ADT7490ARQZ-R7 datasheet(39/75 Pages) ONSEMI

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ADT7490

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Actual Changes in PWM Output

(Advanced Acoustics Settings)

While the automatic fan control algorithm describes the

general response of the PWM output, it is also necessary to

note that the enhanced acoustics registers (0x62, 0x63, and

0x3C) can be used to set/clamp the maximum rate of change

of PWM output for a given temperature zone. This means that

if TRANGE is programmed with an AFC slope that is quite

steep, a relatively small change in temperature could cause a

large change in PWM output and possibly an audible change

in fan speed, which can be noticeable/annoying to end users.

Decreasing the speed of the PWM output changes by

programming the smoothing on the appropriate temperature

channels (Register 0x62 and Register 0x63) changes how

fast the fan speed increases/decreases in the event of a

temperature spike. Slowly the PWM duty cycle increases

until the PWM duty cycle reaches the appropriate duty cycle

as defined by the AFC curve.

Figure 57 shows PWM duty cycle vs. temperature for each

TRANGE setting. Figure 57B shows how each TRANGE

setting affects fan speed vs. temperature. As can be seen

from the graph, the effect on fan speed is nonlinear.

Figure 57. TRANGE vs. Actual Fan Speed

(Not PWM Drive) Profile

TEMPERATURE ABOVE TMIN

20406080

100

120

100

TEMPERATURE ABOVE TMIN

(B)

(A)

100

120

100

25C

805C

53.35C

405C

325C

26.65C

205C

165C

13.35C

105C

85C

6.675C

55C

45C

3.335C

2.55C

25C

805C

53.35C

405C

325C

26.65C

205C

165C

13.35C

105C

85C

6.675C

55C

45C

3.335C

2.55C

The graphs in Figure 57 assume the fan starts from 0%

PWM duty cycle. Clearly, the minimum PWM duty cycle,

PWMMIN, needs to be factored in to see how the loop

actually performs in the system. Figure 58 shows how

TRANGE is affected when the PWMMIN value is set to 20%.

It can be seen that the fan actually runs at about 45% fan

speed when the temperature exceeds TMIN.

Figure 58. TRANGE and % Fan Speed Slopes with

PWMMIN = 20%

TEMPERATURE ABOVE TMIN

100

120

100

TEMPERATURE ABOVE TMIN

(A)

(B)

100

120

100

25C

805C

53.35C

405C

325C

26.65C

205C

165C

13.35C

105C

85C

6.675C

55C

45C

3.335C

2.55C

25C

805C

53.35C

405C

325C

26.65C

205C

165C

13.35C

105C

85C

6.675C

55C

45C

3.335C

2.55C

Step 7: TTHERM for Temperature Channels

TTHERM is the absolute maximum temperature allowed

on a temperature channel. For PECI temperature channels,

the equivalent parameter is TCONTROL. Above this

temperature, a component such as the CPU or VRM may be

operating beyond its safe operating limit. When the

temperature measured exceeds TTHERM, all fans are driven

at 100% PWM duty cycle (full speed) to provide critical

system cooling.

The fans remain running at 100% until the temperature

drops below TTHERM minus hysteresis, where hysteresis is

the number programmed into the hysteresis registers (0x6D

and 0x6E). The default hysteresis value is 4

°C.

The TTHERM limit should be considered the maximum

worst−case operating temperature of the system. Because

ADT7490ARQZ-R7 Datasheet(PDF) 39 Page - ON Semiconductor

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