Step 11: Monitoring Therm - Analog Devices ADT7473 Manual

ADT7473
Figure 69 shows how T increases when the current tempera-
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ture is above T and below the low temperature limit, and
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T is below the high temperature limit and below the
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operating point. Once the temperature rises above the low
temperature limit, T stays the same.
MIN
THERM
LIMIT
HIGH TEMP
LIMIT
OPERATING
POINT
HYSTERESIS
ACTUAL
TEMP
LOW TEMP
LIMIT
T
MIN
Figure 69. Increasing T
Example: Preventing T
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Because T is dynamically adjusted, it is undesirable for T
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to reach full scale (127°C) because the fan would never switch
on. As a result, T is allowed to vary only within a specified
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range:
1. The lowest possible value for T
complement mode) or −64°C (Offset 64 mode).
2. T cannot exceed the high temperature limit.
MIN
THERM
LIMIT
HIGH TEMP
LIMIT
OPERATING
POINT
HYSTERESIS
ACTUAL
TEMP
T
MIN
LOW TEMP
LIMIT
DECREASE HERE DUE TO
SHORT CYCLE ONLY
T1(n) – T1 (n – 1) = 0.5°C
OR 0.75°C = > T
DECREASES BY 1°C
EVERY SHORT CYCLE
Figure 68. Effect of Exceeding Operating Point Minus Hysteresis Temperature
for Quieter Operation
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from Reaching Full Scale
is –127°C (twos
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NO CHANGE IN T
DUE TO ANY CYCLE BECAUSE
T1(n) – T1 (n – 1)
AND T1(n) < OP = > T
DECREASE HERE DUE TO
LONG CYCLE ONLY
T1(n) – T1 (n – 1) 0.25°C
AND T1(n) > OP = > T
MIN MIN
DECREASES BY 1°C
EVERY LONG CYCLE
3. If the temperature is below T
is running at minimum speed and dynamic T
disabled.
THERM
LIMIT
OPERATING
POINT
LOW TEMP
LIMIT
HIGH TEMP
LIMIT
T
MIN
Figure 70. T

STEP 11: MONITORING THERM

Using the operating point limit ensures that the dynamic T
control mode is operating in the best possible acoustic position
while ensuring that the temperature never exceeds the maxi-
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mum operating temperature. Using the operating point limit
allows T to be independent of system-level issues because of
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its self-corrective nature. In PC design, the operating point for
the chassis is usually the worst-case internal chassis
temperature.
The optimal operating point for the processor is determined by
monitoring the thermal monitor in the Intel Pentium 4 proces-
sor. To do this, the PROCHOT output of the Pentium 4 is
connected to the THERM input of the ADT7473.
Rev. 0 | Page 50 of 76
HERE
MIN
0.25°C
MIN
STAYS THE SAME
, the fan is switched off or
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HYSTERESIS
ACTUAL
TEMP
T
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FROM INCREASING
Adjustments Limited by the High Temperature Limit
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control is
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PREVENTED
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