ADT7463ARQZ-R7 datasheet(29/52 Pages) ONSEMI

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REV. C

ADT7463

–29–

Fan TACH Limit Registers

The fan TACH limit registers are 16-bit values consisting of

two bytes.

Reg. 0x54 TACH1 Minimum Low Byte = 0xFF Default

Reg. 0x55 TACH1 Minimum High Byte = 0xFF Default

Reg. 0x56 TACH2 Minimum Low Byte = 0xFF Default

Reg. 0x57 TACH2 Minimum High Byte = 0xFF Default

Reg. 0x58 TACH3 Minimum Low Byte = 0xFF Default

Reg. 0x59 TACH3 Minimum High Byte = 0xFF Default

Reg. 0x5A TACH4 Minimum Low Byte = 0xFF Default

Reg. 0x5B TACH4 Minimum High Byte = 0xFF Default

Fan Speed Measurement Rate

The fan TACH readings are normally updated once every second.

The FAST bit (Bit 3) of Configuration Register 3 (Reg. 0x78),

when set, updates the fan TACH readings every 250 ms.

If any of the fans are not being driven by a PWM channel but

are powered directly from 5 V or 12 V, 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.

Calculating Fan Speed

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

revolution being measured), fan speed is calculated by

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

where

Fan TACH Reading = 16-Bit Fan Tachometer Reading

Example:

TACH1 High Byte (Reg 0x29) = 0x17

TACH1 Low Byte (Reg 0x28) = 0xFF

What is Fan 1 speed in RPM?

Fan 1 TACH Reading = 0x17FF = 6143 Decimal

RPM = (f � 60)/Fan 1 TACH Reading

RPM = (90000 � 60)/6143

Fan Speed = 879 RPM

Fan Pulses per Revolution

Different fan models can output either 1, 2, 3, or 4 Tach pulses

per revolution. Once the number of fan Tach pulses has been

determined, it is programmed into the fan pulses per revolution

be used to determine the number or pulses/revolution output by

a given fan. By plotting fan speed measurements at 100% speed

with different pulses/revolution setting, the smoothest graph with

the lowest ripple determines the correct pulses/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

The ADT7463 is capable of measuring the speed of 2-wire fans,

i.e., fans without TACH outputs. To do this, the fan must be

interfaced as shown in the Fan Drive Circuitry section of the

data sheet. In this case, the TACH inputs need to be repro-

grammed as analog inputs, AIN.

CONFIGURATION REGISTER 2 (REG. 0x73)

Bit 3 (AIN4) = 1, Pin 14 is reconfigured to measure the speed

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

capacitor.

Bit 2 (AIN3) = 1, Pin 9 is reconfigured to measure the speed of

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

capacitor.

Bit 1 (AIN2) = 1, Pin 12 is reconfigured to measure the speed

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

capacitor.

Bit 0 (AIN1) = 1, Pin 11 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, users can select the sensing threshold for the

AIN signal.

CONFIGURATION REGISTER 4 (REG. 0X7D)

<3:2> AINL These two bits define the input threshold for

2-wire fan speed measurements.

00 = �20 mV

01 = �40 mV

10 = �80 mV

11 = �130 mV

Fan Spin-Up

The ADT7463 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. Once two pulses are detected, the

PWM duty cycle goes to the expected running value, e.g.,

33%. The advantage of this is that fans have different spin-up

characteristics and takes different times to overcome inertia. The

ADT7463 just runs the fans fast enough to overcome inertia and

are quieter on spin-up than fans programmed to spin-up for a

given spin-up time.

Fan Start-Up Timeout

To prevent false interrupts being generated as a fan spins up

(since it is below running speed), the ADT7463 includes a fan

start-up timeout function. This is the time limit allowed for two

TACH pulses to be detected on spin-up. For example, if 2-second

fan start-up timeout is chosen and no TACH pulses occur within

2 seconds of the start of spin-up, a fan fault is detected and flagged

in the interrupt status registers.

Rev. 4 | Page 29 of 52 | www.onsemi.com

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