ADT7476A
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46
Step 8 − T
HYST
for Temperature Channels
T
HYST
is the amount of extra cooling a fan provides after
the temperature measured has dropped back below T
MIN
before the fan turns off. The premise for temperature
hysteresis (T
HYST
) is that without it, the fan would merely
chatter, or cycle on and off regularly, whenever the
temperature hovers around the T
MIN
setting.
The T
HYST
value chosen determines the amount of time
needed for the system to cool down or heat up as the fan is
turning on and off. Values of hysteresis are programmable in
the range 1
°
C to 15
°
C. Larger values of T
HYST
prevent the
fans from chattering on and off. The T
HYST
default value is
set at 4
°
C.
The T
HYST
setting applies not only to the temperature
hysteresis for fan on/off, but the same setting is used for the
T
THERM
hysteresis value, described in Step 6 - T
RANGE
for
Temperature Channels. Therefore, programming
Register 0x6D and Register 0x6E sets the hysteresis for
both fan on/off and the THERM function.
In some applications, it is required that fans not turn off
below T
MIN
but remain running at PWM
MIN
. Bits [7:5] of
Enhance Acoustics Register 1 (0x62) allow the fans to be
turned off or to be kept spinning below T
MIN
. If the fans are
always on, the T
HYST
value has no effect on the fan when the
temperature drops below T
MIN
.
THERM Hysteresis
Any hysteresis programmed via Register 0x6D and
Register 0x6E also applies hysteresis on the appropriate
THERM channel.
Figure 65. The T
HYST
Value Applies to Fan On/Off Hysteresis and THERM Hysteresis
THERMAL CALIBRATION
REMOTE1 =
AMBIENT TEMP
100%
0%
T
MIN
T
RANGE
PWM
MIN
MUX
S
TACHOMETER1
MEASUREMENT
S
S
PWM
MIN
PWM
MIN
THERMAL CALIBRATION
100%
0%
T
MIN
T
RANGE
THERMAL CALIBRATION
100%
0%
T
MIN
T
RANGE
LOCAL =
VRM TEMP
REMOTE2 =
CPU TEMP
TACHOMETER2
MEASUREMENT
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
TACHOMETER3
AND 4
MEASUREMENT
PWM
CONFIG
PWM
GENERATOR
PWM1
PWM2
PWM3
TACH3
TACH2
TACH1
CPU FAN SINK
FRONT CHASSIS
REAR CHASSIS
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
PWM
CONFIG
PWM
GENERATOR
PWM
CONFIG
PWM
GENERATOR
T
MIN
100%
0%
PWM DUTY CYCLE
T
THERM
T
RANGE
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