ADT7467ARQZ-R7 datasheet(53/77 Pages) ONSEMI

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ADT7467

Rev. 3 | Page 53 of 77 | www.onsemi.com

TMIN

THERM

LIMIT

OPERATING

POINT

HIGH TEMP

LIMIT

LOW TEMP

LIMIT

HYSTERESIS

DECREASE HERE DUE TO

SHORT CYCLE ONLY

T1(n) – T1 (n – 1) = 0.5°C

OR 0.75°C = > TMIN

DECREASES BY 1°C

EVERY SHORT CYCLE

DECREASE HERE DUE TO

LONG CYCLE ONLY

T1(n) – T1 (n – 1) ≤ 0.25°C

AND T1(n) > OP = > TMIN

DECREASES BY 1°C

EVERY LONG CYCLE

NO CHANGE IN TMIN HERE

DUE TO ANY CYCLE, BECAUSE

T1(n) – T1 (n – 1) ≤ 0.25°C

AND T1(n) < OP = > TMIN

STAYS THE SAME

ACTUAL

TEMP

Figure 73. Effect of Exceeding Operating Point Minus Hysteresis Temperature

Figure 74 shows how TMIN increases when the current tempera-

ture is above TMIN but below the low temperature limit, and how

TMIN is below the high temperature limit and below the

operating point. Once the temperature rises above the low

temperature limit, TMIN remains fixed.

TMIN

OPERATING

POINT

HIGH TEMP

LIMIT

LOW TEMP

LIMIT

ACTUAL

TEMP

HYSTERESIS

THERM

LIMIT

Figure 74. Increasing TMIN for Quiet Operation

Preventing TMIN from Reaching Full Scale

TMIN is dynamically adjusted; therefore, it is undesirable for TMIN

to reach full scale (127°C), because the fan would never switch on.

As a result, TMIN is allowed to vary only within a specified range.

•

The lowest possible value for TMIN is –127°C (twos

complement mode) or −64°C (Offset 64 mode).

•

TMIN cannot exceed the high temperature limit.

•

If the temperature is below TMIN, the fan switches off or

runs at minimum speed and dynamic TMIN control is

disabled.

TMIN PREVENTED

FROM INCREASING

TMIN

OPERATING

POINT

HIGH TEMP

LIMIT

LOW TEMP

LIMIT

ACTUAL

TEMP

HYSTERESIS

THERM

LIMIT

Figure 75. TMIN Adjustments Limited by the High Temperature Limit

STEP 11: MONITORING THERM

Using the operating point limit ensures that the dynamic TMIN

control mode operates in the best possible acoustic position and

that the temperature never exceeds the maximum operating

temperature. Using the operating point limit allows TMIN to be

independent of system-level issues because of 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 processor.

To do this, the PROCHOT output of the Pentium 4 is con-

nected to the THERM input of the ADT7467.

The operating point for the processor can be determined by

allowing the current temperature to be copied to the operating

point register when the PROCHOT output pulls the THERM

input low on the ADT7467. This reveals the maximum

temperature at which the Pentium 4 can run before clock

modulation occurs.

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