ADM1027 datasheet(32/56 Pages) AD

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REV. A

–32–

ADM1027

Step 5

Determine TRANGE for Each Temperature Channel

TRANGE is the range of temperature over which automatic fan

control occurs once the programmed TMIN temperature has

been exceeded. TRANGE is actually a temperature slope and not

an arbitrary value, i.e., a TRANGE of 40

∞C only holds true for

PWMMIN = 33%. If PWMMIN is increased or decreased, the

effective TRANGE is changed, as described later.

TEMPERATURE

TMIN

100%

PWMMIN

TRANGE

Figure 32. TRANGE Parameter Affects Cooling Slope

The TRANGE or fan control slope is determined by the following

procedure:

1. Determine the maximum operating temperature for that

channel, e.g., 70

∞C.

2. Determine experimentally the fan speed (PWM duty cycle

value) will not exceed that temperature at the worst-case

operating points, e.g., 70

∞C is reached when the fans are

running at 50% PWM duty cycle.

3. Determine the slope of the required control loop to meet

these requirements.

4. Use best fit approximation to determine the most suitable

TRANGE value. There is ADM1027 evaluation software avail-

able to calculate the best fit value; ask your local Analog

Devices representative for more details.

TMIN

100%

33%

50%

30 C

40 C

Figure 33. Adjusting PWMMIN Affects TRANGE

TRANGE is implemented as a slope, which means as PWMMIN is

changed, TRANGE changes, but the actual slope remains the

same. The higher the PWMMIN value, the smaller the effective

TRANGE will be, i.e., the fan will reach full speed (100%) at a

lower temperature.

TMIN

100%

33%

50%

30 C

40 C

25%

10%

45 C

54 C

Figure 34. Increasing PWMMIN Changes Effective TRANGE

For a given TRANGE value, the temperature at which the fan

will run full-speed for different PWMMIN values can easily be

calculated:

MaxD.C.

MinD.C.

MAX

MIN

RANGE

()¥

170

where:

TMAX = Temperature at which the fan runs full-speed

TMIN = Temperature at which the fan will turn on

MaxD.C. = Maximum duty cycle (100%) = 255 decimal

MinD.C. = PWMMIN

TRANGE = PWM duty cycle versus temperature slope

Example: Calculate TMAX, given TMIN = 30

∞C, TRANGE = 40∞C,

and PWMMIN = 10% duty cycle = 26 decimal

MaxD C

MinD C

MAX

MIN

RANGE

()¥

170

MAX

= ∞ +-

()¥ ∞

100

170

MAX

= ∞ +-

()¥ ∞

255

170

TMAX = 84

∞C (effective TRANGE = 54∞C)

Example: Calculate TMAX, given TMIN = 30

∞C, TRANGE = 40∞C

and PWMMIN = 25% duty cycle = 64 decimal

MaxD C

MinD C

MAX

MIN

RANGE

()¥

170

MAX

= ∞ +-

()¥ ∞

100

170

MAX

= ∞ +-

()¥ ∞

255

170

TMAX = 75

∞C (effective TRANGE = 45∞C)

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