GENERATOR
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
PWM
GENERATOR
PWM
CONFIG
PWM
MIN
MUX
THERMAL CALIBRATION
100%
0%
T
MIN
T
RANGE
THERMAL CALIBRATION
100%
0%
TMIN
T
RANGE
THERMAL CALIBRATION
100%
0%
T
MIN
T
RANGE
PWM
MIN
PWM
MIN
PWM
CONFIG
PWM
CONFIG
REMOTE 1 =
AMBIENT TEMP
LOCAL =
VRM TEMP
REMOTE 2 =
CPU TEMP
PWM1
TACH1
CPU
FANSINK
FRONT
CHASSIS
REAR
CHASSIS
PWM3
TACH3
PWM2
TACH2
T
RANGE
Figure 35. Understanding How T
THERM
Relates to Automatic Fan Control
Rev. 3 | Page 34 of 56 | www.onsemi.com
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ADM1029ARQZ-R7
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