ADT7462ACPZ-R7 datasheet(44/81 Pages) ONSEMI

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ADT7462

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When the temperature falls below the operating

temperature, TMIN stays the same. Even when the

temperature starts to increase slowly, TMIN stays the same,

because the temperature increases at a rate of

≤0.25°C per

cycle.

Figure 71. Effect of Exceeding Operating Point Minus

Hysteresis Temperature

TMIN

OPERATING POINT

HIGH TEMP LIMIT

LOW TEMP LIMIT

HYSTERESIS

DECREASE HERE DUE TO

SHORT CYCLE ONLY

T1(n) – T1 (n – 1) ≤ 0.255C

OR 0.755C = > TMIN

DECREASES BY 15C

EVERY SHORT CYCLE

DECREASE HERE DUE TO

LONG CYCLE ONLY

T1(n) – T1 (n – 1) ≤ 0.255C

AND T1(n) > OP = > TMIN

DECREASES BY 15C

EVERY LONG CYCLE

NO CHANGE IN TMIN HERE

DUE TO ANY CYCLE BECAUSE

T1(n) – T1 (n – 1) ≤ 0.255C

AND T1(n) < OP = > TMIN

STAYS THE SAME

ACTUAL

TEMP

THERM LIMIT

Example 3: Temperature Below Low Limit, TMIN Increased

When the temperature drops below the low temperature

limit, TMIN may increase, as shown in Figure 72. Increasing

TMIN has the effect of running the fan more slowly and,

therefore, more quietly. The long cycle diagram in Figure 69

shows the conditions that need to be true for TMIN to

increase. The following is a quick summary of those

conditions and the reasons they need to be true:

TMIN may increase, if

• The measured temperature has fallen below the low

temperature limit. This means the user must choose the

low limit carefully. It should not be so low that the

temperature never falls below it, because TMIN would

never increase and the fans would run faster than

necessary.

• TMIN is below the high temperature limit. TMIN is

never allowed to increase above the high temperature

limit. As a result, the high limit should be sensibly

chosen, because it determines how high TMIN can go.

• TMIN is below the operating point temperature. TMIN

should never be allowed to increase above the operating

point temperature, because the fans do not switch on

until the temperature rises above the operating point.

• The temperature is above TMIN. The dynamic TMIN

control is turned off below TMIN.

Figure 72 shows how TMIN increases when the current

temperature is above TMIN and below the low temperature

limit, and TMIN is below the high temperature limit and

below the operating point. When the temperature rises above

the low temperature limit, TMIN stays the same.

Figure 72. Increasing TMIN for Quieter Operation

TMIN

OPERATING POINT

HIGH TEMP LIMIT

LOW TEMP

LIMIT

ACTUAL

TEMP

HYSTERESIS

THERM LIMIT

Example 4: Preventing TMIN from Reaching Full Scale

Because TMIN is dynamically adjusted, it is undesirable

for TMIN to reach full scale (191°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 −64°C.

• TMIN cannot exceed the high temperature limit.

• If the temperature is below TMIN, the fan is switched

off or is running at minimum speed, and dynamic TMIN

control is disabled.

Figure 73. TMIN Adjustments Limited by High

Temperature Limit

TMIN PREVENTED

FROM INCREASING

ACTUAL

TEMP

HYSTERESIS

TMIN

OPERATING POINT

HIGH TEMP LIMIT

LOW TEMP

LIMIT

THERM LIMIT

Enabling Dynamic TMIN Control Mode

Bits [1:0] of Dynamic TMIN Control Register 1 (0x0B)

enable/disable dynamic TMIN control on the temperature

channels (see Table 39).

Dynamic TMIN Control Register 1 (0x0B)

Bit 1 (Remote 2 En) = 1 enables dynamic TMIN control on

the Remote 2 temperature channel. The chosen TMIN value

is dynamically adjusted based on the current temperature,

operating point, and high and low limits for this zone.

Bit 1 (Remote 2 En) = 0 disables dynamic TMIN control.

The TMIN value chosen is not adjusted and the channel

behaves as described in the Automatic Fan Control

Overview section.

ADT7462ACPZ-R7 Datasheet(PDF) 44 Page - ON Semiconductor

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