ADT7463ARQZ ON Semiconductor, ADT7463ARQZ Datasheet - Page 29

• Manufacturer Part Number: ADT7463ARQZ
• Description: IC REMOTE THERMAL CTRLR 24-QSOP
• Manufacturer: ON Semiconductor
• Series: dBCool®r

Specifications of ADT7463ARQZ

• Function: Fan Control, Temp Monitor
• Topology: ADC, Comparator, Fan Speed Counter, Multiplexer, Register Bank
• Sensor Type: External & Internal
• Sensing Temperature: -40°C ~ 120°C, External Sensor
• Output Type: SMBus™
• Output Alarm: No
• Output Fan: Yes
• Voltage - Supply: 3 V ~ 5.5 V
• Operating Temperature: -40°C ~ 120°C
• Mounting Type: Surface Mount
• Package / Case: 24-QSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Fan TACH Limit Registers
The fan TACH limit registers are 16-bit values consisting of
two bytes.
Reg. 0x54 TACH1 Minimum Low Byte = 0xFF Default
Reg. 0x55 TACH1 Minimum High Byte = 0xFF Default
Reg. 0x56 TACH2 Minimum Low Byte = 0xFF Default
Reg. 0x57 TACH2 Minimum High Byte = 0xFF Default
Reg. 0x58 TACH3 Minimum Low Byte = 0xFF Default
Reg. 0x59 TACH3 Minimum High Byte = 0xFF Default
Reg. 0x5A TACH4 Minimum Low Byte = 0xFF Default
Reg. 0x5B TACH4 Minimum High Byte = 0xFF Default
Fan Speed Measurement Rate
The fan TACH readings are normally updated once every second.
The FAST bit (Bit 3) of Configuration Register 3 (Reg. 0x78),
when set, updates the fan TACH readings every 250 ms.
If any of the fans are not being driven by a PWM channel but
are powered directly from 5 V or 12 V, 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.
Calculating Fan Speed
Assuming a fan with a two pulses/revolution (and two pulses/
revolution being measured), fan speed is calculated by
where
Example:
TACH1 High Byte (Reg 0x29) = 0x17
TACH1 Low Byte (Reg 0x28) = 0xFF
What is Fan 1 speed in RPM?
Fan 1 TACH Reading = 0x17FF = 6143 Decimal
RPM = (f � 60)/Fan 1 TACH Reading
RPM = (90000 � 60)/6143
Fan Speed = 879 RPM
Fan Pulses per Revolution
Different fan models can output either 1, 2, 3, or 4 Tach pulses
per revolution. Once the number of fan Tach pulses has been
determined, it is programmed into the fan pulses per revolution
register (Reg. 0x7B) for each fan. Alternatively, this register can
be used to determine the number or pulses/revolution output by
a given fan. By plotting fan speed measurements at 100% speed
with different pulses/revolution setting, the smoothest graph with
the lowest ripple determines the correct pulses/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.
REV. C
Fan Speed (RPM) = (90,000 � 60)/Fan TACH Reading
Fan TACH Reading = 16-Bit Fan Tachometer Reading
Rev. 4 | Page 29 of 52 | www.onsemi.com
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2-Wire Fan Speed Measurements
The ADT7463 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.
AIN Switching Threshold
Having configured the TACH inputs as AIN inputs for 2-wire
measurements, users can select the sensing threshold for the
AIN signal.
CONFIGURATION REGISTER 4 (REG. 0X7D)
<3:2> AINL These two bits define the input threshold for
Fan Spin-Up
The ADT7463 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 pulses are detected, the
PWM duty cycle goes to the expected running value, e.g.,
33%. The advantage of this is that fans have different spin-up
characteristics and takes different times to overcome inertia. The
ADT7463 just runs the fans fast enough to overcome inertia and
are 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 ADT7463 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-second
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.
2-wire fan speed measurements.
00 = �20 mV
01 = �40 mV
10 = �80 mV
11 = �130 mV
00 = 1 Pulse per Revolution.
01 = 2 Pulses per Revolution.
10 = 3 Pulses per Revolution.
11 = 4 Pulses per Revolution.
ADT7463