ADT7467ARQZ-R7 datasheet(52/77 Pages) ONSEMI

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ADT7467

Rev. 3 | Page 52 of 77 | www.onsemi.com

Figure 71 shows the steps taken during the long cycle.

WAIT 2n

MONITORING

CYCLES

IS T1(n) < LOW TEMP LIMIT

AND

TMIN < HIGH TEMP LIMIT

AND

TMIN < OP1

AND

T1(n) > TMIN

IS T1(n) > OP1

YES

INCREASE

TMIN BY 1°C

YES

DECREASE TMIN

BY 1°C

CURRENT

TEMPERATURE

MEASUREMENT

T1(n)

OPERATING

POINT

TEMPERATURE

OP1

DO NOT

CHANGE

Figure 71. Long Cycle Steps

The following examples illustrate circumstances that may cause

TMIN to increase, decrease, or stay the same.

Normal Operation—No TMIN Adjustment

•

If measured temperature never exceeds the programmed

operating point minus the hysteresis temperature, TMIN is

not adjusted, that is, it remains at its current setting.

•

If measured temperature never drops below the low

temperature limit, TMIN is not adjusted.

TMIN

THERM LIMIT

OPERATING

POINT

HIGH TEMP

LIMIT

LOW TEMP

LIMIT

ACTUAL

TEMP

HYSTERESIS

Figure 72. Temperature Between Operating Point and Low Temperature Limit

Because neither the operating point minus the hysteresis

temperature nor the low temperature limit has been exceeded,

the TMIN value is not adjusted and the fan runs at a speed

determined by the fixed TMIN and TRANGE values, defined in the

automatic fan speed control mode in the Enhancing System

Acoustics section.

Operating Point Exceeded—TMIN Reduced

When the measured temperature is below the operating point

temperature minus the hysteresis, TMIN remains the same.

Once the temperature exceeds the operating temperature minus

the hysteresis (OP1 − Hyst), TMIN decreases during the short

cycle (see Figure 70) at a rate determined by the programmed

value of n. This rate also depends on the amount that the

temperature has increased between this monitoring cycle and

the last monitoring cycle. For example, if the temperature has

increased by 1°C, then TMIN is reduced by 2°C. Decreasing TMIN

has the effect of increasing the fan speed, thus providing more

cooling to the system.

If the temperature slowly increases only in the range (OP1 − Hyst),

that is, the change in temperature is ≤0.25°C per short monitor-

ing cycle, TMIN does not decrease. This allows small changes in

temperature in the desired operating zone without changing

TMIN. The long cycle makes no change to TMIN in the temperature

range (OP − Hyst), because the temperature has not exceeded

the operating temperature.

Once the temperature exceeds the operating temperature, TMIN

reduces by 1°C per long cycle as long as the temperature

remains above the operating temperature. This takes place in

addition to the decrease in TMIN that occurs during the short

cycle. In Figure 73, because the temperature is increasing at a

rate of ≤0.25°C per short cycle, no reduction in TMIN takes place

during the short cycle.

Once the temperature falls below the operating temperature,

TMIN remains fixed, even when the temperature starts to

increase slowly, because the temperature only increases at a rate

of ≤0.25°C per cycle.

Increasing the TMIN Cycle

When the temperature drops below the low temperature limit,

TMIN can increase during the long cycle. Increasing TMIN has the

effect of running the fan more slowly and, therefore, more

quietly. The long cycle diagram in Figure 71 shows the

conditions necessary for TMIN to increase.

TMIN can increase if

•

The measured temperature falls below the low temperature

limit. This means that 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 exceed the high temperature limit. As a result,

the high limit should be chosen carefully because it deter-

mines the high limit of TMIN.

•

TMIN is below the operating point temperature. TMIN should

never be allowed to increase above the operating point

temperature, because the fans would not switch on until

the temperature rose above the operating point.

•

The temperature is above TMIN. The dynamic TMIN control

is turned off below TMIN.

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