ADM1031ARQZ-R7 datasheet(13/30 Pages) ONSEMI

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ADM1031

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Figure 22. PWM Duty Cycle vs. Temperature Slope

(TRANGE)

TEMPERATURE (°C)

100

TMIN

510

TMAX = TMIN + TRANGE

TRANGE = 5°C

TRANGE = 10°C

TRANGE = 20°C

TRANGE = 40°C

TRANGE = 80°C

Figure 23 shows how, for a given TRANGE, changing the

TMIN value affects the loop. Increasing the TMIN value

increases the TMAX (temperature at which the fan runs full

speed) value, since TMAX = TMIN + TRANGE. Note, however,

that the PWM duty cycle vs. temperature slope remains

exactly the same. Changing the TMIN value merely shifts the

control slope. The TMIN can be changed in increments of 4°C.

Figure 23. Effect of Increasing TMIN Value on

Control Loop

TEMPERATURE (°C)

100

TMIN

TMAX = TMIN + TRANGE

TRANGE = 40°C

Fan Spin−Up

As mentioned in the How Does the Control Loop Work?

section, once the temperature being measured exceeds the

TMIN value programmed, the fan turns on at minimum speed

(default = 33% duty cycle). However, the problem with fans

being driven by PWM is that 33% duty cycle is not enough

to reliably start the fan spinning. The solution is to spin the

fan up for a predetermined time, and once the fan has spun

up, its running speed can be reduced in line with the

temperature being measured.

The ADM1031 allows fan spin−up times between 200 ms

and 8 seconds. Bits <2:0> of Fan Characteristics Register 1

(Register 0

×20) and Fan Characteristic Register 2 (Register

×21) program the fan spin−up times.

Table 5. Fan Spin−Up Times

Bits 2:0

Spin−Up Time

(Fan Characteristics Registers 1, 2)

000

200 ms

001

400 ms

010

600 ms

011

800 ms

100

1 sec

101

2 sec (Default)

110

4 sec

111

8 sec

Once the automatic fan speed control loop parameters

have been chosen, the ADM1031 device can be

programmed. The ADM1031 is placed into automatic fan

speed control mode by setting Bit 7 of Configuration

×00). The device powers up in

automatic fan speed control mode by default. The control

mode offers further flexibility in that the user can decide

which temperature channel/channels control each fan.

Table 6. Auto Mode Fan Behavior

Bits 6, 5

Control Operation (Configuration Register 1)

Remote Temperature 1 Controls Fan 1

Remote Temperature 2 Controls Fan 2

Remote Temperature 1 Controls Fan 1 and 2

Remote Temperature 2 Controls Fan 1 and 2

Maximum Speed Calculated by Local and Remote

Temperature Channels Controls Fans 1 and 2

When Bit 5 and Bit 6 of Configuration Register 1 are both

set to 1, increased flexibility is offered. The local and remote

temperature channels can have independently programmed

control loops with different control parameters. Whichever

control loop calculates the fastest fan speed based on the

temperature being measured, drives the fans.

Figure 24 and Figure 25 show how the fan’s PWM duty

cycle is determined by two independent control loops. This

is the type of auto mode fan behavior seen when Bit 5 and

Bit 6 of Configuration Register 1 are set to 11. Figure 24

shows the control loop for the local temperature channel. Its

TMIN value has been programmed to 20°C, and its TRANGE

value is 40

°C. The local temperature’s TMAX is thus 60°C.

Figure 25 shows the control loop for the remote temperature

channel. Its TMIN value has been set to 0°C, while its

TRANGE = 80°C. Therefore, the remote temperature’s TMAX

value is 80

°C.

Consider if both temperature channels measure 40

°C.

Both control loops calculate a PWM duty cycle of 66%.

Therefore, the fan is driven at 66% duty cycle. If both

temperature channels measure 20

°C, the local channel

calculates 33% PWM duty cycle, while the Remote 1

channel calculates 50% PWM duty cycle. Thus, the fans are

driven at 50% PWM duty cycle. Consider the local

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