AN-613 APPLICATION NOTE One Technology Way P.O. Box 9106 Norwood, MA Tel: 781/ Fax: 781/

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1 a AN-613 APPLICATION NOTE One Technology Way P.O. Box 916 Norwood, MA Tel: 781/ Fax: 781/ Programming the Automatic Fan Speed Control Loop By Mary Burke AUTOMATIC FAN SPEED CONTROL The ADT746/ADT7463 have a local temperature sensor and two remote temperature channels that may be connected 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 (). 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 temperature. Reducing fan speed can also decrease system current consumption. The automatic fan speed control mode is very flexible owing to the number of programmable parameters, including and, as discussed in detail later. The and values for a temperature channel and thus for a given fan are critical since these define the thermal characteristics of the system. 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 SECTION The aim of this application note is not only to provide the system designer with an understanding of the automatic 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 RAMP CONTROL ENHANCEMENT 1 REMOTE 1 TEMP TACHOMETER 1 MUX RAMP CONTROL ENHANCEMENT 2 LOCAL TEMP TACHOMETER 2 RAMP CONTROL ENHANCEMENT 3 REMOTE 2 TEMP TACHOMETER 3 AND 4 Figure 1. Automatic Fan Control Block Diagram REV. 28 SCILLC. All rights reserved. Publication Order Number: April 28 - Rev. 1 AN613/D

2 system. 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 OVERVIEW Figure 1 gives a top-level overview of the automatic fan control circuitry on the ADT746/ADT7463. From a systems level perspective, up to three system temperatures can be monitored and used to control three outputs. The three outputs can be used to control up to four fans. The ADT746/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 characteristics of each temperature channel. For example, one may decide to run the CPU fan when CPU temperature increases above 6 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 () 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 duty cycle, fan speed failure thresholds, and even ramp control of the outputs. This ultimately allows graceful fan speed changes that are less perceptible to the system user. STEP 1: DETERING THE HARDWARE URATION During 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 system thermal/mechanical engineer. Ask the following questions: 1. What ADT746/ADT7463 functionality will be used? 2 or SMBALERT? 2.5 V voltage monitoring or SMBALERT? 2.5 V voltage monitoring or processor power monitoring? TACH4 fan speed measurement or overtemperature THERM function? 5 V voltage monitoring or overtemperature THERM function? 12 V voltage monitoring or VID5 input? The ADT746/ADT7463 offers multifunctional pins that can be reconfigured to suit different system requirements and physical layouts. These multifunction pins are software programmable. Various pinout options are discussed in a separate application note. 2. 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. 3. Is the CPU fan to be controlled using the ADT746/ ADT7463 or will it run at full speed of the time? If run at, it will free up a output, but the system will be louder. REMOTE 1 = AMBIENT TEMP TACHOMETER 1 1 TACH1 CPU FAN SINK LOCAL = VRM TEMP MUX TACHOMETER 2 2 TACH2 FRONT 3 REMOTE 2 = CPU TEMP TACHOMETER 3 AND 4 TACH3 REAR Figure 2. Hardware Configuration Example 2 REV. Rev. 1 Page 2 of 27

3 FRONT FAN TACH2 1 TACH1 REAR FAN 3 TACH3 VID[:4]/VID[.5] D2+ 5(VRM9)/6(VRM1) D2 THERM PROCHOT AMBIENT TEMPERATURE D1+ D1 ADT VSB 5V 12V/VID5 SDA ADP316x VRM CONTROLLER V COMP CURRENT V CORE SCL SMBALERT GND Figure 3. Recommended Implementation 1 4. Where will the ADT746/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 ADT746/ADT7463 close to the VRM controller circuitry allows the VRM temperature to be monitored using the local temperature channel. RECOMMENDED IMPLEMENTATION 1 Configuring the ADT746/ADT7463 as in Figure 3 provides the systems designer with the following features: 1. Six VID Inputs (VID to VID5) for VRM1 Support. 2. Two Outputs for Fan Control of up to Three Fans. (The front and rear chassis fans are connected in parallel.) 3. Three TACH Fan Speed Measurement Inputs. 4. V CC Measured Internally through Pin CPU Core Voltage Measurement (V CORE ) V Measurement Input Used to Monitor CPU Current (connected to V COMP output of ADP316x VRM controller). This is used to determine CPU power consumption V Measurement Input. 8. VRM temperature uses local temperature sensor. 9. CPU Temperature Measured Using Remote 1 Temperature Channel. 1. 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. Rev. 1 Page 3 of 27

4 FRONT FAN TACH2 2 1 TACH1 REAR FAN 3 TACH3 VID[:4]/VID[.5] D2+ 5(VRM9)/6(VRM1) D2 THERM PROCHOT AMBIENT TEMPERATURE D1+ D1 ADT VSB 5V 12V/VID5 SDA ADP316x VRM CONTROLLER V COMP CURRENT V CORE GND SCL Figure 4. Recommended Implementation 2 RECOMMENDED IMPLEMENTATION 2 Configuring the ADT746/ADT7463 as in Figure 4 provides the systems designer with the following features: 1. Six VID Inputs (VID to VID5) for VRM1 Support. 2. Three Outputs for Fan Control of up to Three Fans. (All three fans can be individually controlled.) 3. Three TACH Fan Speed Measurement Inputs. 4. V CC Measured Internally through Pin CPU Core Voltage Measurement (V CORE ) V Measurement Input Used to Monitor CPU Current (connected to V COMP output of ADP316x VRM Controller). This is used to determine CPU power consumption V Measurement Input. 8. VRM Temperature Uses Local Temperature Sensor. 9. CPU Temperature Measured Using Remote 1 Temperature Channel. 1. 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. Rev. 1 Page 4 of 27

5 STEP 2: URING THE MUX WHICH TEMPERATURE CONTROLS WHICH FAN? After the system hardware configuration is determined, the fans can be assigned to particular temperature channels. 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 control, can run manually (under software control), or can run at the fastest speed calculated by multiple temperature channels. The MUX is the bridge between temperature measurement channels and the three outputs. Bits <7:5> (BHVR bits) of registers x5c, x5d, and x5e ( configuration registers) control the behavior of the fans connected to the 1, 2, and 3 outputs. The values selected for these bits determine how the MUX connects a temperature measurement channel to a output. AUTOMATIC FAN CONTROL MUX OPTIONS <7:5> (BHVR) REGISTERS x5c, x5d, x5e = Remote 1 Temp controls x 1 = Local Temp controls x 1 = Remote 2 Temp controls x 11 = Fastest Speed calculated by Local and Remote 2 Temp controls x 11 = Fastest Speed calculated by all three temperature channels controls x The "Fastest Speed Calculated" options pertain to the ability to control one 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 6 C or if the local temperature exceeds 45 C. OTHER MUX OPTIONS <7:5> (BHVR) REGISTERS x5c, x5d, x5e 11 = x runs full speed (default) 1 = x disabled 111 = Manual Mode. x is run under software control. In this mode, duty cycle registers (registers x3 to x32) are writable and control the outputs. MUX REMOTE 1 = AMBIENT TEMP TACHOMETER 1 1 TACH1 CPU FAN SINK LOCAL = VRM TEMP MUX TACHOMETER 2 2 TACH2 FRONT 3 REMOTE 2 = CPU TEMP TACHOMETER 3 AND 4 TACH3 REAR Figure 5. Assigning Temperature Channels to Fan Channels Rev. 1 Page 5 of 27

6 MUX URATION EXAMPLE This is an example of how to configure the MUX in a system using the ADT746/ADT7463 to control three fans. The CPU fan sink is controlled by 1, the front chassis fan is controlled by 2, and the rear chassis fan is controlled by 3. The MUX is configured for the following fan control behavior: 1 (CPU fan sink) is controlled by the fastest speed calculated by the Local (VRM Temp) and Remote 2 (processor) temperature. In this case, the CPU fan sink is also being used to cool the VRM. EXAMPLE MUX SETTINGS <7:5> (BHVR) 1 URATION REG x5c 11 = Fastest speed calculated by Local and Remote 2 Temp controls 1. <7:5> (BHVR) 2 URATION REG x5d = Remote 1 Temp controls 2. <7:5> (BHVR) 3 URATION REG x5e = Remote 1 Temp controls 3. These settings configure the MUX, as shown in Figure 6. 2 (front chassis fan) is controlled by the Remote 1 temperature (ambient). 3 (rear chassis fan) is controlled by the Remote 1 temperature (ambient). 1 CPU FAN SINK REMOTE 2 = CPU TEMP MUX TACHOMETER 1 TACH1 2 FRONT LOCAL = VRM TEMP REMOTE 1 = AMBIENT TEMP TACHOMETER 2 TACHOMETER 3 AND 4 TACH2 3 TACH3 REAR Figure 6. MUX Configuration Example Rev. 1 Page 6 of 27

7 STEP 3: DETERING SETTING FOR EACH CHANNEL is the temperature at which the fans will start to turn on under automatic fan control. The speed at which the fan runs at is programmed later. The values chosen will be temperature channel specific, e.g., 25 C for ambient channel, 3 C for VRM temperature, and 4 C for processor temperature. is an 8-bit twos complement value that can be programmed in 1 C increments. There is a register associated with each temperature measurement channel: Remote 1, Local, and Remote 2 Temp. Once the value is exceeded, the fan turns on and runs at minimum duty cycle. The fan will turn off once temperature has dropped below T HYST (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 ADT746/ADT7463 data sheet for more details. In some cases, primarily for psychoacoustic reasons, it is desirable that the fan never switches off below. Bits <7:5> of enhance acoustics Register 1 (Reg. x62), when set, keeps the fans running at minimum duty cycle if the temperature should fall below. REGISTERS Reg. x67 Remote 1 Temp = x5a (9 C default) Reg. x68 Local Temp = x5a (9 C default) Reg. x69 Remote 2 Temp = x5a (9 C default) ENHANCE ACOUSTICS REG 1 (REG. x62) Bit 7 (3) =, 3 is OFF ( duty cycle) when Temp is below T HYST. Bit 7 (3) = 1, 3 runs at 3 minimum duty cycle below T HYST. Bit 6 (2) =, 2 is OFF ( duty cycle) when Temp is below T HYST. Bit 6 (2) = 1, 2 runs at 2 minimum duty cycle below T HYST. Bit 5 (1) =, 1 is OFF ( duty cycle) when Temp is below T HYST. Bit 5 (1) = 1, 1 runs at 1 minimum duty cycle below T HYST. DUTY CYCLE REMOTE 2 = CPU TEMP T TRANGE RAMP CONTROL ENHANCEMENT TACHOMETER 1 1 TACH1 CPU FAN SINK MUX RAMP CONTROL ENHANCEMENT 2 FRONT LOCAL = VRM TEMP T TRANGE TACHOMETER 2 TACH2 REMOTE 1 = AMBIENT TEMP T TRANGE RAMP CONTROL ENHANCEMENT TACHOMETER 3 AND 4 3 TACH3 REAR REV. Figure 7. Understanding the Parameter 7 Rev. 1 Page 7 of 27

8 STEP 4: DETERING FOR EACH (FAN) OUTPUT is the minimum 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 temperature rises above. For maximum system acoustic benefit, 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 setting should be in the 2 to 33% duty cycle range. This value can be found through fan validation. DUTY CYCLE TEMPERATURE Figure 8. Determines Minimum Duty Cycle It is important to note that more than one output can be controlled from a single temperature measurement channel. For example, Remote 1 Temp can control 1 and 2 outputs. If two different fans are used on and 2, then the fan characteristics can be set up differently. As a result, Fan 1 driven by 1 can have a different value than that of Fan 2 connected to 2. Figure 9 illustrates this as 1 (front fan) is turned on at a minimum duty cycle of 2, whereas 2 (rear fan) turns on at a minimum of 4 duty cycle. Note, however, that both fans turn on at exactly the same temperature, defined by. DUTY CYCLE TEMPERATURE Figure 9. Operating Two Different Fans from a Single Temperature Channel PROGRAMG THE REGISTERS The registers are 8-bit registers that allow the minimum duty cycle for each output to be configured anywhere from to. This allows minimum duty cycle to be set in steps of.39%. The value to be programmed into the register is given by: Value (decimal) = /.39 Example 1: For a minimum duty cycle of 5, Value (decimal) = 5/.39 = 128 decimal Value = 128 decimal or hex Example 2: For a minimum duty cycle of 33%, Value (decimal) = 33/.39 = 85 decimal Value = 85 decimal or 54 hex REGISTERS Reg. x64 1 Min Duty Cycle = x (5 default) Reg. x65 2 Min Duty Cycle = x (5 default) Reg. x66 3 Min Duty Cycle = x (5 default) FAN SPEED AND DUTY CYCLE It should be noted that duty cycle does not directly correlate to fan speed in RPM. Running a fan at 33% duty cycle does not equate to running the fan at 33% speed. Driving a fan at 33% duty cycle actually runs the fan at closer to 5 of its full speed. This is because fan speed in %RPM relates to the square root of duty cycle. Given a square wave as the drive signal, fan speed in RPM equates to: % fan speed = duty cycle 1 Rev. 1 Page 8 of 27

9 STEP 5: DETERING FOR EACH TEMPERATURE CHANNEL is the range of temperature over which automatic fan control occurs once the programmed temperature has been exceeded. is actually a temperature slope and not an arbitrary value, i.e., a of 4 C only holds true for = 33%. If is increased or decreased, the effective is changed, as described later. is implemented as a slope, which means as is changed, changes but the actual slope remains the same. The higher the value, the smaller the effective will be, i.e., the fan will reach full speed () at a lower temperature. DUTY CYCLE DUTY CYCLE 5 33% 25% 1 3 C 4 C 45 C 54 C TEMPERATURE Figure 1. Parameter Affects Cooling Slope The or fan control slope is determined by the following procedure: 1. Determine the maximum operating temperature for that channel, e.g., 7 C. 2. Determine experimentally the fan speed ( duty cycle value) that will not exceed the temperature at the worst-case operating points, e.g., 7 C is reached when the fans are running at 5 duty cycle. 3. Determine the slope of the required control loop to meet these requirements. 4. Use best fit approximation to determine the most suitable value. ADT746/ADT7463 evaluation software is available to calculate the best fit value. Ask your local Analog Devices representative for more details. DUTY CYCLE 5 33% 3 C 4 C Figure 11. Adjusting Affects Figure 12. Increasing Changes Effective For a given value, the temperature at which the fan will run at full speed for different values can easily be calculated: T MAX = + ((Max D. C. Min D. C.) /17 where Rev. 1 Page 9 of 27 T MAX = Temperature at which the fan runs full speed = Temperature at which the fan will turn on Max D. C. = Maximum duty cycle () = 255 decimal Min D. C. = = duty cycle versus temperature slope Example: Calculate T MAX, given = 3 C, = 4 C, and = 1 duty cycle = 26 decimal T MAX = + (Max D. C. Min D. C.) /17 T MAX = 3 C + ( 1) 4 C/17 T MAX = 3 C + (255 26) 4 C/17 T MAX = 84 C (effective = 54 C) Example: Calculate T MAX, given = 3 C, = 4 C, and = 25% duty cycle = 64 decimal T MAX = + (Max D. C. Min D. C.) /17 T MAX = 3 C + ( 25%) 4 C/17 T MAX = 3 C + (255 64) 4 C/17 T MAX = 75 C (effective = 45 C) Example: Calculate T MAX, given = 3 C, = 4 C, and = 33% duty cycle = 85 decimal T MAX = + (Max D. C. Min D. C.) /17 T MAX = 3 C + ( 33%) 4 C/17 T MAX = 3 C + (255 85) 4 C/17 T MAX = 7 C (effective = 4 C)

10 Example: Calculate T MAX, given = 3 C, = 4 C, and = 5 duty cycle = 128 decimal T MAX = + (Max D. C. Min D. C.) /17 T MAX = 3 C + ( 5) 4 C/17 T MAX = 3 C + ( ) 4 C/17 T MAX = 6 C (effective = 3 C) SELECTING A SLOPE The value can be selected for each temperature channel: Remote 1, Local, and Remote 2 Temp. Bits <7:4> ( ) of registers x5f to x61 define the value for each temperature channel. Table I. Selecting a Value Bits <7:4>* 2 C C C 11 4 C 1 5 C C 11 8 C C C C 11 2 C C C (default) C C 1111 C * Register x5f configures Remote 1 Register x6 configures Local Register x61 configures Remote 2 SUMMARY OF FUNCTION When using the automatic fan control function, the temperature at which the fan reaches full speed can be calculated by T MAX = + (1) Equation 1 only holds true when = 33% duty cycle. Increasing or decreasing will change the effective, although the fan control will still follow the same duty cycle to temperature slope. The effective for different values can be calculated using Equation 2. Remember that % duty cycle does not correspond to %RPM. %RPM relates to the square root of the duty cycle. DUTY CYCLE % FAN SPEED % OF MAX % fan speed = duty cycle C TEMPERATURE ABOVE 1 2 C TEMPERATURE ABOVE Figure 13. vs. Actual Fan Speed Profile 2.5 C 3.33 C 4 C 5 C 6.67 C 8 C 1 C 13.3 C 16 C 2 C 26.6 C 32 C 4 C 53.3 C C 2.5 C 3.33 C 4 C 5 C 6.67 C 8 C 1 C 13.3 C 16 C 2 C 26.6 C 32 C 4 C 53.3 C C Figure 13 shows duty cycle versus temperature for each setting. The lower graph shows how each 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 duty cycle. Clearly, the minimum duty cycle,, needs to be factored in to see how the loop actually performs in the system. Figure 14 shows how is affected when the value is set to 2. It can be seen that the fan will actually run at about 45% fan speed when the temperature exceeds. T MAX = + (Max D. C. Min D. C.) /17 (2) where: (Max D. C. Min D. C.) /17 = effective value. Rev. 1 Page 1 of 27

11 DUTY CYCLE % FAN SPEED % OF MAX 1 2 C TEMPERATURE ABOVE 1 2 C TEMPERATURE ABOVE Figure 14., % Fan Speed Slopes with = C 3.33 C 4 C 5 C 6.67 C 8 C 1 C 13.3 C 16 C 2 C 26.6 C 32 C 4 C 53.3 C C 2.5 C 3.33 C 4 C 5 C 6.67 C 8 C 1 C 13.3 C 16 C 2 C 26.6 C 32 C 4 C 53.3 C C EXAMPLE: DETERING FOR EACH TEMPERATURE CHANNEL The following example is used to show how, settings might be applied to three different thermal zones. In this example, the following values apply: = C for Ambient Temperature = 53.3 C for CPU Temperature = 4 C for VRM Temperature This example uses the MUX configuration described in Step 2, with the ADT746/ADT7463 connected as shown in Figure 6. Both CPU temperature and VRM temperature drive the CPU fan connected to 1. Ambient temperature drives the front chassis fan and rear chassis fan connected to 2 and 3. The front chassis fan is configured to run at = 2. The rear chassis fan is configured to run at = 3. The CPU fan is configured to run at = 1. DUTY CYCLE % FAN SPEED % MAX RPM TEMPERATURE ABOVE TEMPERATURE ABOVE Figure 15., % Fan Speed Slopes for VRM, Ambient, and CPU Temperature Channels Rev. 1 Page 11 of 27

12 STEP 6: DETERING T THERM FOR EACH TEMPERATURE CHANNEL T THERM 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 T THERM, all fans are driven at duty cycle (full speed) to provide critical system cooling. The fans remain running until the temperature drops below T THERM hysteresis. The hysteresis value is the number programmed into hysteresis registers x6d and x6e. The default hysteresis value is 4 C. The T THERM limit should be considered the maximum worst-case operating temperature of the system. Since exceeding any T THERM limit runs all fans at, 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 T THERM limits are nonmaskable and affect the fan speed no matter what automatic fan control settings are configured. This allows some flexibility since a value can be selected based on its slope, while a hard limit, e.g., 7 C, can be programmed as T MAX (the temperature at which the fan reaches full speed) by setting T THERM to 7 C. THERM REGISTERS Reg. x6a Remote 1 THERM limit = x64 (1 C default) Reg. x6b Local Temp THERM limit = x64 (1 C default) Reg. x6c Remote 2 THERM limit = x64 (1 C default) HYSTERESIS REGISTERS Reg. x6d Remote 1, Local Hysteresis Register <7:4> = Remote 1 Temp Hysteresis (4 C default) <3:> = Local Temp Hysteresis (4 C default) Reg. x6e Remote 2 Temp Hysteresis Register <7:4> = Remote 2 Temp Hysteresis (4 C default) DUTY CYCLE Since each hysteresis setting is four bits, hysteresis values are programmable from 1 C to 15 C. It is not recommended that hysteresis values ever be programmed to C, as this actually disables hysteresis. In effect, this would cause the fans to cycle between normal speed and speed, creating unsettling acoustic noise. T THERM REMOTE 2 = CPU TEMP TACHOMETER 1 1 TACH1 CPU FAN SINK LOCAL = VRM TEMP MUX TACHOMETER 2 2 TACH2 FRONT 3 REMOTE 1 = AMBIENT TEMP TACHOMETER 3 AND 4 TACH3 REAR Figure 16. Understanding How T THERM Relates to Automatic Fan Control Figure 16. Understanding Rev. 1 How Page 12 T of 27 Relates to Automatic Fan Control

13 STEP 7: DETERING T HYST FOR EACH TEMPERATURE CHANNEL T HYST is the amount of extra cooling a fan provides after the temperature measured has dropped back below before the fan turns off. The premise for temperature hysteresis (T HYST ) is that without it, the fan would merely chatter, or cycle on and off regularly, whenever temperature is hovering at about the setting. The T HYST 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 T HYST prevent the fans from chattering on and off as previously described. The T HYST default value is set at 4 C. DUTY CYCLE T HYST Note that the T HYST setting applies not only to the temperature hysteresis for fan turn on/off, but the same setting is used for the T THERM hysteresis value described in Step 6. So programming registers x6d and x6e sets the hysteresis for both fan on/off and the THERM function. HYSTERESIS REGISTERS Reg. x6d Remote 1, Local Hysteresis Register <7:4> = Remote 1 Temp Hysteresis (4 C default) <3:> = Local Temp Hysteresis (4 C default) Reg. x6e 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 but remain running at. Bits <7:5> of Enhance Acoustics Register 1 (Reg. x62) allow the fans to be turned off, or to be kept spinning below. If the fans are always on, the T HYST value has no effect on the fan when the temperature drops below. T THERM REMOTE 2 = CPU TEMP T TRANGE RAMP CONTROL TACHOMETER 1 1 TACH1 CPU FAN SINK LOCAL = VRM TEMP T TRANGE MUX RAMP CONTROL TACHOMETER 2 2 TACH2 FRONT RAMP CONTROL 3 REMOTE 1 = AMBIENT TEMP T TRANGE TACHOMETER 3 AND 4 TACH3 REAR Figure 17. The T HYST Value Applies to Fan On/Off Hysteresis and THERM Hysteresis Rev. 1 Page 13 of 27

14 ENHANCE ACOUSTICS REG 1 (REG. x62) Bit 7 (3) =, 3 is OFF ( duty cycle) when Temp is below T HYST. Bit 7 (3) = 1, 3 runs at 3 minimum duty cycle below T HYST. Bit 6 (2) =, 2 is OFF ( duty cycle) when Temp is below T HYST. Bit 6 (2) = 1, 2 runs at 2 minimum duty cycle below T HYST. Bit 5 (1) =, 1 is OFF ( duty cycle) when Temp is below T HYST. Bit 5 (1) = 1, 1 runs at 1 minimum duty cycle below T HYST. DYNAMIC CONTROL MODE In addition to the automatic fan speed control mode described in the previous section, the ADT746/ADT7463 have a mode that extends the basic automatic fan speed control loop. Dynamic control allows the ADT746/ADT7463 to intelligently adapt the system s cooling solution for best system performance or lowest possible system acoustics, depending on user or design requirements. 3. Worst-Case Chassis Airflow. The same motherboard can be used in a number of different chassis configurations. The design of the chassis and physical location of fans and components determine the system 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. VENTS I/O CARDS GOOD CPU AIRFLOW FAN FAN POWER SUPPLY CPU DRIVE BAYS VENTS GOOD VENTING = GOOD AIR EXCHANGE VENTS I/O CARDS POOR CPU AIRFLOW FAN POWER SUPPLY CPU DRIVE BAYS POOR VENTING = POOR AIR EXCHANGE Figure 18. Chassis Airflow Issues AIM OF THIS SECTION This section has two primary goals: 1. To show how dynamic control alleviates the need for designing for worst-case conditions. 2. To illustrate how the dynamic control function significantly reduces system design and validation time. DESIGNING FOR WORST-CASE CONDITIONS When designing a system, you always design for worstcase conditions. In PC design, the worst-case conditions include, but are not limited to: 1. 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 4 C air temperature at 1, ft to 2 C air temperature at sea level, relative air density is increased by 4. This means that the fan can spin 4 slower, and make less noise, at sea level than at 1, ft while keeping the system at the same temperature at both locations. 2. Worst-Case Fan. Due to manufacturing tolerances, fan speeds in RPM are normally quoted with a tolerance of ±2. The designer needs to assume that the fan RPM can be 2 below tolerance. This translates to reduced system airflow and elevated system temperature. Note that fans 2 out of tolerance will negatively impact system acoustics since they run faster and generate more noise. 4. Worst-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). 5. Worst-Case Peripheral Power Consumptions. The tendency is to design to data sheet maximums for these components (again overcooling the system). 6. 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 variations 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. THERMAL INTERFACE MATERIAL HEAT SINK INTEGRATED HEAT SPREADER Rev. 1 Page 14 of 27 SUBSTRATE PROCESSOR EPOXY THERMAL INTERFACE MATERIAL SA T A TIMS T TIM CTIM TIMC T TIM JTIM Figure 19. Thermal Model T S T C T J CS CA JA

15 The design usually accounts for worst-case conditions in all of these cases. Note, however, that the actual system is almost never operated at worst-case conditions. The alternative to designing for the worst case is to use the dynamic control function. solution to maintain each zone temperature as closely as possible to their target operating points. OPERATING POINT REGISTERS Reg. x33 Remote 1 Operating Point = x64 (1 C) Reg. x34 Local Temp Operating Point = x64 (1 C) Reg. x35 Remote 2 Operating Point = x64 (1 C) DYNAMIC CONTROL OVERVIEW Dynamic Control mode builds upon the basic automatic fan control loop by adjusting the value based on system performance and measured temperature. Why is this important? Instead of designing for the worst case, the system thermals can be defined as operating zones. The ADT746/ADT7463 will self-adjust its fan control loop to maintain an operating zone temperature or system target temperature. For example, you can specify that the ambient temperature in a system should be maintained at 5 C. If the temperature is below 5 C, the fans may not need to run or may run very slowly. If the temperature is higher than 5 C, the fans need to throttle up. How is this different from the automatic fan control mode? DUTY CYCLE T LOW OPERATING POINT T HIGH T THERM Figure 2. Dynamic Control Loop TEMPERATURE Figure 2 shows an overview of the parameters that affect the operation of the dynamic control loop. A brief description of each parameter follows: The challenge presented by any thermal design is finding the right settings to suit the system s fan control solution. This can involve designing for the worst case (as previously outlined), followed by weeks of system thermal characterization, and finally fan acoustic optimization (for psycho-acoustic reasons). Getting the most benefit from the automatic fan control mode involves characterizing the system to find the best and settings for the control loop, and the best value for the quietest fan speed setting. Using the ADT746/ADT7463 s dynamic control mode shortens the characterization time and alleviates tweaking the control loop settings because the device can self-adjust during system operation. DYNAMIC CONTROL THE SPECIFICS The dynamic control mode is operated by specifying the operating zone temperatures required for the system. Associated with this control mode are three operating point registers, one for each temperature channel. This allows the system thermal solution to be broken down into distinct thermal zones, e.g., CPU operating temperature = 7 C, VRM operating temperature = C, ambient operating temperature = 5 C. The ADT746/ADT7463 will dynamically alter the control 1. T LOW. If temperature drops below the T LOW limit, an error flag is set in a status register and an SMBALERT interrupt can be generated. 2. T HIGH. If temperature exceeds the T HIGH limit, an error flag gets set in a status register and an SMBALERT interrupt can be generated. 3.. This is the temperature at which the fan turns on under automatic fan speed control. 4. Operating Point. This temperature defines the target temperature or optimal operating point for a particular temperature zone. The ADT746/ADT7463 attempt to maintain system temperature at about the operating point by adjusting the parameter of the control loop. 5. T THERM. If temperature exceeds this critical limit, the fans can be run at for maximum cooling. 6.. This programs the duty cycle versus temperature control slope. DYNAMIC CONTROL PROGRAMG Since the dynamic control mode is a basic extension of the automatic fan control mode, the automatic fan control mode parameters should be programmed first. Follow the seven steps in the Automatic Fan Control section of the ADT746/ADT7463 data sheet before proceeding with dynamic control mode programming. Rev. 1 Page 15 of 27

16 STEP 8: DETERING THE OPERATING POINT FOR EACH TEMPERATURE CHANNEL The operating point for each temperature channel is the optimal temperature for that thermal zone. The hotter each zone is allowed to be, the quieter the system since the fans are not required to run at all of the time. The ADT746/ADT7463 will increase/decrease fan speeds as necessary to maintain operating point temperature. This allows for system-to-system variation and removes the need for worst-case design. As long as a sensible operating point value is chosen, any value can be selected in the system characterization. If the value is too low, the fans will run sooner than required, and the temperature will be below the operating point. In response, the ADT746/ADT7463 will increase to keep the fans off for longer and allow the temperature zone to get closer to the operating point. Likewise, too high a value will cause the operating point to be exceeded, and in turn, the ADT746/ADT7463 will reduce to turn the fans on earlier to cool the system. PROGRAMG OPERATING POINT REGISTERS There are three operating point registers, one associated with each temperature channel. These 8-bit registers allow the operating point temperatures to be programmed with 1 C resolution. OPERATING POINT REGISTERS Reg. x33 Remote 1 Operating Point = x64 (1 C) Reg. x34 Local Temp Operating Point = x64 (1 C) Reg. x35 Remote 2 Operating Point = x64 (1 C) OPERATING POINT REMOTE 2 = CPU TEMP TACHOMETER 1 1 TACH1 CPU FAN SINK MUX 2 FRONT LOCAL = VRM TEMP TACHOMETER 2 TACH2 3 REMOTE 1 = AMBIENT TEMP TACHOMETER 3 AND 4 TACH3 REAR Figure 21. Operating Point Value Dynamically Adjusts Automatic Fan Control Settings Rev. 1 Page 16 of 27

17 STEP 9: DETERING THE HIGH AND LOW LIMITS FOR EACH TEMPERATURE CHANNEL The low limit defines the temperature at which the value will start to be increased if temperature falls below this value. This has the net effect of reducing the fan speed, allowing the system to get hotter. An interrupt can be generated when the temperature drops below the low limit. The high limit defines the temperature at which the value will start to be reduced if temperature increases above this value. This has the net effect of increasing fan speed in order to cool down the system. An interrupt can be generated when the temperature rises above the high limit. PROGRAMG HIGH AND LOW LIMITS There are six limit registers; a high limit and low limit are associated with each temperature channel. These 8-bit registers allow the high and low limit temperatures to be programmed with 1 C resolution. TEMPERATURE LIMIT REGISTERS Reg. x4e Remote 1 Temp Low Limit = x81 Reg. x4f Remote 1 Temp High Limit = x7f Reg. x5 Local Temp Low Limit = x81 Reg. x51 Local Temp High Limit = x7f Reg. x52 Remote 2 Temp Low Limit = x81 Reg. x53 Remote 2 Temp High Limit = x7f Figure 22. Dynamic Control in Operation Rev. 1 Page 17 of 27

18 HOW DOES DYNAMIC CONTROL WORK? The basic premise is as follows: 1. Set the target temperature for the temperature zone, which could be, for example, the Remote 1 thermal diode. This value is programmed to the Remote 1 operating temperature register. 2. As the temperature in that zone (Remote 1 temperature) rises toward and exceeds the operating point temperature, is reduced and the fan speed increases. 3. As the temperature drops below the operating point temperature, is increased, reducing the fan speed. The loop operation is not as simple as described above. There are a number of conditions governing situations in which can increase or decrease. SHORT CYCLE AND LONG CYCLE The ADT746/ADT7463 implement two loops, a short cycle and a long cycle. The short cycle takes place every n monitoring cycles. The long cycle takes place every 2n monitoring cycles. The value of n is programmable for each temperature channel. The bits are located at the following register locations: Table II. Cycle Bit Assignments CODE Short Cycle Long Cycle 8 cycles (1 s) 16 cycles (2 s) 1 16 cycles (2 s) 32 cycles (4 s) 1 32 cycles (4 s) 64 cycles (8 s) cycles (8 s) 128 cycles (16 s) cycles (16 s) 256 cycles (32 s) cycles (32 s) 512 cycles (64 s) cycles (64 s) 124 cycles (128 s) cycles (128 s) 248 cycles (256 s) Care should be taken in choosing the cycle time. A long cycle time means that the is not updated very often; if your system has very fast temperature transients, the dynamic control loop will always be lagging. If you choose a cycle time that is too fast, the full benefit of changing may not have been realized and you change again on the next cycle; in effect you would be overshooting. It is necessary to carry out some calibration to identify the most suitable response time. Remote 1 = CYR1 = Bits <2:> of Calibration Control Register 2 (Addr = x37) Local = CYL = Bits <5:3> of Calibration Control Register 2 (Addr = x37) Remote 2 = CYR2 = Bits <7:6> of Calibration Control Register 2 and Bit of Calibration Control Register 1 (Addr = x36) Rev. 1 Page 18 of 27

19 SHORT CYCLE Figure 23 displays the steps taken during the short cycle. WAIT n MONITORING CYCLES CURRENT TEMPERATURE T1(n) OPERATING POINT TEMPERATURE OP1 IS T1(n) > (OP1 HYS) YES NO DO NOTHING PREVIOUS TEMPERATURE T1 (n 1) IS T1(n) T1(n 1).25 C NO YES DO NOTHING (i.e., SYSTEM IS COOLING OFF OR CONSTANT.) IS T1(n) T1(n 1) =.5.75 C IS T1(n) T1(n 1) = C IS T1(n) T1(n 1) > 2. C DECREASE by 1 C DECREASE by 2 C DECREASE by 4 C LONG CYCLE Figure 24 displays the steps taken during the long cycle. Figure 23. Short Cycle WAIT 2n MONITORING CYCLES CURRENT TEMPERATURE T1(n) OPERATING POINT TEMPERATURE OP1 IS T1(n) OP1 NO YES DECREASE by 1 C IS T1(n) < LOW TEMP LIMIT AND < HIGH TEMP LIMIT AND < OP1 AND T1(n) > YES INCREASE by 1 C NO DO NOT CHANGE Figure 24. Long Cycle Rev. 1 Page 19 of 27

20 EXAMPLES The following are examples of some circumstances that may cause to increase or decrease or stay the same. NORMAL OPERATION NO ADJUSTMENT 1. If measured temperature never exceeds the programmed operating point hysteresis temperature, then is not adjusted, i.e., remains at its current setting. 2. If measured temperature never drops below the low temperature limit, then is not adjusted. THERM LIMIT HIGH TEMP LIMIT OPERATING POINT LOW TEMP LIMIT HYSTERESIS ACTUAL TEMP Figure 25. Temperature between Operating Point and Low Temperature Limit Since neither the operating point hysteresis temperature nor the low temperature limit has been exceeded, the value is not adjusted and the fan runs at a speed determined by the fixed and values defined in the automatic fan speed control mode. OPERATING POINT EXCEEDED REDUCED When the measured temperature is below the operating point temperature less the hysteresis, remains the same. Once the temperature exceeds the operating temperature less the hysteresis (OP Hys), the starts to decrease. This occurs during the short cycle; see Figure 23. The rate with which decreases depends on the programmed value of n. It also depends on how much the temperature has increased between this monitoring cycle and the last monitoring cycle, i.e., if the temperature has increased by 1 C, then is reduced by 2 C. Decreasing has the effect of increasing the fan speed, thus providing more cooling to the system. If the temperature is only slowly increasing in the range (OP Hys), i.e.,.25 C per short monitoring cycle, then does not decrease. This allows small changes in temperature in the desired operating zone without changing. The long cycle makes no change to in the temperature range (OP Hys) since the temperature has not exceeded the operating temperature. Once the temperature exceeds the operating temperature, the long cycle will cause to reduce by 1 C every long cycle while the temperature remains above the operating temperature. This takes place in addition to the decrease in that would occur due to the short cycle. In Figure 26, since the temperature is only increasing at a rate less than or equal to.25 C per short cycle, no reduction in takes place during the short cycle. Once the temperature has fallen below the operating temperature, stays the same. Even when the temperature starts to increase slowly, stays the same because the temperature increases at a rate.25 C per cycle. THERM LIMIT HIGH TEMP LIMIT OPERATING POINT HYSTERESIS LOW TEMP LIMIT ACTUAL TEMP NO CHANGE IN HERE DUE TO ANY CYCLE SINCE T1(n) T1 (n 1).25 C AND T1(n) < OP = > STAYS THE SAME DECREASE HERE DUE TO SHORT CYCLE ONLY T1(n) T1 (n 1) =.5 C OR.75 C = > DECREASES BY 1 C EVERY SHORT CYCLE DECREASE HERE DUE TO LONG CYCLE ONLY T1(n) T1 (n 1).25 C AND T1(n) > OP = > DECREASES BY 1 C EVERY LONG CYCLE Figure 26. Effect of Exceeding Operating Point Hysteresis Temperature Rev. 1 Page 2 of 27

21 INCREASE CYCLE When the temperature drops below the low temperature limit, can increase in the long cycle. Increasing has the effect of running the fan slower and therefore quieter. The long cycle diagram in Figure 24 shows the conditions that need to be true for to increase. Here is a quick summary of those conditions and the reasons they need to be true. can increase if 1. The measured temperature has fallen below the low temperature limit. This means the user must choose the low limit carefully. It should not be so low that the temperature will never fall below it because would never increase and the fans would run faster than necessary. AND 2. is below the high temperature limit. is never allowed to increase above the high temperature limit. As a result, the high limit should be sensibly chosen because it determines how high can go. AND 3. is below the operating point temperature. should never be allowed to increase above the operating point temperature since the fans would not switch on until the temperature rose above the operating point. AND Figure 27 shows how increases when the current temperature is above and below the low temperature limit, and is below the high temperature limit and below the operating point. Once the temperature rises above the low temperature limit, stays the same. WHAT PREVENTS FROM REACHING FULL SCALE? Since is dynamically adjusted, it is undesirable for to reach full scale (127 C) because the fan would never switch on. As a result, is allowed to vary only within a specified range: 1. The lowest possible value to is 127 C. 2. cannot exceed the high temperature limit. 3. If the temperature is below, the fan is switched off or is running at minimum speed and dynamic control is disabled. THERM LIMIT OPERATING POINT LOW TEMP LIMIT HIGH TEMP LIMIT HYSTERESIS ACTUAL TEMP PREVENTED FROM INCREASING Figure 28. Adjustments Limited by the High Temperature Limit 4. The temperature is above. The dynamic control is turned off below. THERM LIMIT HIGH TEMP LIMIT OPERATING POINT HYSTERESIS LOW TEMP LIMIT ACTUAL TEMP Figure 27. Increasing for Quieter Operation Rev. 1 Page 21 of 27

22 STEP 1: DETERING WHETHER TO MONITOR THERM Using the operating point limit ensures that the dynamic control mode is operating in the best possible acoustic position while ensuring that the temperature never exceeds the maximum operating temperature. Using the operating point limit allows the to be independent of system level issues because of its selfcorrective nature. In PC design, the operating point for the chassis is usually the worst-case internal chassis temperature. The optimal operating point for the processor is determined by monitoring the thermal monitor in the Intel Pentium 4 processor. To do this, the PROCHOT output of the Pentium 4 is connected to the THERM input of the ADT746/ADT7463. The operating point for the processor can be determined by allowing the current temperature to be copied to the operating point register when the PROCHOT output pulls the THERM input low on the ADT746/ADT7463. This gives the maximum temperature at which the Pentium 4 can be run before clock modulation occurs. ENABLING THERM TRIP POINT AS THE OPERATING POINT Bits <4:2> of dynamic control Register 1 (Reg. x36) enable/disable THERM monitoring to program the operating point. DYNAMIC CONTROL REGISTER 1 (x36) <2> PHTR2 = 1 copies the Remote 2 current temperature to the Remote 2 operating point register if THERM gets asserted. The operating point will contain the temperature at which THERM is asserted. This allows the system to run as quietly as possible without system performance being affected. PHTR2 = ignores any THERM assertions. The Remote 2 operating point register will reflect its programmed value. <3> PHTL = 1 copies the local current temperature to the local temperature operating point register if THERM gets asserted. The operating point will contain the temperature at which THERM is asserted. This allows the system to run as quietly as possible without system performance being affected. PHTL = ignores any THERM assertions. The local temperature operating point register will reflect its programmed value. <4> PHTR1 = 1 copies the Remote 1 current temperature to the Remote 1 operating point register if THERM gets asserted. The operating point will contain the temperature at which THERM is asserted. This allows the system to run as quietly as possible without affecting system performance. PHTR1 = ignores any THERM assertions. The Remote 1 operating point register will reflect its programmed value. ENABLING DYNAMIC CONTROL MODE Bits <7:5> of dynamic control Register 1 (Reg. x36) enable/disable dynamic control on the temperature channels. DYNAMIC CONTROL REGISTER 1 (x36) <5> R2T = 1 enables dynamic control on the Remote 2 temperature channel. The chosen value will be dynamically adjusted based on the current temperature, operating point, and high and low limits for this zone. R2T = disables dynamic control. The value chosen will not be adjusted and the channel will behave as described in the Automatic Fan Control section. <6> LT = 1 enables dynamic control on the local temperature channel. The chosen value will be dynamically adjusted based on the current temperature, operating point, and high and low limits for this zone. LT = disables dynamic control. The value chosen will not be adjusted and the channel will behave as described in the Automatic Fan Control section. <7> R1T = 1 enables dynamic control on the Remote 1 temperature channel. The chosen value will be dynamically adjusted based on the current temperature, operating point, and high and low limits for this zone. R1T = disables dynamic control. The value chosen will not be adjusted and the channel will behave as described in the Automatic Fan Control section. Rev. 1 Page 22 of 27

23 ENHANCING SYSTEM ACOUSTICS Automatic fan speed control mode reacts instantaneously to changes in temperature, i.e., the duty cycle will respond immediately to temperature change. Any impulses in temperature can cause an impulse in fan noise. For psycho-acoustic reasons, the ADT746/ ADT7463 can prevent the output from reacting instantaneously to temperature changes. Enhanced acoustic mode will control the maximum change in duty cycle in a given time. The objective is to prevent the fan from cycling up and down and annoying the system user. ACOUSTIC ENHANCEMENT MODE OVERVIEW Figure 29 gives a top-level overview of the automatic fan control circuitry on the ADT746/ADT7463 and where acoustic enhancement fits in. Acoustic enhancement is intended as a post-design tweak made by a system or mechanical engineer evaluating best settings for the system. Having determined the optimal settings for the thermal solution, the engineer can adjust the system acoustics. The goal is to implement a system that is acoustically pleasing without causing user annoyance due to fan cycling. It is important to realize that although a system may pass an acoustic noise requirement spec, (e.g., 36 db), if the fan is annoying, it will fail the consumer test. THE APPROACH There are two different approaches to implementing system acoustic enhancement. The first method is temperature-centric. It involves smoothing transient temperatures as they are measured by a temperature source, e.g., Remote 1 temperature. The temperature values used to calculate the duty cycle values would be smoothed, reducing fan speed variation. However, this approach would cause an inherent delay in updating fan speed and would cause the thermal characteristics of the system to change. It would also cause the system fans to stay on longer than necessary, since the fan s reaction is merely delayed. The user would also have no control over noise from different fans driven by the same temperature source. Consider controlling a CPU cooler fan (on 1) and a chassis fan (on 2) using Remote 1 temperature. Because the Remote 1 temperature is smoothed, both fans would be updated at exactly the same rate. If the chassis fan is much louder than the CPU fan, there is no way to improve its acoustics without changing the thermal solution of the CPU cooling fan. The second approach is fan-centric. The idea is to control the duty cycle driving the fan at a fixed rate, e.g., 6%. Each time the duty cycle is updated, it is incremented by a fixed 6%. As a result, the fan ramps ACOUSTIC ENHANCEMENT REMOTE 2 = CPU TEMP T TRANGE TACHOMETER 1 1 TACH1 CPU FAN SINK LOCAL = VRM TEMP T TRANGE MUX TACHOMETER 2 2 TACH2 FRONT 3 REMOTE 1 = AMBIENT TEMP T TRANGE TACHOMETER 3 AND 4 TACH3 REAR Figure 29. Acoustic Enhancement Smooths Fan Speed Variations under Automatic Fan Speed Control Rev. 1 Page 23 of 27