ADT7468 datasheet(30/80 Pages) AD

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ADT7468

Rev. 0 | Page 30 of 80

Driving 2-Wire Fans

The ADT7468 can support 2-wire fans only when low fre-

quency PWM mode is selected in Configuration Register 5,

Bit 2. If this bit is not set to 1, the ADT7468 cannot measure the

speed of 2-wire fans.

Figure 39 shows how a 2-wire fan can be connected to the

ADT7468. 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, which is ac-coupled into the

ADT7468 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.

ADT7468

PWM

TACH

5V OR

12V FAN

NDT3055L

3.3V

10k

Ω

TYPICAL

1N4148

0.01

µF

RSENSE

Ω

TYPICAL

Figure 39. Driving a 2-Wire Fan

Figure 40 shows a typical plot of the sensing waveform at the

TACH/AIN pin.

Figure 40. Fan Speed Sensing Waveform at TACH/AIN Pin

For fans that draw more current, such as larger desktop or

server fans, RSENSE can be reduced for the same programmed

threshold. The smaller the threshold programmed the better,

because more voltage is developed across the fan and the fan

spins faster. 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.

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 populated,

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 = 0W

C1 = UNPOPULATED

FOR 2-WIRE FANS:

POPULATE R4, C1

R1, R2, R3 UNPOPULATED

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

TACH Inputs

Pins 9, 11, 12, and 14 (when configured as TACH inputs) are

open-drain TACH inputs intended for fan speed measurement.

Signal conditioning in the ADT7468 accommodates the slow

rise and fall times typical of fan tachometer outputs. The

maximum 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

4.7k

Ω

TYPICAL

TACH

OUTPUT

FAN SPEED

COUNTER

TACH

ADT7468

Figure 42. Fan with TACH Pull-Up to VCC

ADT7468 Datasheet(PDF) 30 Page - Analog Devices

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