EVAL-ADT7467EBZ ON Semiconductor, EVAL-ADT7467EBZ Datasheet - Page 31

• Manufacturer Part Number: EVAL-ADT7467EBZ
• Description: BOARD EVALUATION FOR ADT7467
• Manufacturer: ON Semiconductor
• Series: dBCool®r

Specifications of EVAL-ADT7467EBZ

• Sensor Type: Temperature
• Sensing Range: -40°C ~ 120°C
• Interface: SMBus (2-Wire/I²C)
• Sensitivity: ±1.5°C
• Voltage - Supply: 3 V ~ 5.5 V
• Embedded: No
• Utilized Ic / Part: ADT7467
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Fan Speed Measurement Rate
The fan TACH readings are normally updated once every
second.
When set, the FAST bit (Bit 3) of Configuration Register 3
(0x78) updates the fan TACH readings every 250 ms.
If a fan is powered directly from 5 V or 12 V and is not driven by
a PWM channel, 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. For
optimal results, the associated dc bit should always be set when
using 4-wire fans.
Calculating Fan Speed
Assuming a fan with two pulses per revolution (and two pulses
per revolution being measured), fan speed is calculated by
where Fan TACH Reading is the 16-bit fan tachometer reading.
Example
What is Fan 1 speed in RPM?
Fan Pulses per Revolution
Different fan models can output either one, two, three, or four
TACH pulses per revolution. Once the number of fan TACH
pulses has been determined, it can be programmed into the fan
pulses per revolution register (0x7B) for each fan. Alternatively,
this register can be used to determine the number of pulses per
revolution output for a given fan. By plotting fan speed measure-
ments at 100% speed with different pulses per revolution setting,
the smoothest graph with the lowest ripple determines the
correct pulses per 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
Fan Speed (RPM) = (90,000 × 60)/Fan TACH Reading
TACH1 high byte (Register 0x29) = 0x17
TACH1 low byte (Register 0x28) = 0xFF
Fan 1 TACH Reading = 0x17FF = 6143 (decimal)
RPM = (f × 60)/Fan 1 TACH Reading
RPM = (90,000 × 60)/6143
Fan Speed = 879 RPM
00 = 1 pulse per revolution
01 = 2 pulses per revolution
10 = 3 pulses per revolution
11 = 4 pulses per revolution
Rev. 3 | Page 31 of 77 | www.onsemi.com
2-Wire Fan Speed Measurements
(Low Frequency Mode Only)
The ADT7467 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 (0x73)
Bit 3 (AIN4) = 1, Pin 9 is reconfigured to measure the speed of
a 2-wire fan using an external sensing resistor and coupling
capacitor.
Bit 2 (AIN3) = 1, Pin 4 is reconfigured to measure the speed of
a 2-wire fan using an external sensing resistor and coupling
capacitor.
Bit 1 (AIN2) = 1, Pin 7 is reconfigured to measure the speed of
a 2-wire fan using an external sensing resistor and coupling
capacitor.
Bit 0 (AIN1) = 1, Pin 6 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 (0x7D)
<3:2> AINL, input threshold for 2-wire fan speed
measurements
FAN SPIN-UP
The ADT7467 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. Then, 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 differ-
ent times to overcome inertia. The ADT7467 runs the fans just
fast enough to overcome inertia and is quieter during spin-up
than other 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 ADT7467 includes a
fan start-up timeout function. During this time, the ADT7467
looks for two TACH pulses. If two TACH pulses are not detected,
an interrupt is generated. Using Configuration Register 1 (0x40)
00 = ±20 mV
01 = ±40 mV
10 = ±80 mV
11 = ±130 mV
ADT7467