ADT7476ARQZ-R7 datasheet(42/67 Pages) ONSEMI

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Figure 62. 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

Example: Determining TRANGE for Each Temperature

Channel

The following example shows how the different TMIN and

TRANGE settings can be applied to three different thermal

zones. In this example, the following TRANGE values apply:

TRANGE = 80°C for ambient temperature

TRANGE = 53.33°C for CPU temperature

TRANGE = 40°C for VRM temperature

This example uses the mux configuration described in

Step 2 − Configuring the Mux with the ADT7476 connected

as shown in Figure 52. Both CPU temperature and VRM

temperature drive the CPU fan connected to PWM1.

Ambient temperature drives the front chassis fan and rear

chassis fan connected to PWM2 and PWM3. The front

chassis fan is configured to run at PWMMIN = 20%. The rear

chassis fan is configured to run at PWMMIN = 30%. The

CPU fan is configured to run at PWMMIN = 10%.

Note: The control range for 4−wire fans is much wider

than that of 3−wire fans. In many cases, 4−wire fans can start

with a PWM drive of as little as 20% or less. In extreme

cases, some 3−wire fans cannot run unless a PWM drive of

60% or more is applied.

Figure 63. TRANGE and % Fan Speed Slopes for VRM,

Ambient, and CPU Temperature Channels

TEMPERATURE ABOVE TMIN

1020

3040

100

TEMPERATURE ABOVE TMIN

100

1020

3040

100

VRM TEMPERATURE

AMBIENT TEMPERATURE

CPU TEMPERATURE

VRM TEMPERATURE

CPU TEMPERATURE

AMBIENT TEMPERATURE

Step 7—TTHERM for Temperature Channels

TTHERM is the absolute maximum temperature allowed

on a temperature channel. Above this temperature, a

component such as the CPU or VRM can operate 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

exceeding any TTHERM limit runs all fans at 100%, it has

very negative acoustic effects. Ultimately, this limit should

be set up as a fail−safe, and users should ensure that it is not

exceeded under normal system operating conditions.

Note: TTHERM limits are nonmaskable and affect the fan

speed no matter how automatic fan control settings are

configured. This allows some flexibility, because a TRANGE

value can be selected based on its slope, while a hard limit

(such as 70

°C), can be programmed as TMAX (the

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