ADM1034 datasheet(26/40 Pages) AD

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ADM1034

Rev. 0 | Page 26 of 40

FAN INPUTS

Pin 2 and Pin 4 are TACH inputs intended for fan speed

measurement. These inputs are open-drain.

Signal conditioning on the ADM1034 accommodates the slow

rise and fall time of typical tachometer outputs. The maximum

input signal range is from 0 V to 5 V, even when VCC is less than

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

exceeding 0 V to 5 V, either resistive attenuation of the fan

signal or diode clamping must be used to keep the fan inputs

within an acceptable range.

Figure 36 to Figure 39 show examples of possible fan input

circuits. If the fan TACH has a resistive pull-up to VCC, it can

be connected directly to the fan output.

5V OR 12V

FAN

DRIVE X

FAN SPEED

COUNTER

ADM1034

TACH X

PULL-UP

4.7k

Ω

TYP

TACH

OUTPUT

VCC

100k

Ω

TYP

Figure 36. Fan with TACH Pull-Up to +VCC

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

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

Zener diode, as shown in Figure 37. The Zener voltage should

be chosen so that it is greater than VIH 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

FAN

DRIVE X

FAN SPEED

COUNTER

ADM1034

TACH X

PULL-UP

4.7k

Ω

TYP

TACH

OUTPUT

VCC

100k

Ω

TYP

ZD1*

*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8

× V

Figure 37. Fan with TACH Pull-Up to Voltage > 5 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 38. Alternatively, a resistive

attenuator may be used, as shown in Figure 39.

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. 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 gives a high input voltage of 3.83 V.

12V

ADM1034

FAN SPEED

COUNTER

FAN(0–7)

PULL-UP TYP

<1 k

Ω OR

TOTEM POLE

TACH

OUTPUT

VCC

* CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8

× V

ZD1

ZENER*

10k

Ω

Figure 38. Fan with Strong TACH. Pull-Up to >VCC or Totem-Pole Output,

Clamped with Zener and Resistor.

12V

FAN SPEED

COUNTER

ADM1034

FAN(0–7)

<1 k

Ω

TACH

OUTPUT

VCC

R1*

*SEE TEXT

Figure 39. 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. This is because the fan may be spinning at 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 81.92 kHz

oscillator into the input of a 16-bit counter for one complete

revolution of the fan. Therefore, the accumulated count is

actually proportional to the fan tachometer period and inversely

proportional to the fan speed.

The number of poles in the fan must be programmed in

Configuration Register 3 (Address 0x03). Bits <3:0> set the

number of poles for Fan 1, and Bits <7:4> set the number of

poles for Fan 2. This number must be an even number only,

because there cannot be an uneven number of poles in a fan. A

TACH period is output for every two poles. Therefore, the

number of poles must be known so that the ADM1034 can

measure for a full revolution.

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