ADT7463ARQZ-R7 datasheet(27/52 Pages) ONSEMI

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REV. C

ADT7463

–27–

Driving 2-Wire Fans

Figure 33 shows how a 2-wire fan may be connected to the

ADT7463. This circuit allows the speed of a 2-wire fan to be

measured, even though the fan has no dedicated TACH signal.

A series resistor, RSENSE, in the fan circuit converts the fan

commutation pulses into a voltage. This is ac-coupled into the

ADT7463 through the 0.01

µF capacitor. On-chip signal

conditioning allows accurate monitoring of fan speed. The

value of RSENSE chosen depends upon the programmed input

threshold and the current drawn by the fan. For fans drawing

approximately 200 mA, a 2

Ω RSENSE value is suitable when the

threshold is programmed as 40 mV. For fans that draw more

current, such as larger desktop or server fans, RSENSE may be

reduced for the same programmed threshold. The smaller the

threshold programmed the better, since more voltage is developed

across the fan and the fan spins faster. Figure 34 shows a typical

plot of the sensing waveform at the TACH/AIN pin. The most

important thing is that the voltage spikes (either negative going

or positive going) are more than 40 mV in amplitude. This

allows fan speed to be reliably determined.

ADT7463

TACH/AIN

PWM

10k�

TYPICAL

3.3V

NDT3055L

5V OR

12V FAN

RSENSE

2�

TYPICAL

0.01�F

1N4148

Figure 33. Driving a 2-Wire Fan

Figure 34. Fan Speed Sensing Waveform at TACH/AIN Pin

LAYING OUT 2-WIRE AND 3-WIRE FANS

Figure 35 shows how to lay out a common circuit arrangement

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

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

3.3V OR 5V

TACH/AIN

PWM

12V OR 5V

MMBT2222

FOR 3-WIRE FANS:

POPULATE R1, R2, R3

R4 = 0�

C1 = UNPOPULATED

FOR 2-WIRE FANS:

POPULATE R4, C1

R1, R2, R3 UNPOPULATED

1N4148

Figure 35. Planning for 2-Wire or 3-Wire Fans on a PCB

TACH Inputs

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

for fan speed measurement.

Signal conditioning in the ADT7463 accommodates the slow rise

and fall times typical of fan tachometer outputs. The maximum

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 36a to 36d 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 36a.

12V

ADT7463

FAN SPEED

COUNTER

TACH

OUTPUT

PULL-UP

4.7k�

TYP

VCC

Figure 36a. 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 36b. 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.

Rev. 4 | Page 27 of 52 | www.onsemi.com

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