ADM1027 datasheet(25/56 Pages) AD

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REV. A

ADM1027

–25–

2-Wire Fan Speed Measurements

The ADM1027 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.

FAN SPIN-UP

The ADM1027 has a unique fan spin-up function. It will spin

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

detected on the TACH input. Once two pulses have been

detected, the PWM duty cycle will go to the expected running

value, e.g., 33%. The advantage of this is that fans have dif-

ferent spin-up characteristics and will take different times to

overcome inertia. The ADM1027 just runs the fans fast enough

to overcome inertia and will be 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 ADM1027 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

seconds 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.

PWM1 CONFIGURATION (REG. 0x5C)

<2:0> SPIN

These bits control the start-up timeout for

PWM1.

000 = No startup timeout

001 = 100 ms

010 = 250 ms (default)

011 = 400 ms

101 = 1 sec

110 = 2 sec

111 = 4 sec

PWM2 CONFIGURATION (REG. 0x5D)

<2:0> SPIN

These bits control the start-up timeout for

PWM2.

000 = No startup timeout

001 = 100 ms

010 = 250 ms (default)

011 = 400 ms

101 = 1 sec

110 = 2 sec

111 = 4 sec

PWM3 CONFIGURATION (REG. 0x5E)

<2:0> SPIN

These bits control the start-up timeout for

PWM3.

000 = No startup timeout

001 = 100 ms

010 = 250 ms (default)

011 = 400 ms

101 = 1 sec

110 = 2 sec

111 = 4 sec

Disabling Fan Start-Up Timeout

Although fan start-up makes fan spin-ups much quieter than

fixed-time spin-ups, the option is there to use fixed spin-up times.

Bit 5 (FSPDIS) = 1 in Configuration Register 1 (Reg. 0x40)

disables the spin-up for two TACH pulses. Instead, the fan will

spin up for the fixed time as selected in registers 0x5C to 0x5E.

ADM1027 Datasheet(PDF) 25 Page - Analog Devices

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