ADT7468 datasheet(33/80 Pages) AD

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ADT7468

Rev. 0 | Page 33 of 80

2-Wire Fan Speed Measurements (Low Frequency Mode

Only)

The ADT7468 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 (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, a user can select the sensing threshold for the

AIN signal.

Configuration Register 4 (Reg. 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 ADT7468 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 TACH pulses have been

detected, 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 different times to

overcome inertia. The ADT7468 runs the fans just fast enough

to overcome inertia and is quieter on spin-up than fans

programmed to spin up for a given spin-up time.

Fan Startup Timeout

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

(because it is below running speed), the ADT7468 includes a

fan startup timeout function. During this time, the ADT7468

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

detected, then an interrupt is generated.

Using Configuration Register 4 (0x40) Bit 5 (FSPDIS), this

functionality can be changed (see the Disabling Fan Startup

Timeout section).

PWM1 Configuration (Reg. 0x5C)

<2:0> SPIN

, startup timeout for PWM1.

000 = No startup timeout

001 = 100 ms

010 = 250 ms default

011 = 400 ms

100 = 667 ms

101 = 1 s

110 = 2 s

111 = 4 s

PWM2 Configuration (Reg. 0x5D)

<2:0> SPIN

, startup timeout for PWM2.

000 = No startup timeout

001 = 100 ms

010 = 250 ms default

011 = 400 ms

100 = 667 ms

101 = 1 s

110 = 2 s

111 = 4 s

PWM3 Configuration (Reg. 0x5E)

<2:0> SPIN

, start-up timeout for PWM3.

000 = No startup timeout

001 = 100 ms

010 = 250 ms default

011 = 400 ms

100 = 667 ms

101 = 1 s

110 = 2 s

111 = 4 s

Disabling Fan Startup Timeout

Although fan startup makes fan spin-ups much quieter than

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

Setting Bit 5 (FSPDIS) to 1 in Configuration Register 1

(Reg. 0x40) disables the spin-up for two TACH pulses. Instead,

the fan spins up for the fixed time as selected in Reg. 0x5C to

Reg. 0x5E.

ADT7468 Datasheet(PDF) 33 Page - Analog Devices

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