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ADT7460ARQ-ADT7460ARQZ
dBCOOL™ Thermal Management Controller and Voltage Monitor
AN-613APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: 781/329-4700 • Fax: 781/326-8703 •
Programming the Automatic Fan Speed Control Loop
By Mary Burke
AUTOMATIC FAN SPEED CONTROLThe ADT7460/ADT7463 have a local temperature sensor
and two remote temperature channels that may be con-
nected to an on-chip diode-connected transistor on a
CPU. These three temperature channels may be used as
the basis for automatic fan speed control to drive fans
using pulsewidth modulation (PWM). In general, the
greater the number of fans in a system, the better the
cooling, but this is to the detriment of system acoustics.
Automatic fan speed control reduces acoustic noise
by optimizing fan speed according to measured tem-
perature. Reducing fan speed can also decrease system
current consumption. The automatic fan speed control
mode is very flexible owing to the number of program-
mable parameters, including TMIN and TRANGE, as
discussed in detail later. The TMIN and TRANGE values for a
temperature channel and thus for a given fan are critical
since these define the thermal characteristics of the sys-
tem. The thermal validation of the system is one of the
most important steps of the design process, so these
values should be carefully selected.
AIM OF THIS SECTIONThe aim of this application note is not only to provide
the system designer with an understanding of the auto-
matic fan control loop, but to also provide step-by-step
guidance as to how to most effectively evaluate and
select the critical system parameters. To optimize the
system characteristics, the designer needs to give some
forethought to how the system will be configured, i.e.,
the number of fans, where they are located, and what
temperatures are being measured in the particular
AN-613system. The mechanical or thermal engineer who is
tasked with the actual system evaluation should also be
involved at the beginning of the process.
AUTOMATIC FAN CONTROL OVERVIEWFigure 1 gives a top-level overview of the automatic fan
control circuitry on the ADT7460/ADT7463. From a sys-
tems level perspective, up to three system temperatures
can be monitored and used to control three PWM out-
puts. The three PWM outputs can be used to control up
to four fans. The ADT7460/ADT7463 allow the speed of
four fans to be monitored. Each temperature channel
has a thermal calibration block. This allows the
designer to individually configure the thermal character-
istics of each temperature channel. For example, one
may decide to run the CPU fan when CPU temperature
increases above 60°C, and a chassis fan when the local
temperature increases above 45°C. Note that at this
stage, you have not assigned these thermal calibration
settings to a particular fan drive (PWM) channel. The
right side of the Block Diagram (Figure 1) shows controls
that are fan-specific. The designer has individual control
over parameters such as minimum PWM duty cycle, fan
speed failure thresholds, and even ramp control of the
PWM outputs. This ultimately allows graceful fan speed
changes that are less perceptible to the system user.
STEP 1: DETERMINING THE HARDWARE CONFIGURATIONDuring system design, the motherboard sensing and
control capabilities should not be an afterthought, but
addressed early in the design stages. Decisions about
how these capabilities are used should involve the sys-
tem thermal/mechanical engineer. Ask the following
questions:What ADT7460/ADT7463 functionality will be usedPWM2 or SMBALERT2.5 V voltage monitoring or SMBALERT2.5 V voltage monitoring or processor power
monitoringTACH4 fan speed measurement or over-
temperature THERM function5 V voltage monitoring or overtemperature
THERM function12 V voltage monitoring or VID5 input
The ADT7460/ADT7463 offers multifunctional pins that
can be reconfigured to suit different system require-
ments and physical layouts. These multifunction pins
are software programmable. Various pinout options
are discussed in a separate application note.How many fans will be supported in system, three or
four This will influence the choice of whether to use
the TACH4 pin or to reconfigure it for the THERM
function.Is the CPU fan to be controlled using the ADT7460/
ADT7463 or will it run at full speed 100% of the time
If run at 100%, it will free up a PWM output, but the
system will be louder.
AN-613Figure 3.Recommended Implementation 1Where will the ADT7460/ADT7463 be physically
located in the system
This influences the assignment of the temperature
measurement channels to particular system thermal
zones. For example, locating the ADT7460/ADT7463
close to the VRM controller circuitry allows the VRM
temperature to be monitored using the local tem-
perature channel.
RECOMMENDED IMPLEMENTATION 1Configuring the ADT7460/ADT7463 as in Figure 3 pro-
vides the systems designer with the following features:Six VID Inputs (VID0 to VID5) for VRM10 Support.Two PWM Outputs for Fan Control of up to Three
Fans. (The front and rear chassis fans are connected
in parallel.)Three TACH Fan Speed Measurement Inputs.VCC Measured Internally through Pin 4.CPU Core Voltage Measurement (VCORE).2.5 V Measurement Input Used to Monitor CPU Cur-
rent (connected to VCOMP output of ADP316x VRM
controller). This is used to determine CPU power
consumption.5 V Measurement Input.VRM temperature uses local temperature sensor.CPU Temperature Measured Using Remote 1 Tem-
perature Channel.
10.Ambient Temperature Measured through Remote 2
Temperature Channel.
11.If not using VID5, this pin can be reconfigured as the
12 V monitoring input.
12.Bidirectional THERM Pin. Allows monitoring of
PROCHOT output from Intel® P4 processor, for
example, or can be used as an overtemperature
THERM output.
13.SMBALERT System Interrupt Output.
AN-613
RECOMMENDED IMPLEMENTATION 2Configuring the ADT7460/ADT7463 as in Figure 4 pro-
vides the systems designer with the following features:Six VID Inputs (VID0 to VID5) for VRM10 Support.Three PWM Outputs for Fan Control of up to Three
Fans. (All three fans can be individually controlled.)Three TACH Fan Speed Measurement Inputs.VCC Measured Internally through Pin 4.CPU Core Voltage Measurement (VCORE).2.5 V Measurement Input Used to Monitor CPU Cur-
rent (connected to VCOMP output of ADP316x VRM
Controller). This is used to determine CPU power
consumption.5 V Measurement Input.VRM Temperature Uses Local Temperature Sensor.CPU Temperature Measured Using Remote 1 Tem-
perature Channel.
10.Ambient Temperature Measured through Remote 2
Temperature Channel.
11.If not using VID5, this pin can be reconfigured as the
12 V monitoring input.
12.BIDIRECTIONAL THERM Pin. Allows monitoringPROCHOT output from Intel P4 processor, for
example, or can be used as an overtemperature
THERM output.
Figure 4.Recommended Implementation 2
AN-613
STEP 2: CONFIGURING THE MUX—WHICH
TEMPERATURE CONTROLS WHICH FANAfter the system hardware configuration is determined,
the fans can be assigned to particular temperature chan-
nels. Not only can fans be assigned to individual
channels, but the behavior of fans is also configurable.
For example, fans can be run under automatic fan con-
trol, can run manually (under software control), or can
run at the fastest speed calculated by multiple tempera-
ture channels. The MUX is the bridge between
temperature measurement channels and the three PWM
outputs.
Bits <7:5> (BHVR bits) of registers 0x5C, 0x5D, and 0x5E
(PWM configuration registers) control the behavior of
the fans connected to the PWM1, PWM2, and PWM3 out-
puts. The values selected for these bits determine how
the MUX connects a temperature measurement channel
to a PWM output.
AUTOMATIC FAN CONTROL MUX OPTIONS
<7:5> (BHVR) REGISTERS 0x5C, 0x5D, 0x5E000 = Remote 1 Temp controls PWMx
001 = Local Temp controls PWMx
010 = Remote 2 Temp controls PWMx
101 = Fastest Speed calculated by Local and Remote 2
Temp controls PWMx
110 = Fastest Speed calculated by all three temperature
channels controls PWMx
The "Fastest Speed Calculated" options pertain to the
ability to control one PWM output based on multiple
temperature channels. The thermal characteristics of
the three temperature zones can be set to drive a
single fan. An example would be if the fan turns on
when Remote 1 temperature exceeds 60°C or if the local
temperature exceeds 45°C.
OTHER MUX OPTIONS
<7:5> (BHVR) REGISTERS 0x5C, 0x5D, 0x5E011 = PWMx runs full speed (default)
100 = PWMx disabled
111 = Manual Mode. PWMx is run under software control.
In this mode, PWM duty cycle registers (registers 0x30 to
0x32) are writable and control the PWM outputs.
AN-613
MUX CONFIGURATION EXAMPLEThis is an example of how to configure the MUX in a
system using the ADT7460/ADT7463 to control three
fans. The CPU fan sink is controlled by PWM1, the front
chassis fan is controlled by PWM 2, and the rear chassis
fan is controlled by PWM3. The MUX is configured for
the following fan control behavior:
PWM1 (CPU fan sink) is controlled by the fastest speed
calculated by the Local (VRM Temp) and Remote 2 (pro-
cessor) temperature. In this case, the CPU fan sink is
also being used to cool the VRM.
PWM2 (front chassis fan) is controlled by the Remote 1
temperature (ambient).
PWM3 (rear chassis fan) is controlled by the Remote 1
temperature (ambient).
EXAMPLE MUX SETTINGS
<7:5> (BHVR) PWM1 CONFIGURATION REG 0x5C101 = Fastest speed calculated by Local and Remote 2
Temp controls PWM1.
<7:5> (BHVR) PWM2 CONFIGURATION REG 0x5D000 = Remote 1 Temp controls PWM2.
<7:5> (BHVR) PWM3 CONFIGURATION REG 0x5E000 = Remote 1 Temp controls PWM3.
These settings configure the MUX, as shown in Figure 6.
AN-613
STEP 3: DETERMINING TMIN SETTING FOR EACH
THERMAL CALIBRATION CHANNELTMIN is the temperature at which the fans will start to
turn on under automatic fan control. The speed at which
the fan runs at TMIN is programmed later. The TMIN values
chosen will be temperature channel specific, e.g., 25°C
for ambient channel, 30°C for VRM temperature, and
40°C for processor temperature.
TMIN is an 8-bit twos complement value that can be pro-
grammed in 1°C increments. There is a TMIN register
associated with each temperature measurement channel:
Remote 1, Local, and Remote 2 Temp. Once the TMIN
value is exceeded, the fan turns on and runs at minimum
PWM duty cycle. The fan will turn off once temperature
has dropped below TMIN – THYST (detailed later).
To overcome fan inertia, the fan is spun up until two
valid tach rising edges are counted. See the Fan Startup
Timeout section of the ADT7460/ADT7463 data sheet
for more details. In some cases, primarily for psycho-
acoustic reasons, it is desirable that the fan never
switches off below TMIN. Bits <7:5> of enhance acoustics
Register 1 (Reg. 0x62), when set, keeps the fans running
at PWM minimum duty cycle if the temperature should
fall below TMIN.
TMIN REGISTERSReg. 0x67 Remote 1 Temp TMIN = 0x5A (90°C default)
Reg. 0x68 Local Temp TMIN = 0x5A (90°C default)
Reg. 0x69 Remote 2 Temp TMIN = 0x5A (90°C default)
ENHANCE ACOUSTICS REG 1 (REG. 0x62)
Bit 7 (MIN3) = 0, PWM3 is OFF (0% PWM duty cycle)when Temp is below TMIN – THYST.
Bit 7 (MIN3) = 1, PWM3 runs at PWM3 minimum dutycycle below TMIN – THYST.
Bit 6 (MIN2) = 0, PWM2 is OFF (0% PWM duty cycle)when Temp is below TMIN – THYST.
Bit 6 (MIN2) = 1, PWM2 runs at PWM2 minimum dutycycle below TMIN – THYST.
Bit 5 (MIN1) = 0, PWM1 is OFF (0% PWM duty cycle)when Temp is below TMIN – THYST.
Bit 5 (MIN1) = 1, PWM1 runs at PWM1 minimum dutycycle below TMIN – THYST.
AN-613
STEP 4: DETERMINING PWMMIN FOR EACH PWM (FAN)
OUTPUTPWMMIN is the minimum PWM duty cycle at which each
fan in the system will run. It is also the “start” speed for
each fan under automatic fan control once the tempera-
ture rises above TMIN. For maximum system acoustic
benefit, PWMMIN should be as low as possible. Starting
the fans at higher speeds than necessary will merely
make the system louder than necessary. Depending on
the fan used, the PWMMIN setting should be in the 20% to
33% duty cycle range. This value can be found through
fan validation.
Figure 8.PWMMIN Determines Minimum PWM
Duty Cycle
It is important to note that more than one PWM
output can be controlled from a single temperature
measurement channel. For example, Remote 1 Temp
can control PWM1 and PWM2 outputs. If two different
fans are used on PWM and PWM2, then the fan charac-
teristics can be set up differently. As a result, Fan 1
driven by PWM1 can have a different PWMMIN value than
that of Fan 2 connected to PWM2. Figure 9 illustrates
this as PWM1MIN (front fan) is turned on at a minimum
duty cycle of 20%, whereas PWM2MIN (rear fan) turns
on at a minimum of 40% duty cycle. Note, however,
that both fans turn on at exactly the same tempera-
ture, defined by TMIN.
Figure 9.Operating Two Different Fans from a Single
Temperature Channel
PROGRAMMING THE PWMMIN REGISTERSThe PWMMIN registers are 8-bit registers that allow the
minimum PWM duty cycle for each output to be config-
ured anywhere from 0% to 100%. This allows minimum
PWM duty cycle to be set in steps of 0.39%.
The value to be programmed into the PWMMIN register is
given by:
Value (decimal) = PWMMIN/0.39
Example 1: For a minimum PWM duty cycle of 50%,
Value (decimal) = 50/0.39 = 128 decimal
Value = 128 decimal or 80 hex
Example 2: For a minimum PWM duty cycle of 33%,
Value (decimal) = 33/0.39 = 85 decimal
Value = 85 decimal or 54 hex
PWMMIN REGISTERSReg. 0x64 PWM1 Min Duty Cycle = 0x80 (50% default)
Reg. 0x65 PWM2 Min Duty Cycle = 0x80 (50% default)
Reg. 0x66 PWM3 Min Duty Cycle = 0x80 (50% default)
FAN SPEED AND PWM DUTY CYCLEIt should be noted that PWM duty cycle does not
directly correlate to fan speed in RPM. Running a fan at
33% PWM duty cycle does not equate to running the fan
at 33% speed. Driving a fan at 33% PWM duty cycle
actually runs the fan at closer to 50% of its full speed.
This is because fan speed in %RPM relates to the square
root of PWM duty cycle. Given a PWM square wave as
the drive signal, fan speed in RPM equates to:
AN-613
STEP 5: DETERMINING TRANGE FOR EACH TEMPERATURE
CHANNELTRANGE is the range of temperature over which automatic
fan control occurs once the programmed TMIN tempera-
ture has been exceeded. TRANGE is actually a temperature
slope and not an arbitrary value, i.e., a TRANGE of 40°C
only holds true for PWMMIN = 33%. If PWMMIN is in-
creased or decreased, the effective TRANGE is changed, as
described later.
Figure 10.TRANGE Parameter Affects Cooling Slope
The TRANGE or fan control slope is determined by the fol-
lowing procedure:Determine the maximum operating temperature for
that channel, e.g., 70°C.Determine experimentally the fan speed (PWM duty
cycle value) that will not exceed the temperature at
the worst-case operating points, e.g., 70°C is reached
when the fans are running at 50% PWM duty cycle.Determine the slope of the required control loop to
meet these requirements.Use best fit approximation to determine the most
suitable TRANGE value. ADT7460/ADT7463 evaluation
software is available to calculate the best fit value.
Ask your local Analog Devices representative for
more details.
TRANGE is implemented as a slope, which means as
PWMMIN is changed, TRANGE changes but the actual slope
remains the same. The higher the PWMMIN value, the
smaller the effective TRANGE will be, i.e., the fan will reach
full speed (100%) at a lower temperature.
Figure 12.Increasing PWMMIN Changes Effective
TRANGE
For a given TRANGE value, the temperature at which the
fan will run at full speed for different PWMMIN values can
easily be calculated:
TMAX = TMIN + ((Max D. C. – Min D. C.) � TRANGE /170
where
TMAX = Temperature at which the fan runs full speed
TMIN = Temperature at which the fan will turn on
Max D. C. = Maximum duty cycle (100%) = 255 decimal
Min D. C. = PWMMIN
TRANGE = PWM duty cycle versus temperature slope
Example: Calculate TMAX, given TMIN = 30°C, TRANGE =
40°C, and PWMMIN = 10% duty cycle = 26 decimal
TMAX = TMIN + (Max D. C. – Min D. C.) � TRANGE /170
TMAX = 30°C + (100% – 10%) � 40°C/170
TMAX = 30°C + (255 – 26) � 40°C/170
TMAX = 84°C (effective TRANGE = 54°C)
Example: Calculate TMAX, given TMIN = 30°C, TRANGE =
40°C, and PWMMIN = 25% duty cycle = 64 decimal
TMAX = TMIN + (Max D. C. – Min D. C.) � TRANGE /170
TMAX = 30°C + (100% – 25%) � 40°C/170
TMAX = 30°C + (255 – 64) � 40°C/170
TMAX = 75°C (effective TRANGE = 45°C)
Example: Calculate TMAX, given TMIN = 30°C, TRANGE =
40°C, and PWMMIN = 33% duty cycle = 85 decimal
TMAX = TMIN + (Max D. C. – Min D. C.) � TRANGE /170
AN-613
Example: Calculate TMAX, given TMIN = 30°C, TRANGE =
40°C, and PWMMIN = 50% duty cycle = 128 decimal
TMAX = TMIN + (Max D. C. – Min D. C.) � TRANGE /170
TMAX = 30°C + (100% – 50%) � 40°C/170
TMAX = 30°C + (255 – 128) � 40°C/170
TMAX = 60°C (effective TRANGE = 30°C)
SELECTING A TRANGE SLOPEThe TRANGE value can be selected for each temperature
channel: Remote 1, Local, and Remote 2 Temp. Bits
<7:4> (TRANGE) of registers 0x5F to 0x61 define the TRANGE
value for each temperature channel.
Table I.Selecting a TRANGE Value*Register 0x5F configures Remote 1 TRANGE
Register 0x60 configures Local TRANGE
Register 0x61 configures Remote 2 TRANGE
SUMMARY OF TRANGE FUNCTIONWhen using the automatic fan control function, the tem-
perature at which the fan reaches full speed can be
calculated by
TMAX = TMIN + TRANGE(1)
Equation 1 only holds true when PWMMIN = 33% PWM
duty cycle.
Increasing or decreasing PWMMIN will change the effec-
tive TRANGE, although the fan control will still follow the
same PWM duty cycle to temperature slope. The effec-
tive TRANGE for different PWMMIN values can be
calculated using Equation 2.
TMAX = TMIN + (Max D. C. – Min D. C.) � TRANGE /170(2)
where:
(Max D. C. – Min D. C.) � TRANGE /170 = effective TRANGE
Remember that %PWM duty cycle does not correspond
to %RPM. %RPM relates to the square root of the PWM
duty cycle.
Figure 13.TRANGE vs. Actual Fan Speed Profile
Figure 13 shows PWM duty cycle versus temperature for
each TRANGE setting. The lower graph shows how each
TRANGE setting affects fan speed versus temperature. As
can be seen from the graph, the effect on fan speed is
nonlinear. The graphs in Figure 13 assume that the fan
starts from 0% PWM duty cycle. Clearly, the minimum
PWM duty cycle, PWMMIN, needs to be factored in to see
how the loop actually performs in the system. Figure 14
shows how TRANGE is affected when the PWMMIN value is
set to 20%. It can be seen that the fan will actually run at
about 45% fan speed when the temperature exceeds TMIN.
AN-613Figure 14.TRANGE, % Fan Speed Slopes with
PWMMIN = 20%
EXAMPLE: DETERMINING TRANGE FOR EACH
TEMPERATURE CHANNELThe following example is used to show how TMIN, TRANGE
settings might be applied to three different thermal
zones. In this example, the following TRANGE values
apply:
TRANGE = 80°C for Ambient Temperature
TRANGE = 53.3°C for CPU Temperature
TRANGE = 40°C for VRM Temperature
This example uses the MUX configuration described
in Step 2, with the ADT7460/ADT7463 connected as
shown in Figure 6. Both CPU temperature and VRM tem-
perature drive the CPU fan connected to PWM1.
Ambient temperature drives the front chassis fan and
rear chassis fan connected to PWM2 and PWM3.
The front chassis fan is configured to run at
PWMMIN = 20%. The rear chassis fan is configured to run
at PWMMIN = 30%.
The CPU fan is configured to run at PWMMIN = 10%.
Figure 15.TRANGE, % Fan Speed Slopes for VRM,
Ambient, and CPU Temperature Channels
AN-613
STEP 6: DETERMINING TTHERM FOR EACH TEMPERATURE
CHANNELTTHERM is the absolute maximum temperature allowed
on a temperature channel. Above this temperature, a
component such as the CPU or VRM may be operating
beyond its safe operating limit. When the temperature
measured exceeds TTHERM, all fans are driven at 100% PWM
duty cycle (full speed) to provide critical system cooling.
The fans remain running 100% until the temperature drops
below TTHERM – hysteresis. The hysteresis value is the
number programmed into hysteresis registers 0x6D and
0x6E. The default hysteresis value is 4°C.
The TTHERM limit should be considered the maximum
worst-case operating temperature of the system. Since
exceeding any TTHERM limit runs all fans at 100%, it has
very negative acoustic effects. Ultimately, this limit
should be set up as a failsafe, and one should ensure
that it is not exceeded under normal system operating
conditions.
Note that the TTHERM limits are nonmaskable and affect
the fan speed no matter what automatic fan control set-
tings are configured. This allows some flexibility since a
TRANGE value can be selected based on its slope, while a
“hard limit,” e.g., 70°C, can be programmed as TMAX (the
temperature at which the fan reaches full speed) by set-
ting TTHERM to 70°C.
THERM REGISTERSReg. 0x6A Remote 1 THERM limit = 0x64 (100°C default)
Reg. 0x6B Local Temp THERM limit = 0x64 (100°C
default)
Reg. 0x6C Remote 2 THERM limit = 0x64 (100°C default)
HYSTERESIS REGISTERS
Reg. 0x6D Remote 1, Local Hysteresis Register<7:4> = Remote 1 Temp Hysteresis (4°C default)
<3:0> = Local Temp Hysteresis (4°C default)
Reg. 0x6E Remote 2 Temp Hysteresis Register<7:4> = Remote 2 Temp Hysteresis (4°C default)
Since each hysteresis setting is four bits, hysteresis values
AN-613
STEP 7: DETERMINING THYST FOR EACH TEMPERATURE
CHANNELTHYST 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 tem-
perature is hovering at about the TMIN setting.
The THYST value chosen will determine 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 as pre-
viously described. The THYST default value is set at 4°C.
Figure 17.The THYST Value Applies to Fan On/Off Hysteresis and THERM Hysteresis
Note that the THYST setting applies not only to the
temperature hysteresis for fan turn on/off, but the same
setting is used for the TTHERM hysteresis value described
in Step 6. So programming registers 0x6D and 0x6E sets
the hysteresis for both fan on/off and the THERM function.
HYSTERESIS REGISTERS
Reg. 0x6D Remote 1, Local Hysteresis Register<7:4> = Remote 1 Temp Hysteresis (4°C default)
<3:0> = Local Temp Hysteresis (4°C default)
Reg. 0x6E Remote 2 Temp Hysteresis Register<7:4> = Remote 2 Temp Hysteresis (4°C default)
Note that in some applications, it is required that the
fans not turn off below TMIN but remain running at
PWMMIN. Bits <7:5> of Enhance Acoustics Register 1
(Reg. 0x62) allow the fans to be turned off, or to be kept
AN-613
ENHANCE ACOUSTICS REG 1 (REG. 0x62)
Bit 7 (MIN3) = 0, PWM3 is OFF (0% PWM duty cycle)when Temp is below TMIN – THYST.
Bit 7 (MIN3) = 1, PWM3 runs at PWM3 minimum dutycycle below TMIN – THYST.
Bit 6 (MIN2) = 0, PWM2 is OFF (0% PWM duty cycle)when Temp is below TMIN – THYST.
Bit 6 (MIN2) = 1, PWM2 runs at PWM2 minimum dutycycle below TMIN – THYST.
Bit 5 (MIN1) = 0, PWM1 is OFF (0% PWM duty cycle)when Temp is below TMIN – THYST.
Bit 5 (MIN1) = 1, PWM1 runs at PWM1 minimum dutycycle below TMIN – THYST.
DYNAMIC TMIN CONTROL MODEIn addition to the automatic fan speed control mode de-
scribed in the previous section, the ADT7460/ADT7463
have a mode that extends the basic automatic fan speed
control loop. Dynamic TMIN control allows the
ADT7460/ADT7463 to intelligently adapt the system’s
cooling solution for best system performance or lowest
possible system acoustics, depending on user or design
requirements.
AIM OF THIS SECTIONThis section has two primary goals:To show how dynamic TMIN control alleviates the
need for designing for worst-case conditions.To illustrate how the dynamic TMIN control function
significantly reduces system design and validation
time.
DESIGNING FOR WORST-CASE CONDITIONSWhen designing a system, you always design for worst-
case conditions. In PC design, the worst-case conditions
include, but are not limited to:Worst-Case Altitude. A computer can be operated at
different altitudes. The altitude affects the relative air
density, which will alter the effectiveness of the fan
cooling solution. For example, comparing 40°C air
temperature at 10,000 ft 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 ft 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 toler-
ance of ±20%. The designer needs to assume that the
fan RPM can be 20% below tolerance. This translates
to reduced system airflow and elevated system tem-Worst-Case Chassis Airflow. The same motherboard
can be used in a number of different chassis configu-
rations. The design of the chassis and physical
location of fans and components determine the sys-
tem 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, e.g., placing a
computer against a wall can block the air ducts and
reduce system airflow.
Figure 18.Chassis Airflow IssuesWorst-Case Processor Power Consumption. This is a
data sheet maximum that does not necessarily reflect
the true processor power consumption. Designing for
worst-case CPU power consumption results in that
the processor getting overcooled (generating excess
system noise).Worst-Case Peripheral Power Consumptions. The
tendency is to design to data sheet maximums for
these components (again overcooling the system).Worst-Case Assembly. Every system manufactured is
unique because of manufacturing variations. Heat
sinks may be loose fitting or slightly misaligned. Too
much or too little thermal grease may be used, or varia-
tions in application pressure for thermal interface
material can affect the efficiency of the thermal solution.
How can this be accounted for in every system Again,
the system is designed for the worst case.
SUBSTRATE
HEAT
SINK
THERMAL
INTERFACE
MATERIAL
INTEGRATED
HEAT
SPREADER�CTIM
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