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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.
Rev. 3 | Page 25 of 56 | www.onsemi.com
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