ADT7476ARQZ-R7 datasheet(28/67 Pages) ONSEMI

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ADT7476

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

voltage greater than 5.5 V, the fan output can be clamped

with a Zener diode, as shown in Figure 42. The Zener diode

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

TACH input but less than 5.5 V, allowing for the voltage

tolerance of the Zener. A value between 5.0 V and 5.5 V is

suitable.

Figure 42. Fan with Strong TACH Pullup to > 5.5 V,

(for Example, 12 V) Clamped with Zener Diode

12V

VCC

PULLUP

4.7kΩ

TYPICAL

TACH

OUTPUT

FAN SPEED

COUNTER

TACH

ADT7476A

ZD1*

*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8

× VCC

If the fan has a strong pullup (less than 1 k

W) to 12 V or

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

the Zener current, as shown in Figure 43.

Figure 43. Fan with Strong TACH Pullup to >VCC

or Totem−Pole Output, Clamped with

Zener Diode and Resistor

5V OR 12V

VCC

PULLUP TYP

<1kΩ OR

TOTEM POLE

TACH

OUTPUT

FAN SPEED

COUNTER

TACH

ADT7476A

ZD1

ZENER*

FAN

*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8

× VCC

10kΩ

Alternatively, a resistive attenuator can be used, as shown

in Figure 44. R1 and R2 should be chosen such that:

(eq. 4)

2V t VPULLUP R2 RPULLUP ) R1 ) R2 t 5.5 V

The fan inputs have an input resistance of nominally

160 k

W to ground, which should be taken into account when

calculating resistor values.

With a pullup voltage of 12 V and pullup resistor less than

1 k

W, suitable values for R1 and R2 are 100 kW and 40 kW,

respectively. This gives a high input voltage of 3.42 V.

Figure 44. Fan with Strong TACH Pullup to >VCC or

Totem−Pole Output, Attenuated with R1/R2

12V

VCC

<1kΩ

TACH

OUTPUT

FAN SPEED

COUNTER

TACH

ADT7476

R2*

*SEE TEXT

R1*

The fan counter does not count the fan TACH output

pulses directly because the fan speed could be less than

1000 RPM, and it takes 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 TACH output (Figure 45), so the accumulated count

is actually proportional to the fan tachometer period and

inversely proportional to the fan speed.

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

settings of TACH pulses per revolution register (0x7B). This

(default), three, or four TACH pulses to be counted.

Figure 45. Fan Speed Measurement

CLOCK

PWM

TACH

Fan TachometerReading Registers

The fan tachometer readings are 16−bit values consisting

of a 2−byte read from the ADT7476.

Reading Fan Speed from the ADT7476

The measurement of fan speeds involves a 2−register read

for each measurement. The low byte should be read first.

This causes the high byte to be frozen until both high and

low byte registers have been read, preventing erroneous

TACH readings. The fan tachometer reading registers report

back the number of 11.11

ms period clocks (90 kHz

oscillator) gated to the fan speed counter from the rising

edge of the first fan TACH pulse to the rising edge of the

third fan TACH pulse (assuming two pulses per revolution

are being counted).

Because the device is essentially measuring the fan TACH

period, the higher the count value, the slower the fan is

actually running. A 16−bit fan tachometer reading of

0xFFFF indicates that either the fan has stalled or is running

very slowly (<100 RPM).

High Limit: > Comparison Performed

ADT7476ARQZ-R7 Datasheet(PDF) 28 Page - ON Semiconductor

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