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temperature measuring 60

°C while the Remote 1

temperature is measuring 70

°C. The PWM duty cycle

calculated by the local temperature control loop is 100%

(because the temperature = TMAX). The PWM duty cycle

calculated by the Remote 1 temperature control loop at 70

°C

is approximately 90%. Therefore, the fan runs full−speed

(100% duty cycle). Remember, that the fan speed is based on

the fastest speed calculated, and is not necessarily based on

the highest temperature measured. Depending on the control

loop parameters programmed, a lower temperature on one

channel, can actually calculate a faster speed than a higher

temperature on the other channel.

Figure 24. Maximum Speed Calculated by Local

Temperature Control Loop Drives Fan

Figure 25. Maximum Speed Calculated by Local

Temperature Control Loop Drives Fan

LOCAL TEMPERATURE (°C)

100

TMIN

TMAX = TMIN + TRANGE

TRANGE = 40°C

REMOTE TEMPERATURE (°C)

100

TMIN

TMAX = TMIN + TRANGE

TRANGE = 80°C

Programming the Automatic Fan Speed Control Loop

1. Program a value for TMIN.

2. Program a value for the slope TRANGE.

3. TMAX = TMIN + TRANGE.

4. Program a value for fan spin−up time.

5. Program the desired automatic fan speed control

mode behavior, that is, which temperature channel

controls the fan.

6. Select automatic fan speed control mode by setting

Bit 7 of Configuration Register 1.

Other Control Loop Parameters

It should be noted that changing the minimum PWM duty

cycle affects the control loop behavior.

Slope 1 of Figure 26 shows TMIN set to 0°C and the

TRANGE chosen is 40°C. In this case, the fan’s PWM duty

cycle varies over the range 33% to 100%. The fan runs

full−speed at 40

°C. If the minimum PWM duty cycle at

which the fan runs at TMIN is changed, its effect can be seen

on Slope 2 and Slope 3. Take Case 2, where the minimum

PWM duty cycle is reprogrammed from 33% (default) to

53%.

Figure 26. Effect of Changing Minimum Duty Cycle

on Control Loop with Fixed TMIN and TRANGE Values

TEMPERATURE (°C)

100

TMIN

TRANGE = 40°C

The fan actually reaches full speed at a much lower

temperature, 28

°C. Case 3 shows that when the minimum

PWM duty cycle is increased to 73%, the temperature at

which the fan runs full speed is 16

°C. Therefore, the effect

of increasing the minimum PWM duty cycle, with a fixed

TMIN and fixed TRANGE, is that the fan actually reaches full

speed (TMAX) at a lower temperature than TMIN + TRANGE.

How can TMAX be calculated?

In automatic fan speed control mode, the register that

holds the minimum PWM duty cycle at TMIN, is the fan

speed configuration register (Register 0

×22). Table 7 shows

the relationship between the decimal values written to the

fan speed configuration register and PWM duty cycle

obtained.

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