ADM1031ARQZ-R7 datasheet(12/30 Pages) ONSEMI

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ADM1031

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speed. Bit 1 (Fan Fault) is set whenever a fan tach failure is

detected. Once cleared, it reasserts on subsequent fan tach

failures.

Bit 2 and Bit 3 of Status Register 1 and Status Register 2

are the Remote 1 and Remote 2 Temperature High and Low

status bits. Exceeding the high or low temperature limits for

the external channel sets these status bits. Reading the status

if the out−of−limit condition still exists on the next

monitoring cycle. Bit 6 and Bit 7 are the Local Temperature

High and Low status bits. These behave exactly the same as

the Remote Temperature High and Low status bits. Bit 4 of

Status Register 1 indicates that the Remote Temperature

THERM limit has been exceeded. This bit gets cleared on a

read of Status Register 1 (see Figure 20). Bit 5 indicates a

remote diode error. This bit is a 1 if a short or open is detected

on the remote temperature channel on powerup. If this bit is

set to 1 on powerup, it cannot be cleared. Bit 6 of Status

×03) indicates that the Local THERM limit has

been exceeded. This bit is cleared on a read of Status

as an input. This bit can also be cleared on a read of Status

Figure 20. Operation of THERM and INT Signals

5°

INT REARMED

STATUS REG. READ

TEMP

THERM LIMIT

THERM

INT

Figure 20 shows the interaction between INT and

THERM. Once a critical temperature THERM limit is

exceeded, both INT and THERM assert low. Reading the

status registers clears the interrupt and the INT pin goes

high. However, the THERM pin remains asserted until the

measured temperature falls 5

°C below the exceeded

THERM limit. This feature can be used to CPU throttle or

drive a fan full speed for maximum cooling. Note that the

INT pin for that interrupt source is not rearmed until the

temperature has fallen below the THERM limit –5

°C. This

prevents unnecessary interrupts from tying up valuable CPU

resources.

Fan Control Modes of Operation

The ADM1031 has four different modes of operation.

These modes determine the behavior of the system.

1. Automatic Fan Speed Control Mode.

2. Filtered Automatic Fan Speed Control Mode.

3. PWM Duty Cycle Select Mode (directly sets fan

speed under software control).

4. RPM Feedback Mode.

Automatic Fan Speed Control

The ADM1031 has a local temperature channel and two

remote temperature channels, which can be connected to an

on−chip diode−connected transistor on a CPU. These three

temperature channels can be used as the basis for an

automatic fan speed control loop to drive fans using pulse

width modulation (PWM).

How Does the Control Loop Work?

The automatic fan speed control loop is shown in Figure 21.

Figure 21. Automatic Fan Speed Control Loop

MAX

FAN

SPEED

MIN

TEMPERATURE

TMIN

TMAX = TMIN + TRANGE

SPIN−UP FOR SECONDS

TMIN is the temperature at which the fan should switch on

and run at minimum speed. The fan only turns on once the

temperature being measured rises above the TMIN value

programmed. The fan spins up for a predetermined time

(default = 2 seconds). See the Fan Spin−Up section for more

details.

TRANGE is the temperature range over which the

ADM1031 automatically adjusts the fan speed. As the

temperature increases beyond TMIN, the PWM_OUT duty

cycle increases accordingly. The TRANGE parameter

actually defines the fan speed vs. temperature slope of the

control loop.

TMAX is the temperature at which the fan is at its

maximum speed. At this temperature, the PWM duty cycle

driving the fan is 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.

A hysteresis value of 5

°C is included in the control loop

to prevent the fan continuously switching on and off if the

temperature is close to TMIN. The fan continues to run until

the temperature drops 5

°C below TMIN.

Figure 22 shows the different control slopes determined

by the TRANGE value chosen, and programmed into the

ADM1031. TMIN is set to 0°C to start all slopes from the

same point. The figure shows how changing the TRANGE

value affects the PWM duty cycle vs. temperature slope.

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