ADM1031 datasheet(20/32 Pages) AD

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ADM1031

–20–

1. Run the fan at 53% PWM duty cycle in Software Mode. Clear

Bits 5 and 7 of Configuration Register 1 (Reg 0x00) to enter

PWM duty cycle mode. Write 0x08 to the Fan Speed Config

2. Measure the fan RPM. This represents the fan RPM below

which the RPM mode will fail to operate. Do NOT program a

lower RPM than this value when using RPM Feedback mode.

3. Ensure that Speed Range, N, = 2 when using RPM Feed-

back mode.

Fans come in a variety of different options. One distinguishing

feature of fans is the number of poles that a fan has internally.

The most common fans available have four, six, or eight poles.

The number of poles the fan has generally affects the number of

pulses per revolution the fan outputs.

If the ADM1031 is used to drive fans other than 4-pole fans that

output 2 tach pulses/revolution, then the fan speed measurement

equation needs to be adjusted to calculate and display the cor-

rect fan speed, and also to program the correct count value in

RPM Feedback Mode.

FAN SPEED MEASUREMENT EQUATIONS

For a 4-pole fan (2 tach pulses/rev):

Fan RPM = (f

× 60)/Count × N

For a 6-pole fan (3 tach pulses/rev):

Fan RPM = (f

× 60)/(Count × N × 1.5)

For an 8-pole fan (4 tach pulses/rev):

Fan RPM = (f

× 60)/(Count × N × 2)

If in doubt as to the number of poles the fans used have, or the

number of tach output pulses/rev, consult the fan manufacturer’s

data sheet, or contact the fan vendor for more information.

FAN DRIVE USING PWM CONTROL

The external circuitry required to drive a fan using PWM con-

trol is extremely simple. A single NMOS FET is the only drive

transistor required. The specifications of the MOSFET depend

on the maximum current required by the fan being driven.

Typical notebook fans draw a nominal 170 mA, and so SOT

devices can be used where board space is a constraint. If driving

several fans in parallel from a single PWM output, or driving

larger server fans, the MOSFET will need to handle the higher

current requirements. The only other stipulation is that the

MOSFET should have a gate voltage drive, VGS < 3.3 V, for

direct interfacing to the PWM_OUT pin. The MOSFET should

also have a low on-resistance to ensure that there is not signifi-

cant voltage drop across the FET. This would reduce the

maximum operating speed of the fan.

Figure 18 shows how a 3-wire fan may be driven using

PWM control.

NDT3055L

PWM_OUT

5V OR 12V

FAN

TACH/AIN

ADM1031

3.3V

10k

TYPICAL

TACH

10k

TYPICAL

3.3V

Figure 18. Interfacing the ADM1031 to a 3-Wire Fan

The NDT3055L n-type MOSFET was chosen since it has 3.3 V

gate drive, low on-resistance, and can handle 3.5 A of current.

Other MOSFETs may be substituted based on the system’s fan

drive requirements.

NDT3055L

PWM_OUT

5V OR 12V

FAN

TACH/AIN

ADM1031

3.3V

10k

TYPICAL

TACH

RSENSE

TYPICAL)

0.01 F

Figure 19. Interfacing the ADM1031 to a 2-Wire Fan

Figure 19 shows how a 2-wire fan may be connected to the

ADM1031. This circuit allows the speed of the 2-wire fan to be

measured even though the fan has no dedicated Tach signal. A

series RSENSE resistor in the fan circuit converts the fan commu-

tation pulses into a voltage. This is ac-coupled into the ADM1031

through the 0.01

µF capacitor. On-chip signal conditioning

allows accurate monitoring of fan speed. For typical notebook

fans drawing approximately 170 mA, a 2

Ω RSENSE value is

suitable. For fans such as desktop or server fans that draw

more current, RSENSE may be reduced. The smaller RSENSE is,

the better, since more voltage will be developed across the fan,

and the fan will spin faster. Figure 20 shows a typical plot of

the sensing waveform at the TACH/AIN pin. The most important

thing is that the negative-going spikes are more than 250 mV

in amplitude. This will be the case for most fans when RSENSE

= 2

Ω. The value of RSENSE can be reduced as long as the volt-

age spikes at the TACH/AIN pin are greater than 250 mV. This

allows fan speed to be reliably determined.

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