ADT7473 Analog Devices, Inc., ADT7473 Datasheet - Page 29

• Manufacturer Part Number: ADT7473
• Description: Dbcool Remote Thermal Monitor And Fan Controller
• Manufacturer: Analog Devices, Inc.

Register 0x57, TACH2 Minimum H
Register 0x58, TACH3 Minimum Low Byte = 0xFF default
Register 0x59, TACH3 Minimum High Byte = 0xFF default
Register 0x5A, TACH4 Minimum Low Byte = 0xFF default
Register 0x5B, T
Fan Speed Measur
The fan TACH readings are normally updated once every
second.
The FAST bit (Bit 3) of Configuration Regist
set, updates t
If any of the fans are not being driven by a P
are powered directly from 5 V or 12 V, their
Configuration Register 3 should be set. This
readings to be taken on a continuous basis f
directly to a dc source. For optimal results, t
should always be set when using 4-wire fans
Calculating Fan Speed
Assuming a fan has two pulses per revolutio
per revoluti
where Fan TACH Reading is the 16-bit fan tachome
Example
TACH1 High Byte (Register 0x29) = 0x17
TACH1 Low Byte (Register 0x28) = 0xFF
What is Fan 1 speed in RPM?
Fan 1 TA
RPM = (f × 60)/Fan 1 TACH Reading
RPM = (90000 ×
Fan
Fan
Diff
TAC
pulses has b
puls
Alter
or p
speed
revo
determines the correct pulses per revolution value.
erent fan models can o
ulses per revolu
es per revolution register
Speed = 879 RP
lution setting, t
Fan Speed (RPM) = (90,000 × 60)/Fan T
Pulses per Re
H pulses per re
natively, this re
measuremen
CH Reading = 0x17FF = 6143 (decimal)
on being measured), fan speed is calculated by
een determined,
he fan TACH readings every 250 ms.
ACH4 Minimum High Byte = 0xFF default
60)/6143
ts at a 100% speed with different pulses per
M
volution
he smoothest graph with the lowest ripple
volution. Once the number of fan TACH
tion output by a given fan. By plotting fan
gister can be used to determine the number
ement Rate
utput either one, two, three, or four
it can be programmed into the fan
(Register 0x7B) for each fan.
igh Byte = 0xFF default
or fans connected
he associated dc bit
WM channel but
.
n (and two pulses
associated dc bit in
er 3 (0x78), when
ACH Reading
allows TACH
ter reading.
Rev. A | Page 29 of 76
TACH Pulses per Rev
Bits [1:0] Fan 1 default = 2 pulses per revolution
Bits [3:2]
Bits [5:4] Fan 3 default = 2 pulses per revolution
Bits [7:6] Fan 4 default = 2 pulses per revolution
Fan Spin-Up
The ADT7473 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 are
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 tim
overcome inertia. The ADT7473 runs the fans just fast
to overcome inertia and is quieter on spin-up than fans
grammed for a gi
Fan Startup Timeout
To prevent the generation of false interrupts as a fan spins up
(because it is below running speed), the ADT7473 includes a
fan start-up timeout function. During this time, the ADT7473
looks for two TACH pulses. If
detected, an interrupt is generated. Using Configuration
Register 1 (0x40), Bit 5 (FSPDIS), this functionality can be
changed (see the Disabling Fan Startup Timeout section).
PWM1, PWM2, PWM3 Configura
(Register 0x5C, Register 0x5D, and Re
Bits [2:0] SPIN, start-up timeout for PWM1 = 0x5C,
PW
M2 = 0x5D, and PWM3 = 0x5E.
000 = No start-up timeout
001 = 100 ms
010 = 250 ms default
011 = 400 ms
100 = 667 ms
101 = 1 sec
110 = 2 sec
111 = 4 sec
00 = 1 pulse per revolution
01 = 2 pulses per revolution
10 = 3 pulses per revolution
11 = 4 pulses per revolution
Fan 2 default = 2 pulses per revolution
ven spin-up time.
olution Register
two TACH pulses are not
tion Registers
gister 0x5E)
ADT7473
es to
enough
pro-