ADT7460ARQ-REEL ON Semiconductor, ADT7460ARQ-REEL Datasheet - Page 31

• Manufacturer Part Number: ADT7460ARQ-REEL
• Description: IC SENSOR TEMP FAN-CTRL 16QSOP
• Manufacturer: ON Semiconductor
• Series: dBCool®r

Specifications of ADT7460ARQ-REEL

• Rohs Status: RoHS non-compliant
• Function: Fan Control, Temp Monitor
• Topology: ADC, Comparator, 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: 16-QSOP

Calculating Fan Spe
Assuming a fan with a two pulses/revolution (and two pulses/
revolution being measured), fan speed is calculated by
where:
Fan TACH Reading = 16-Bit Fan Tachometer Reading
For 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 = 61
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 is deter-
mined, it can be programmed into the fan pulses per revolution
register (Reg. 0x7B) for each fan. Alternatively, this register c
be used to determine the number of pulses/revolution output by
a given fan. By plotting fan speed measureme
with different pulses/revolution settings, the s
with the lowest ripple det
valu
Table
Bit
<1:0>
<3:2>
<5:4>
<7:6>
Table 33. Fan Pulses per Revolution Register Bit Values
Value
00
01
10
11
2-Wire Fan Speed Measurements
The ADT7460 is capable of measuring the speed of 2-wire fans,
that is, fans without TA
interfaced as shown in the Fan Drive Circuit
case, the TACH inputs need to be reprogram
inputs, AIN.
e.
Fan Speed (RPM) = 90,000 × 60/Fan TACH Reading
32. Fan Pulses per Re
Mnemonic
FAN1 Default
FAN2 Default
FAN3 Default
FAN4 Default
ed
CH outputs. To do this, the fan must be
ermines the correct pulses/revolution
volu
Description
2 pulses per revolution
2 pulses per revolution
2 pulses per revolution
2 pulses per revolution
Description
1 pulse per revolution
2 pulses per revolution
3 pulses per revolution
4 pulse
tion Register (Reg. 0x7B)
43d
s per revolution
ry section. In this
med as analog
nts at 100% speed
moothest graph
Rev. C | Page 31 of 52
an
Table 34. Configuration Register 2 (Re
Bit
3
2
1
0
AIN Switching Threshold
Having configured the TACH inputs as AIN inputs for 2-wire
measurements, the user can select the sensing threshold for the
AIN signal.
T
Bit
<3:2>
Fan Spin-Up
The ADT7460 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, for
example, 33%. The advantage of this is that fans have different
spin-up characteristics and take different amounts of time to
overcome inertia. The ADT7460 runs the fans just fast enough
to overcome inertia and is quieter on spin-up than fans
programmed to spinup 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 ADT7460 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.
able 35. Configuration Register 4 (Reg. 0x7D)
M
AIN4
AIN3
AIN2
AIN1
Mnemonic
AINL
nemon
ic
Descrip
1 indicates th
measure the
an external s
capacitor.
1 indicates that Pin 4 is reconfigured to
measure the speed of a 2-wire fan using
an external sensing resistor and coupling
capacitor.
1 indicates that Pin 7 is reconfigured to
measure the speed of a 2-wire fan using
an external sensing resistor and coupling
capacitor.
1 indicates that Pin 6 is reconfigured to
measure the speed of a 2-wire fan using
an external sensing resistor and coupling
capacitor.
Description
These two bits define the input
threshold for 2-wire fan speed
measurements.
00 = ±20 mV
01 = ±40 mV
10 = ±80 mV
11 = ±130 mV
tion
ensing res
speed of a 2-wire fan using
at Pin 9 is
g. 0x73)
istor and coup
reconfigured
ADT7460
to
ling