ADM1026JSTZ-R7 datasheet(23/55 Pages) ONSEMI

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ADM1026

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When the temperature measured by any of the sensors

exceeds the corresponding TMIN, the fan is spun up for

2 seconds with the fan drive set to maximum (full scale from

the DAC or 100% PWM duty cycle). The fan speed is then

set to the minimum as previously defined. As the

temperature increases, the fan drive increases until the

temperature reaches TMIN + 20C.

The fan drive at any temperature up to 20C above TMIN

is given by:

(eq. 13)

PWM + PWMMIN ) 100 * PWMMIN

TACTUAL * TMIN

(eq. 14)

DAC + DACMIN ) 240 * DACMIN

TACTUAL * TMIN

For simplicity of the automatic fan speed algorithm, the

DAC code increases linearly up to 240, not its full scale of

255. However, when the temperature exceeds TMIN +20C,

the DAC output jumps to full scale. To ensure that the

maximum cooling capacity is always available, the fan drive

is always set by the sensor channel demanding the highest

fan speed.

If the temperature falls, the fan does not turn off until the

temperature measured by all three temperature sensors has

fallen to their corresponding TMIN − 4C. This prevents the

fan from cycling on and off continuously when the

temperature is close to TMIN.

Whenever a fan starts or stops during automatic fan speed

control, a one-off interrupt is generated at the INT output.

This is described in more detail in the section on the

ADM1026 Interrupt Structure.

Figure 41. Automatic PWM Fan Control Transfer

Function

PWM

OUTPUT

MIN

TEMPERATURE

TMIN

SPIN UP FOR 2 SECONDS

100%

TMIN ć 45C

TMIN + 205C

Figure 42. Automatic DAC Fan Control Transfer

Function

DAC

OUTPUT

MIN

TEMPERATURE

TMIN

255

TMIN - 45C

TMIN + 205C

SPIN UP FOR 2 SECONDS

240

Fan Inputs

Pins 3 to 6 and 9 to 12 may be configured as fan speed

measuring inputs by clearing the corresponding bit(s) of

Configuration Register 2 (Address 01h), or as

general-purpose logic inputs/outputs by setting bits in this

which means all the inputs are set for fan speed

measurement.

Signal conditioning in the ADM1026 accommodates the

slow rise and fall times typical of fan tachometer outputs.

The fan tach inputs have internal 10 k

W pullup resistors to

3.3 V STBY. In the event that these inputs are supplied from

fan outputs that exceed the supply, either resistive

attenuation of the fan signal or diode clamping must be

included to keep inputs within an acceptable range.

Figure 43 through Figure 47 show circuits for common fan

tach outputs.

If the fan tach output is open-drain or has a resistive pullup

to VCC, then it can be connected directly to the fan input, as

shown in Figure 44.

Figure 43. Fan with Tach Pullup to +VCC

12 V

FAN SPEED

COUNTER

FAN(0–7)

PULLUP

4.7k

TYP

VCC

If the fan output has a resistive pullup to +12 V (or other

voltage greater than 3.3 V STBY), the fan output can be

clamped with a Zener diode, as shown in Figure 46. The

Zener voltage should be chosen so that it is greater than VIH

but less than 3.3 V STBY, allowing for the voltage tolerance

of the Zener.

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