ADT7467ARQZ-R7 datasheet(31/77 Pages) ONSEMI

31 / 77 page

ADT7467

Rev. 3 | Page 31 of 77 | www.onsemi.com

Fan Speed Measurement Rate

The fan TACH readings are normally updated once every

second.

When set, the FAST bit (Bit 3) of Configuration Register 3

(0x78) updates the fan TACH readings every 250 ms.

If a fan is powered directly from 5 V or 12 V and is not driven by

a PWM channel, its associated dc bit in Configuration Register 3

should be set. This allows TACH readings to be taken on a

continuous basis for fans connected directly to a dc source. For

optimal results, the associated dc bit should always be set when

using 4-wire fans.

Calculating Fan Speed

Assuming a fan with two pulses per revolution (and two pulses

per revolution being measured), fan speed is calculated by

Fan Speed (RPM) = (90,000 × 60)/Fan TACH Reading

where Fan TACH Reading is the 16-bit fan tachometer reading.

Example

TACH1 high byte (Register 0x29) = 0x17

TACH1 low byte (Register 0x28) = 0xFF

What is Fan 1 speed in RPM?

Fan 1 TACH Reading = 0x17FF = 6143 (decimal)

RPM = (f × 60)/Fan 1 TACH Reading

RPM = (90,000 × 60)/6143

Fan Speed = 879 RPM

Fan Pulses per Revolution

Different fan models can output either one, two, three, or four

TACH pulses per revolution. Once the number of fan TACH

pulses has been determined, it can be programmed into the fan

pulses per revolution register (0x7B) for each fan. Alternatively,

this register can be used to determine the number of pulses per

revolution output for a given fan. By plotting fan speed measure-

ments at 100% speed with different pulses per revolution setting,

the smoothest graph with the lowest ripple determines the

correct pulses per revolution value.

Fan Pulses per Revolution Register

<1:0> Fan 1 default = 2 pulses per revolution

<3:2> Fan 2 default = 2 pulses per revolution

<5:4> Fan 3 default = 2 pulses per revolution

<7:6> Fan 4 default = 2 pulses per revolution

00 = 1 pulse per revolution

01 = 2 pulses per revolution

10 = 3 pulses per revolution

11 = 4 pulses per revolution

2-Wire Fan Speed Measurements

(Low Frequency Mode Only)

The ADT7467 is capable of measuring the speed of 2-wire fans,

that is, fans without TACH outputs. To do this, the fan must be

interfaced as shown in the Driving 2-Wire Fans section. In this

case, the TACH inputs should be reprogrammed as analog

inputs, AIN.

Configuration Register 2 (0x73)

Bit 3 (AIN4) = 1, Pin 9 is reconfigured to measure the speed of

a 2-wire fan using an external sensing resistor and coupling

capacitor.

Bit 2 (AIN3) = 1, Pin 4 is reconfigured to measure the speed of

a 2-wire fan using an external sensing resistor and coupling

capacitor.

Bit 1 (AIN2) = 1, Pin 7 is reconfigured to measure the speed of

a 2-wire fan using an external sensing resistor and coupling

capacitor.

Bit 0 (AIN1) = 1, Pin 6 is reconfigured to measure the speed of

a 2-wire fan using an external sensing resistor and coupling

capacitor.

AIN Switching Threshold

Having configured the TACH inputs as AIN inputs for 2-wire

measurements, a user can select the sensing threshold for the

AIN signal.

Configuration Register 4 (0x7D)

<3:2> AINL, input threshold for 2-wire fan speed

measurements

00 = ±20 mV

01 = ±40 mV

10 = ±80 mV

11 = ±130 mV

FAN SPIN-UP

The ADT7467 has a unique fan spin-up function. It spins the

fan at 100% PWM duty cycle until two TACH pulses are detected

on the TACH input. Then, the PWM duty cycle goes to the

expected running value, for example, 33%. The advantage is

that fans have different spin-up characteristics and take differ-

ent times to overcome inertia. The ADT7467 runs the fans just

fast enough to overcome inertia and is quieter during spin-up

than other fans programmed to spin up for a given spin-up time.

Fan Start-Up Timeout

To prevent the generation of false interrupts as a fan spins up

(because it is below running speed), the ADT7467 includes a

fan start-up timeout function. During this time, the ADT7467

looks for two TACH pulses. If two TACH pulses are not detected,

an interrupt is generated. Using Configuration Register 1 (0x40)

ADT7467ARQZ-R7 Datasheet(PDF) 31 Page - ON Semiconductor

ADT7467ARQZ-R7 Datasheet(HTML) 31 Page - ON Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77

Datasheet Download

Link URL