ADM1029ARQZ-R7 datasheet(15/50 Pages) ONSEMI

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ADM1029

For the most accurate fan failure indication, the oscillator

frequency should be chosen to give as large a limit value as

possible without the counter overranging. A count close to 3/4

full-scale or 191 is the optimum value.

For example, if a fan produces two tach pulses per revolution

and the fan failure speed is to be 600 rpm, the oscillator fre-

quency should be set to 940 Hz. This will give a count at the fail

speed of:

940

60/600/2 = 188

If the oscillator frequency were only 470 Hz, the count would

be 94, while an oscillator frequency of 1880 Hz cannot be used

because the count would be 376 and the counter would overrange.

FAN MONITORING CYCLE TIME

Five complete tach periods are required to carry out a fan speed

measurement Therefore, if the start of a fan measurement just

misses a rising edge, the measurement can take almost six tach

periods for each fan.

The worst-case monitoring cycle time is when both fans are

under speed and the fan speed counter counts up to its maxi-

mum value. The actual count takes 256 oscillator pulses over

four tach periods, plus a further two tach periods or 128 oscilla-

tor pulses before the count starts. The total monitoring cycle

time is therefore:

tMEAS = 384/fOSC(FAN 1) + 384/fOSC(FAN 2)

In order to read a valid result from the Fan Tach Value Registers,

the total monitoring time allowed after starting the monitoring

cycle should be greater than this.

TACH SIGNAL CONDITIONING

Signal conditioning in the ADM1029 accommodates the slow

rise and fall times typical of fan tachometer outputs. The maxi-

mum input signal range is 0 V to 5 V, even if VCC is less than

5 V. In the event that these inputs are supplied from fan outputs

that exceed 0 V to 5 V, either resistive attenuation of the fan

signal or diode clamping must be included to keep inputs within

an acceptable range.

Figures 9a to 9d show circuits for most common fan tach outputs.

If the fan tach output has a resistive pull-up to VCC, it can be

connected directly to the fan input, as shown in Figure 9a.

12V

FAN SPEED

COUNTER

TACH1

OR TACH2

PULL-UP

4.7k

TYP

TACH

OUTPUT

VCC

Figure 9a. Fan with Tach Pull-Up to +VCC

If the fan output has a resistive pull-up to 12 V (or other voltage

greater than 6.5 V), the fan output can be clamped with a Zener

diode, as shown in Figure 9b. The Zener voltage should be

chosen so that it is greater than VIH but less than 6.5 V, allowing

for the voltage tolerance of the Zener. A value of between 3 V

and 5 V is suitable.

12V

FAN SPEED

COUNTER

TACH1

OR TACH2

TACH

OUTPUT

ZD1*

ZENER

PULL-UP

4.7k

TYP

*CHOOSE ZD1 VOLTAGE APPROX. 0.8

VCC

Figure 9b. Fan with Tach. Pull-Up to Voltage >6.5 V (e.g.,

12 V) Clamped with Zener Diode

If the fan has a strong pull-up (less than 1 kΩ) to 12 V, or a

totem-pole output, a series resistor can be added to limit the

Zener current, as shown in Figure 9c. Alternatively, a resistive

attenuator may be used, as shown in Figure 9d.

R1 and R2 should be chosen such that:

2 V < VPULLUP × R2/(RPULLUP + R1 + R2) < 5 V

The fan inputs have an input resistance of nominally 160 kΩ to

ground, so this should be taken into account when calculating

resistor values.

With a pull-up voltage of 12 V and pull-up resistor less than

1 kΩ, suitable values for R1 and R2 would be 100 kΩ and 47 kΩ.

This will give a high input voltage of 3.83 V.

12V

FAN SPEED

COUNTER

TACH1

OR TACH2

PULL-UP

TYP <1k

OR TOTEM-POLE

ZD1

ZENER*

10k

TACH

O/P

*CHOOSE ZD1 VOLTAGE APPROX. 0.8

VCC

Figure 9c. Fan with Strong Tach. Pull-Up to >VCC or

Totem-Pole Output, Clamped with Zener and Resistor

12V

FAN SPEED

COUNTER

TACH1

OR TACH2

TACH

OUTPUT

R1*

R2*

<1k

VCC

*SEE TEXT

Figure 9d. Fan with Strong Tach. Pull-Up to >VCC or

Totem-Pole Output, Attenuated with R1/R2

FAN SPEED CONTROL

Fan speed is controlled using pulsewidth modulation (PWM).

The PWM outputs (Pins 1 and 24) give a pulse output with a

programmable frequency (default 250 Hz) and a duty-cycle

defined by the contents of the relevant fan speed register, or by

the automatic fan speed control when this mode is enabled. The

speed at which a fan runs is determined by fault conditions and

the settings of various control and mask registers.

A fan can only be driven if it is defined as being supported by

the controller in register 02h. The ADM1029 supports up to

two fans, so Bits 0 and 1 of this register are permanently set.

This register is read-only.

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