EVAL-ADM1026EB datasheet(25/56 Pages) AD

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ADM1026

Rev. A | Page 25 of 56

Signal conditioning in the ADM1026 accommodates the slow

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

tach inputs have internal 10 kΩ pull-up 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 44 through Figure 47 show circuits for

common fan tach outputs.

If the fan tach output is open-drain or has a resistive pull-up to

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

in Figure 44.

12V

FAN SPEED

COUNTER

FAN(0–7)

PULL-UP

4.7k

Ω

TYP

TACH

OUTPUT

VCC

Figure 44. Fan with Tach Pull-Up to +VCC

If the fan output has a resistive pull-up 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 45. 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.

12V

FAN SPEED

COUNTER

FAN(0–7)

PULL-UP

4.7k

Ω

TYP

TACH

OUTPUT

VCC

* CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8

× V

ZD1*

ZENER

Figure 45. Fan with Tach Pull-Up to Voltage > VCC (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 46. Alternatively, a resistive

attenuator may be used, as shown in Figure 47.

R1 and R2 should be chosen such that

()

STBY

PULLUP

12V

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 46. Fan with Strong Tach Pull-Up to > VCC or Totem Pole Output,

Clamped with Zener and Resistor

12V

FAN SPEED

COUNTER

FAN(0–7)

<1 k

Ω

TACH

OUTPUT

VCC

R1*

* SEE TEXT

Figure 47. 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 22.5 kHz oscillator into the

input of an 8-bit counter for two periods of the fan tach output,

as shown in Figure 48, so the accumulated count is actually

proportional to the fan tach period and inversely proportional

to the fan speed.

22.5kHz

CLOCK

CONFIGURATION

REG. 1 BIT 0

FAN0

INPUT

FAN0

MEASUREMENT

PERIOD

FAN1

MEASUREMENT

PERIOD

START OF

MONITORING

CYCLE

FAN1

INPUT

Figure 48. Fan Speed Measurement

The monitoring cycle begins when a 1 is written to the monitor

bit (Bit 0 of Configuration Register 1). The INT_Enable (Bit 1)

should be set to 1 to enable the INT output.

EVAL-ADM1026EB Datasheet(PDF) 25 Page - Analog Devices

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