ADM1029ARQZ-R7 datasheet(18/50 Pages) ONSEMI

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AUTOMATIC FAN SPEED CONTROL

The ADM1029 has a local temperature channel and two remote

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

diode-connected transistor on a CPU or a general-purpose

discrete transistor. 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 10.

MAX

MIN

TEMPERATURE

TMIN

TMAX = TMIN + TRANGE

SPIN UP FOR 2 SECONDS

Figure 10. Automatic Fan Speed Control

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

parameters need to be programmed in to the device:

1. TMIN. This is the temperature at which a 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 predeter-

mined time (default = 2 secs). See Fan Spin-Up section

for more details.

2. TRANGE. This will be the temperature range over which the

ADM1029 will automatically adjust fan speed. As the tempera-

ture increases beyond TMIN, the PWM duty cycle will be

increased accordingly. The TRANGE parameter actually defines

the fan speed versus temperature slope of the control loop.

3. TMAX. This is defined as the temperature at which a fan

will be at its maximum 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 register on-chip.

4. Programmable hysteresis is included in the control loop to

prevent the fans continuously switching on and off if the

temperature is close to TMIN. The fans will continue to run

until such time as the temperature drops below TMIN–THYST.

The four MSBs of the TRANGE/THYST registers (Registers

0x88, 0x89, 0x8A) contain a temperature hysteresis value

that can be programmed from 0001 to 1111. This allows a

temperature hysteresis range from 1°C to 15°C for each

temperature measurement channel.

100

TEMPERATURE

– C

TMIN

TMAX = TMIN + TRANGE

T R

TRANGE = 5 C

Figure 11. PWM Duty Cycle vs. Temperature Slopes

(TRANGE)

Figure 11 shows the different control slopes determined by the

TRANGE value chosen, and programmed in to the ADM1029.

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 vs. Temperature Slope.

Figure 12 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 versus Temperature slope remains exactly the same.

Changing the TMIN value merely shifts the control slope.

TEMPERATURE

– C

TMIN

TMAX = TMIN + TRANGE

100

Figure 12. Effect of Increasing TMIN Value on Control Loop

FAN SPIN-UP

As previously mentioned, once the temperature being measured

exceeds the TMIN value programmed, the fan will turn on at

minimum speed (default = 33% duty cycle). However, the prob-

lem with fans being driven by PWM is that 33% duty cycle is

not enough to reliably start the fan spinning. The solution is to

ADM1029

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