ADT7467ARQZ-R7 datasheet(29/77 Pages) ONSEMI

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ADT7467

Rev. 3 | Page 29 of 77 | www.onsemi.com

Note that when the voltage spikes (either negative going or

positive going) are more than 40 mV in amplitude, the fan

speed can be reliably determined.

ADT7467

PWMx

TACHx

5V OR

12V FAN

NDT3055L

3.3V

10kΩ

TYPICAL

1N4148

0.01μF

RSENSE

2Ω

TYPICAL

Figure 39. Driving a 2-Wire Fan

Figure 40. Fan Speed Sensing Waveform at TACHx Pin

LAYING OUT 2-WIRE AND 3-WIRE FANS

Figure 41 shows how to lay out a common circuit arrangement

for 2-wire and 3-wire fans. Some components are not popu-

lated, depending on whether a 2-wire or 3-wire fan is used.

MMBT2222

PWM

1N4148

3.3V OR 5V

12V OR 5V

TACH

FOR 3-WIRE FANS:

POPULATE R1, R2, R3

R4 = 0Ω

C1 = UNPOPULATED

FOR 2-WIRE FANS:

POPULATE R4, C1

R1, R2, R3 UNPOPULATED

R4 = 2Ω

Figure 41. Planning for 2-Wire or 3-Wire Fans on a PCB

TACH Inputs

When configured as TACH inputs, Pin 4, Pin 6, Pin 7, and Pin 9

are open-drain TACH inputs intended for fan speed measurement.

Signal conditioning in the ADT7467 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 when 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.

Figure 42 to Figure 45 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 42.

12V

VCC

PULL-UP

4.7kΩ

TYPICAL

TACH

OUTPUT

FAN SPEED

COUNTER

TACHx

ADT7467

Figure 42. Fan with TACH Pull-Up to VCC

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

voltage that is greater than 5 V), the fan output can be clamped

with a Zener diode, as shown in Figure 43. The Zener diode

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

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

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

12V

VCC

PULL-UP

4.7kΩ

TYPICAL

TACH

OUTPUT

FAN SPEED

COUNTER

TACHx

ADT7467

ZD1*

*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × VCC.

Figure 43. Fan with TACH Pull-Up to a Voltage >5 V (For Example, 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 44.

5V OR 12V

VCC

PULL-UP TYP

<1kΩ OR

TOTEM POLE

TACH

OUTPUT

FAN SPEED

COUNTER

TACHx

ADT7467

ZD1

ZENER*

FAN

*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × VCC.

10kΩ

Figure 44. Fan with Strong TACH Pull-Up to >VCC or Totem-Pole Output,

ADT7467ARQZ-R7 Datasheet(PDF) 29 Page - ON Semiconductor

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