ADM1031ARQZ-R7 datasheet(21/30 Pages) ONSEMI

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ADM1031

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Figure 34 shows how a 3−wire fan can be driven using

PWM control.

Figure 34. Interfacing the ADM1031 to a 3−Wire Fan

5.0 V OR 12 V

FAN

NDT3055L

ADM1031

PWM_OUT

TACH/AIN

TACH

10kW

TYPICAL

10kΩ

TYPICAL

3.3 V

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 can be substituted based on the

system’s fan drive requirements.

Figure 35 shows how a 2−wire fan can 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 commutation pulses into a voltage. This is accoupled

into the ADM1031 through the 0.01

mF capacitor. On−chip

signal conditioning allows accurate monitoring of fan speed.

For typical notebook fans drawing approximately 170 mA,

a 2

W RSENSE value is suitable. For fans such as desktop or

server fans that draw more current, RSENSE can be reduced.

The smaller RSENSE is, the better, since more voltage is

developed across the fan, and the fan then spins faster.

Figure 35. Interfacing the ADM1031 to a 2−Wire Fan

5.0 V OR 12 V

FAN

ADM1031

TACH/AIN

PWM_OUT

TACH

10kW

TYPICAL

0.01μF

3.3 V

NDT3055L

RSENSE

(2W TYPICAL)

Figure 36 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 is the case for most fans when RSENSE = 2 W. The value

of RSENSE can be reduced as long as the voltage spikes at the

TACH/AIN pin are greater than 250 mV. This allows fan

speed to be reliably determined.

Figure 36. Fan Speed Sensing Waveform at

TACH/AIN Pin

Fan Speed Measurement

The fan counter does not count the fan tach output pulses

directly, because the fan speed can be less than 1000 RPM

and it would take several seconds to accumulate a

reasonably large and accurate count. Instead, the period of

the fan revolution is measured by gating an on−chip

11.25 kHz oscillator into the input of an 8−bit counter. The

fan speed measuring circuit is initialized on the rising edge

of a PWM high output if fan speed measurement is enabled

(Bit 2 and Bit 3 of Configuration Register 2 = 1). It then starts

counting on the rising edge of the second tach pulse and

counts for two fan tach periods, until the rising edge of the

fourth tach pulse, or until the counter overranges if the fan

tach period is too long. The measurement cycle repeats until

monitoring is disabled. The fan speed measurement is stored

in the fan speed reading register at address 0

×08, 0×09. The

fan speed count is given by:

Count = (f

× 60)/R × N

where:

f = 11.25 kHz

R = fan speed in RPM.

N = speed range (either 1, 2, 4, or 8)

The frequency of the oscillator can be adjusted to suit the

expected running speed of the fan by varying N, the speed

range. The oscillator frequency is set by Bit 7 and Bit 6 of

Fan Characteristics Register 1 (0

×20) and Fan

Characteristics Register 2 (0

×21) as shown in Table 11.

Figure 37 shows how the fan measurements relate to the

PWM_OUT pulse trains.

Table 11. Oscillator Frequencies

Bit 7

Bit 6

Oscillator Frequency (kHz)

11.25

5.625

2.812

1.406

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