ADM1027 datasheet(23/56 Pages) AD

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REV. A

ADM1027

–23–

FAN SPEED MEASUREMENTS

TACH Inputs

Pins 11, 12, 9, and 14 are open-drain TACH inputs intended

for fan speed measurement.

Signal conditioning in the ADM1027 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 where 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 25a to 25d 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 25a.

5V OR 12V

ADM1027

FAN SPEED

COUNTER

TACH X

TACHO

OUTPUT

PULL-UP

4.7k

TYP

VCC

FAN

Figure 25a. Fan With TACH Pull-Up to +VCC

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

greater than 5 V), then the fan output can be clamped with a

Zener diode, as shown in Figure 25b. The Zener diode voltage

should be chosen so that it is greater than VIH of the TACH

input but less than 5 V, allowing for the voltage tolerance of the

Zener. A value of between 3 V and 5 V is suitable.

5V OR 12V

ADM1027

FAN SPEED

COUNTER

TACH X

TACHO

OUTPUT

PULL-UP

4.7k

TYP

VCC

FAN

ZD1*

*CHOOSE ZD1 VOLTAGE APPROX 0.8

VCC

Figure 25b. Fan with TACH Pull-Up to Voltage

> 5 V (e.g., 12 V) Clamped with Zener Diode

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

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

the Zener current, as shown in Figure 25c. Alternatively, a

resistive attenuator may be used, as shown in Figure 25d.

R1 and R2 should be chosen such that

PULLUP

<¥

()>

The fan inputs have an input resistance of nominally 160 k

W to

ground; 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 k

W, suitable values for R1 and R2 would be 100 kW and

47 k

W. This will give a high input voltage of 3.83 V.

5V OR 12V

ADM1027

FAN SPEED

COUNTER

TACH X

TACHO

OUTPUT

PULL-UP TYP

<1k

TOTEM POLE

VCC

FAN

ZD1*

*CHOOSE ZD1 VOLTAGE APPROX 0.8

VCC

10k

Figure 25c. Fan with Strong TACH Pull-Up to > VCC or

Totem-Pole Output, Clamped with Zener and Resistor

5V OR 12V

ADM1027

FAN SPEED

COUNTER

TACH X

TACHO

OUTPUT

PULL-UP TYP

<1k

VCC

FAN

R2*

*SEE TEXT

R1*

Figure 25d. Fan with Strong TACH Pull-Up to > VCC

or Totem-Pole Output, Attenuated with R1/R2

Fan Speed Measurement

The fan counter does not count the fan TACH output pulses

directly because the fan speed may be less than 1000 RPM and

it would take several seconds to accumulate a reasonably large

and accurate count. Instead, the period of the fan revolution is

measured by gating an on-chip 90 kHz oscillator into the input

of a 16-bit counter for N periods of the fan TACHO output

(Figure 26), so the accumulated count is actually proportional

to the fan tachometer period and inversely proportional to the

fan speed.

CLOCK

PWM

TACH

Figure 26. Fan Speed Measurement

N, the number of pulses counted, is determined by the settings

of Register 0x7B (fan pulses per revolution register). This register

contains two bits for each fan, allowing 1, 2 (default), 3, or 4

TACH pulses to be counted.

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