ADT7462 Datasheet, PDF (47/92 Pages) Analog Devices – Flexible Temperature and Voltage Monitor and System Fan Controller

ADT7462
100%
T_RANGE
HYSTERESIS
0%
T_MIN
T_THERM
Figure 62. How TTHERM Relates to Automatic Fan Control
STEP 7—T_HYST FOR TEMPERATURE CHANNELS
THYST is the amount of extra cooling a fan provides after the
temperature measured has dropped back below TMIN before the
fan turns off. The premise for temperature hysteresis (THYST) is
that without it, the fan would merely chatter or cycle on and off
regularly whenever the temperature hovers near the TMIN
setting.
The THYST value chosen determines the amount of time needed
for the system to cool down or heat up as the fan is turning on
and off. Values of hysteresis are programmable in the range 1°C
to 15°C. Larger values of THYST prevent the fans from chattering
on and off. The THYST default value is set at 4°C.
Hysteresis Register
Register 0x60, Bits [3:0] Local THYST
Register 0x61, Bits [3:0] Remote 1 THYST
Register 0x62, Bits [3:0] Remote 2 THYST
Register 0x63, Bits [3:0] Remote 3 THYST
In some applications, it is required that fans not turn off below TMIN
but remain running at PWMMIN. Bits [1:0] of the PWM1, PWM2
Frequency Register (0x25) and the PWM3, PWM4 Frequency
Register (0x26) allow the fans to be turned off or to be kept
spinning below TMIN. If the fans are always on, the THYST value has
no effect on the fan when the temperature drops below TMIN.
T_RANGE
100%
T_HYST
0%
T_MIN
T_THERM
Figure 63. The THYST Value Applies to Fan On/Off Hysteresis
Dynamic T_MIN Control Mode
In addition to the automatic fan speed control mode described
in the Automatic Fan Control Overview section, the ADT7462
has a mode that extends the basic automatic fan speed control
loop. Dynamic TMIN control allows the ADT7462 to intelligently
adapt the system’s cooling solution for best system performance
or lowest possible system acoustics, depending on user or design
requirements. Use of dynamic TMIN control alleviates the need
to design for worst-case conditions and significantly reduces
system design and validation time.
Designing for Worst-Case Conditions
System design must always allow for worst-case conditions.
In PC design, the worst-case conditions include, but are not
limited to, the following:
Worst-Case Altitude
A computer can be operated at different altitudes. Altitude
affects the relative air density, which alters the effectiveness of
the fan cooling solution. For example, when comparing 40°C
air temperature at 10,000 feet to 20°C air temperature at sea
level, relative air density is increased by 40%. This means that
the fan can spin 40% slower and make less noise at sea level
than at 10,000 feet while keeping the system at the same
temperature at both locations.
Worst-Case Fan
Due to manufacturing tolerances, fan speeds in rpm are
normally quoted with a tolerance of ±20%. The designer
must assume that the fan rpm can be 20% below tolerance.
This translates to reduced system airflow and elevated system
temperature. Note that fans 20% out of tolerance can negatively
impact system acoustics because they run faster and generate
more noise.
Worst-Case Chassis Airflow
The same motherboard can be used in a number of different
chassis configurations. The design of the chassis and the
physical location of fans and components determine the system’s
thermal characteristics. Moreover, for a given chassis, the
addition of add-in cards, cables, or other system configuration
options can alter the system airflow and reduce the
effectiveness of the system cooling solution. The cooling
solution can also be inadvertently altered by the end user.
(For example, placing a computer against a wall can block the
air ducts and reduce system airflow.)
VENTS
I/O CARDS
FAN
POWER ^VENTS
SUPPLY
I/O CARDS
FAN
POWER
SUPPLY
CPU
GOOD CPU AIRFLOW
FAN
DRIVE
BAYS
POOR CPU
AIRFLOW
CPU
DRIVE
BAYS
Rev. 0 | Page 47 of 92
VENTS
GOOD VENTING =
GOOD AIR EXCHANGE
POOR VENTING =
POOR AIR EXCHANGE
Figure 64. Chassis Airflow Issues