ADT7468 Analog Devices, Inc., ADT7468 Datasheet - Page 33

• Manufacturer Part Number: ADT7468
• Description: Dbcool Remote Thermal Controller And Voltage Monitor
• Manufacturer: Analog Devices, Inc.

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.
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 during spin-up than 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 ADT7468 includes a
fan start-up timeout function. During this time, the ADT7468
looks for two TACH pulses. If two TACH pulses are not
detected, then an interrupt is generated.
00 = ±20 mV
01 = ±40 mV
10 = ±80 mV
11 = ±130 mV
Rev. A | Page 33 of 84
Using Configuration Register 4 (0x40) Bit 5 (FSPDIS), this
functionality can be changed (see the Disabling Fan Start-Up
Timeout section).
PWM1 Configuration (Reg. 0x5C)
<2:0> spin, startup timeout for PWM1.
PWM2 Configuration (Reg. 0x5D)
<2:0> spin, startup timeout for PWM2.
PWM3 Configuration (Reg. 0x5E)
<2:0> spin, start-up timeout for PWM3.
Disabling Fan Start-Up Timeout
Although fan start-up times make fan spin-ups much quieter
than fixed-time spin-up times, 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.
000 = No startup timeout
001 = 100 ms
010 = 250 ms default
011 = 400 ms
100 = 667 ms
101 = 1 sec
110 = 2 sec
111 = 4 sec
000 = No startup timeout
001 = 100 ms
010 = 250 ms default
011 = 400 ms
100 = 667 ms
101 = 1 sec
110 = 2 sec
111 = 4 sec
000 = No startup timeout
001 = 100 ms
010 = 250 ms default
011 = 400 ms
100 = 667 ms
101 = 1 sec
110 = 2 sec
111 = 4 sec
ADT7468