ADM1031 datasheet(14/32 Pages) AD

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ADM1031

–14–

It should be noted however, that changing the minimum PWM

duty cycle affects the control loop behavior.

Slope 1 of Figure 10 shows TMIN set to 0

°C and the TRANGE chosen

is 40

°C. In this case, the fan’s PWM duty cycle will vary over the

range 33% to 100%. The fan will run 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 Slopes 2 and 3. Take Case 2, where

the minimum PWM duty cycle is reprogrammed from 33%

(default) to 53%.

TEMPERATURE – C

100

TMIN

Figure 10. Effect of Changing Minimum Duty Cycle on

Control Loop with Fixed TMIN and TRANGE Values

The fan will actually reach full speed at a much lower temperature;

°C. Case 3 shows that when the minimum PWM duty cycle was

increased to 73%, the temperature at which the fan ran full speed

was 16

°C. So the effect of increasing the minimum PWM duty cycle,

with a fixed TMIN and fixed TRANGE, is that the fan will actually

reach 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 Config

the decimal values written to the Fan Speed Config Register and PWM

duty cycle obtained.

Table VII. Programming PWM Duty Cycle

Decimal Value

PWM Duty Cycle

14%

20%

27%

33% (Default)

40%

47%

53%

60%

10 (0x0A)

67%

11 (0x0B)

73%

12 (0x0C)

80%

13 (0x0D)

87%

14 (0x0E)

93%

15 (0x0F)

100%

The temperature at which the fan will run full-speed (100%

duty cycle) is given by:

TMAX = TMIN + ((Max DC – Min DC)

× TRANGE/10)

where,

TMAX

= Temperature at which fan runs full-speed.

TMIN

= Temperature at which fan will turn on.

Max DC = Maximum Duty Cycle (100%) = 15 decimal.

Min DC = Duty Cycle at TMIN, programmed in to Fan Speed

Config Register (default = 33% = 5 decimal).

TRANGE

= PWM Duty Cycle versus Temperature Slope.

Example 1

TMIN

°C, TRANGE = 40°C

Min DC = 53% = 8 Decimal (Table VII)

Calculate TMAX.

TMAX

=TMIN + ((Max DC – Min DC)

× T

RANGE/10)

TMAX

= 0 + ((100% DC – 53% DC)

× 40/10)

TMAX

= 0 + ((15 – 8)

× 4) = 28

TMAX

=28 C (As seen on Slope 2 of Figure 10)

Example 2

TMIN

°C, TRANGE = 40°C

Min DC = 73% = 11 Decimal (Table VII)

Calculate TMAX.

TMAX

=TMIN + ((Max DC – Min DC)

× TRANGE/10)

TMAX

= 0 + ((100% DC – 73% DC)

× 40/10)

TMAX

= 0 + ((15 – 11)

× 4) = 16

TMAX

=16 C (As seen on Slope 3 of Figure 10)

Example 3

TMIN

°C, TRANGE = 40°C

Min DC = 33% = 5 Decimal (Table VII)

Calculate TMAX.

TMAX

=TMIN + ((Max DC – Min DC)

× TRANGE/10)

TMAX

= 0 + ((100% DC – 33% DC)

× 40/10)

TMAX

= 0 + ((15 – 5)

× 4) = 40

TMAX

=40 C (As seen on Slope 1 of Figure 10)

In this case, since the Minimum Duty Cycle is the default 33%,

the equation for TMAX reduces to:

TMAX

=TMIN + ((Max DC – Min DC)

× T

RANGE/10)

TMAX

=TMIN + ((15 – 5)

× T

RANGE/10)

TMAX

=TMIN + (10

× T

RANGE/10)

TMAX

=TMIN + TRANGE

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