ADM1031 datasheet(12/32 Pages) AD

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ADM1031

–12–

AUTOMATIC FAN SPEED CONTROL

The ADM1031 has a local temperature channel and two remote

temperature channels, which may be connected to an on-chip

diode-connected transistor on a CPU. These three temperature

channels may be used as the basis for an automatic fan speed

control loop to drive fans using Pulsewidth Modulation (PWM).

HOW DOES THE CONTROL LOOP WORK?

The Automatic Fan Speed Control Loop is shown in Figure 6.

FAN

SPEED

MAX

MIN

TEMPERATURE

TMIN

SPIN-UP FOR TWO SECONDS

TMAX = TMIN + TRANGE

Figure 6. Automatic Fan Speed Control

In order for the fan speed control loop to work, certain loop

parameters need to be programmed into the device.

1. TMIN. The temperature at which the fan should switch on

and run at minimum speed. The fan will only turn on once

the temperature being measured rises above the TMIN value

programmed. The fan will spin up for a predetermined time

(default = 2 secs). See Fan Spin-Up section for more details.

2. TRANGE. The temperature range over which the ADM1031

will automatically adjust the fan speed. As the temperature

increases beyond TMIN, the PWM_OUT duty cycle will be

increased accordingly. The TRANGE parameter actually defines

the fan speed versus temperature slope of the control loop.

3. TMAX. The temperature at which the fan will be at its maxi-

mum speed. At this temperature, the PWM duty cycle driving

the fan will be 100%. TMAX is given by TMIN + TRANGE.

Since this parameter is the sum of the TMIN and TRANGE

parameters, it does not need to be programmed into a

4. A hysteresis value of 5

°C is included in the control loop to

prevent the fan continuously switching on and off if the tem-

perature is close to TMIN. The fan will continue to run until

such time as the temperature drops 5

°C below TMIN.

Figure 7 shows the different control slopes determined by the

TRANGE value chosen, and programmed into the ADM1031.

TMIN was set to 0

°C to start all slopes from the same point. It

can be seen how changing the TRANGE value affects the PWM

duty cycle versus temperature slope.

TEMPERATURE – C

100

TMIN

406080

T RANGE

TMAX = TMIN + TRANGE

Figure 7. PWM Duty Cycle vs. Temperature Slopes (TRANGE)

Figure 8 shows how, for a given TRANGE, changing the TMIN

value affects the loop. Increasing the TMIN value will increase

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 may be changed in increments of 4

°C.

TEMPERATURE – C

100

TMIN

TMAX = TMIN + TRANGE

Figure 8. Effect of Increasing TMIN Value on Control Loop

FAN SPIN-UP

As was previously mentioned, once the temperature being mea-

sured exceeds the TMIN value programmed, the fan will turn 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 may 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 Registers 1 and

2 (Register 0x20, 0x21) program the fan spin-up times.

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