ADM1031ARQZ-R7 datasheet(16/30 Pages) ONSEMI

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ADM1031

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<7:3> Local Temperature TMIN. These bits set the

temperature at which the fan turns on when under auto fan

speed control. TMIN can be programmed in 4°C increments.

00000 = 0

°C

00001 = 4

°C

00010 = 8

°C

00011 = 12

°C

01000 = 32

°C (Default)

11110 = 120

°C

11111 = 124

°C

<2:0> Local Temperature TRANGE. This nibble sets the

temperature range over which automatic fan speed control

takes place.

000 = 5

°C

001 = 10

°C

010 = 20

°C

011 = 40

°C

100 = 80

°C

TMIN/TRANGE

<7:3> Remote Temperature TMIN. Sets the temperature at

which the fan switches on based on Remote X Temperature

Readings.

00000 = 0

°C

00001 = 4

°C

00010 = 8

°C

00011 = 12

°C

01100 = 48

°C

11110 = 120

°C

11111 = 124

°C

<2:0> Remote Temperature TRANGE. This nibble sets the

temperature range over which the fan is controlled based on

remote temperature readings.

000 = 5

°C

001 = 10

°C

010 = 20

°C

011 = 40

°C

100 = 80

°C

Filtered Control Mode

The automatic fan speed control loop reacts

instantaneously to changes in temperature, that is, the PWM

duty cycle responds immediately to temperature change. In

certain circumstances, we do not want the PWM output to

react instantaneously to temperature changes. If significant

variations in temperature are found in a system, the fan

speed changes, which could be obvious to someone in close

proximity. One way to improve the system’s acoustics

would be to slow down the loop so that the fan ramps slowly

to its newly calculated fan speed. This also ensures that

temperature transients are effectively ignored, and the fan’s

operation is smooth.

There are two means by which to apply filtering to the

automatic fan speed control loop. The first method is to ramp

the fan speed at a predetermined rate, to its newly calculated

value instead of jumping directly to the new fan speed. The

second approach involves changing the on−chip ADC

sample rate, to change the number of temperature readings

taken per second.

The filtered mode on the ADM1031 is invoked by setting

Bit 0 of the fan filter register (Register 0

×23) for Fan 1 and

Bit 1 for Fan 2. Once the fan filter register has been written

to, and all other control loop parameters (such as TMIN,

TRANGE) have been programmed, the device can be placed

into automatic fan speed control mode by setting Bit 7 of

Configuration Register 1 (Register 0

×00) to 1.

Effect of Ramp Rate on Filtered Mode

Bits <6:5> of the fan filter register determine the ramp rate

in filtered mode. The PWM_OUT signal driving the fan has

a period, T, given by the PWM_OUT drive frequency, f,

since T = 1/f. For a given PWM period, T, the PWM period

is subdivided in to 240 equal time slots. One time slot

corresponds to the smallest possible increment in PWM duty

cycle. A PWM signal of 33% duty cycle is thus high for 1/3

× 240 time slots and low for 2/3 × 240 time slots. Therefore,

33% PWM duty cycle corresponds to a signal that is high for

80 time slots and low for 160 time slots.

Figure 27. 33% PWM Duty Cycle Presented

in Time Slots

PWM_OUT

33% DUTY

CYCLE

80 TIME

SLOTS

160 TIME

SLOTS

PWM OUTPUT

(ONE PERIOD =

240 TIME SLOTS

The ramp rates in filtered mode are selectable between 1,

2, 4, and 8. The ramp rates are actually discrete time slots.

For example, if the ramp rate = 8, then eight time slots are

added to the PWM_OUT high duty cycle each time the

PWM_OUT duty cycle needs to be increased. Figure 28

shows how the filtered mode algorithm operates.

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