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| a AN-613 APPLICATION NOTE One Technology Way * P.O. Box 9106 * Norwood, MA 02062-9106 * Tel: 781/329-4700 * Fax: 781/326-8703 * www.analog.com Programming the Automatic Fan Speed Control Loop By Mary Burke AUTOMATIC FAN SPEED CONTROL The ADT7460/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 (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 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 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 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 THERMAL CALIBRATION 100% PWM MIN RAMP CONTROL (ACOUSTIC ENHANCEMENT PWM CONFIG PWM GENERATOR PWM1 REMOTE 1 TEMP TMIN TRANGE 0% TACHOMETER 1 MEASUREMENT THERMAL CALIBRATION 100% PWM MIN RAMP CONTROL (ACOUSTIC ENHANCEMENT PWM CONFIG MUX PWM GENERATOR PWM2 LOCAL TEMP TMIN TRANGE 0% TACHOMETER 2 MEASUREMENT THERMAL CALIBRATION 100% PWM MIN RAMP CONTROL (ACOUSTIC ENHANCEMENT PWM CONFIG PWM GENERATOR PWM3 REMOTE 2 TEMP TMIN TRANGE 0% TACHOMETER 3 AND 4 MEASUREMENT Figure 1. Automatic Fan Control Block Diagram REV. 0 AN-613 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 ADT7460/ADT7463. From a systems level perspective, up to three system temperatures can be monitored and used to control three PWM outputs. 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 characteristics of each temperature channel. For example, one may decide to run the CPU fan when CPU temperature increases above 60C, and a chassis fan when the local temperature increases above 45C. 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 CONFIGURATION 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 ADT7460/ADT7463 functionality will be used? * PWM2 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 ADT7460/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 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. THERMAL CALIBRATION 100% PWM MIN RAMP CONTROL (ACOUSTIC ENHANCEMENT) PWM CONFIG PWM GENERATOR PWM1 CPU FAN SINK TACH1 REMOTE 1 = AMBIENT TEMP TMIN TRANGE 0% TACHOMETER 1 MEASUREMENT PWM MIN MUX RAMP CONTROL (ACOUSTIC ENHANCEMENT) TACHOMETER 2 MEASUREMENT PWM MIN RAMP CONTROL (ACOUSTIC ENHANCEMENT) PWM CONFIG PWM GENERATOR PWM CONFIG PWM GENERATOR THERMAL CALIBRATION 100% PWM2 FRONT CHASSIS TACH2 LOCAL = VRM TEMP TMIN TRANGE 0% THERMAL CALIBRATION 100% PWM3 REAR CHASSIS REMOTE 2 = CPU TEMP TMIN TRANGE 0% TACHOMETER 3 AND 4 MEASUREMENT TACH3 Figure 2. Hardware Configuration Example -2- REV. 0 AN-613 FRONT CHASSIS FAN TACH2 PWM1 TACH1 PWM3 REAR CHASSIS FAN VID[0:4]/VID[0.5] TACH3 D2+ D2- THERM AMBIENT TEMPERATURE 5(VRM9)/6(VRM10) PROCHOT D1+ D1- ADT7463 3.3VSB 5V 12V/VID5 SDA VCOMP CURRENT VCORE GND SCL SMBALERT ADP316x VRM CONTROLLER Figure 3. Recommended Implementation 1 4. Where 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 temperature channel. RECOMMENDED IMPLEMENTATION 1 Configuring the ADT7460/ADT7463 as in Figure 3 provides the systems designer with the following features: 1. 2. 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). 6. 2.5 V Measurement Input Used to Monitor CPU Current (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 Temperature Channel. 7. 8. 9. 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(R) P4 processor, for example, or can be used as an overtemperature THERM output. 13. SMBALERT System Interrupt Output. 3. 4. 5. REV. 0 -3- AN-613 FRONT CHASSIS FAN TACH2 PWM1 PWM2 TACH1 REAR CHASSIS FAN PWM3 VID[0:4]/VID[0.5] TACH3 D2+ D2- THERM 5(VRM9)/6(VRM10) PROCHOT AMBIENT TEMPERATURE D1+ D1- ADT7463 3.3VSB 5V 12V/VID5 SDA VCOMP CURRENT VCORE GND SCL ADP316x VRM CONTROLLER Figure 4. Recommended Implementation 2 RECOMMENDED IMPLEMENTATION 2 Configuring the ADT7460/ADT7463 as in Figure 4 provides the systems designer with the following features: 1. 2. 3. 4. 5. 6. 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 Current (connected to VCOMP output of ADP316x VRM Controller). This is used to determine CPU power consumption. 7. 8. 9. 5 V Measurement Input. VRM Temperature Uses Local Temperature Sensor. CPU Temperature Measured Using Remote 1 Temperature 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. -4- REV. 0 AN-613 STEP 2: CONFIGURING 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 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 outputs. 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, 0x5E 000 = 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 60C or if the local temperature exceeds 45C. OTHER MUX OPTIONS <7:5> (BHVR) REGISTERS 0x5C, 0x5D, 0x5E 011 = 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. MUX THERMAL CALIBRATION 100% PWM MIN RAMP CONTROL (ACOUSTIC ENHANCEMENT) PWM CONFIG PWM GENERATOR PWM1 CPU FAN SINK TACH1 REMOTE 1 = AMBIENT TEMP TMIN TRANGE 0% TACHOMETER 1 MEASUREMENT PWM MIN THERMAL CALIBRATION 100% PWM CONFIG RAMP CONTROL (ACOUSTIC ENHANCEMENT) TACHOMETER 2 MEASUREMENT PWM GENERATOR MUX LOCAL = VRM TEMP PWM2 FRONT CHASSIS TACH2 TMIN TRANGE 0% THERMAL CALIBRATION 100% PWM MIN PWM CONFIG RAMP CONTROL (ACOUSTIC ENHANCEMENT) PWM GENERATOR PWM3 REAR CHASSIS TACH3 REMOTE 2 = CPU TEMP TMIN TRANGE 0% TACHOMETER 3 AND 4 MEASUREMENT Figure 5. Assigning Temperature Channels to Fan Channels REV. 0 -5- AN-613 MUX CONFIGURATION EXAMPLE This 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 (processor) 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 0x5C 101 = Fastest speed calculated by Local and Remote 2 Temp controls PWM1. <7:5> (BHVR) PWM2 CONFIGURATION REG 0x5D 000 = Remote 1 Temp controls PWM2. <7:5> (BHVR) PWM3 CONFIGURATION REG 0x5E 000 = Remote 1 Temp controls PWM3. These settings configure the MUX, as shown in Figure 6. THERMAL CALIBRATION 100% PWM MIN RAMP CONTROL (ACOUSTIC ENHANCEMENT) TACHOMETER 1 MEASUREMENT PWM MIN 100% RAMP CONTROL (ACOUSTIC ENHANCEMENT) TACHOMETER 2 MEASUREMENT PWM MIN 100% RAMP CONTROL (ACOUSTIC ENHANCEMENT) TACHOMETER 3 AND 4 MEASUREMENT PWM CONFIG PWM GENERATOR PWM1 CPU FAN SINK REMOTE 2 = CPU TEMP TMIN TRANGE 0% MUX TACH1 PWM CONFIG PWM GENERATOR THERMAL CALIBRATION PWM2 FRONT CHASSIS LOCAL = VRM TEMP TMIN TRANGE 0% TACH2 PWM CONFIG PWM GENERATOR THERMAL CALIBRATION PWM3 REAR CHASSIS TACH3 REMOTE 1 = AMBIENT TEMP TMIN TRANGE 0% Figure 6. MUX Configuration Example -6- REV. 0 AN-613 STEP 3: DETERMINING T MIN SETTING FOR EACH THERMAL CALIBRATION CHANNEL TMIN 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., 25C for ambient channel, 30C for VRM temperature, and 40C for processor temperature. TMIN is an 8-bit twos complement value that can be programmed in 1C 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 psychoacoustic 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 REGISTERS Reg. 0x67 Remote 1 Temp TMIN = 0x5A (90C default) Reg. 0x68 Local Temp TMIN = 0x5A (90C default) Reg. 0x69 Remote 2 Temp TMIN = 0x5A (90C 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 duty cycle 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 duty cycle 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 duty cycle below TMIN - THYST. 100% PWM DUTY CYCLE 0% TMIN PWM MIN THERMAL CALIBRATION 100% PWM CONFIG RAMP CONTROL (ACOUSTIC ENHANCEMENT PWM GENERATOR PWM1 CPU FAN SINK TACH1 REMOTE 2 = CPU TEMP TMIN TRANGE 0% TACHOMETER 1 MEASUREMENT PWM MIN THERMAL CALIBRATION 100% PWM CONFIG RAMP CONTROL (ACOUSTIC ENHANCEMENT MUX LOCAL = VRM TEMP PWM GENERATOR PWM2 FRONT CHASSIS TMIN TRANGE 0% TACHOMETER 2 MEASUREMENT PWM MIN TACH2 PWM CONFIG THERMAL CALIBRATION 100% RAMP CONTROL (ACOUSTIC ENHANCEMENT PWM GENERATOR PWM3 REAR CHASSIS REMOTE 1 = AMBIENT TEMP TMIN TRANGE 0% TACHOMETER 3 AND 4 MEASUREMENT TACH3 Figure 7. Understanding the TMIN Parameter REV. 0 -7- AN-613 STEP 4: DETERMINING PWM MIN FOR EACH PWM (FAN) OUTPUT PWMMIN 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 temperature 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. 100% PWM DUTY CYCLE M2 PW M1 PW PWM2MIN PWM1MIN 0% TMIN TEMPERATURE Figure 9. Operating Two Different Fans from a Single Temperature Channel PROGRAMMING THE PWM MIN REGISTERS The PWMMIN registers are 8-bit registers that allow the minimum PWM duty cycle for each output to be configured 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: 100% PWM DUTY CYCLE PWMMIN 0% TMIN TEMPERATURE 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 PWM MIN REGISTERS Reg. 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 CYCLE It 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: 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 characteristics 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 temperature, defined by TMIN. % fan speed = PWM duty cycle x 10 -8- REV. 0 AN-613 STEP 5: DETERMINING TRANGE FOR EACH TEMPERATURE CHANNEL TRANGE is the range of temperature over which automatic fan control occurs once the programmed TMIN temperature has been exceeded. TRANGE is actually a temperature slope and not an arbitrary value, i.e., a TRANGE of 40C only holds true for PWMMIN = 33%. If PWMMIN is increased or decreased, the effective TRANGE is changed, as described later. TRANGE 100% PWM DUTY CYCLE 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. 100% PWM DUTY CYCLE 50% 33% 25% 10% 0% 30 C 40 C 45 C 54 C TMIN PWMMIN 0% TMIN TEMPERATURE Figure 10. TRANGE Parameter Affects Cooling Slope The TRANGE or fan control slope is determined by the following procedure: 1. Determine the maximum operating temperature for that channel, e.g., 70C. 2. Determine experimentally the fan speed (PWM duty cycle value) that will not exceed the temperature at the worst-case operating points, e.g., 70C is reached when the fans are running at 50% PWM 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 TRANGE value. ADT7460/ADT7463 evaluation software is available to calculate the best fit value. Ask your local Analog Devices representative for more details. 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.) where TRANGE /170 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 = 30C, TRANGE = 40C, and PWMMIN = 10% duty cycle = 26 decimal 100% TMAX = TMIN + (Max D. C. - Min D. C.) TRANGE /170 TMAX = 30C + (100% - 10%) 40C/170 TMAX = 30C + (255 - 26) 40C/170 TMAX = 84C (effective TRANGE = 54C) Example: Calculate TMAX, given TMIN = 30C, TRANGE = 40C, and PWMMIN = 25% duty cycle = 64 decimal PWM DUTY CYCLE 50% 33% 0% 30 C 40 C TMIN TMAX = TMIN + (Max D. C. - Min D. C.) TRANGE /170 TMAX = 30C + (100% - 25%) 40C/170 TMAX = 30C + (255 - 64) 40C/170 TMAX = 75C (effective TRANGE = 45C) Example: Calculate TMAX, given TMIN = 30C, TRANGE = 40C, and PWMMIN = 33% duty cycle = 85 decimal Figure 11. Adjusting PWMMIN Affects TRANGE TMAX = TMIN + (Max D. C. - Min D. C.) TRANGE /170 TMAX = 30C + (100% - 33%) 40C/170 TMAX = 30C + (255 - 85) 40C/170 TMAX = 70C (effective TRANGE = 40C) REV. 0 -9- AN-613 Example: Calculate TMAX, given TMIN = 30C, TRANGE = 40C, and PWMMIN = 50% duty cycle = 128 decimal Remember that %PWM duty cycle does not correspond to %RPM. %RPM relates to the square root of the PWM duty cycle. TMAX = TMIN + (Max D. C. - Min D. C.) TRANGE /170 TMAX = 30C + (100% - 50%) 40C/170 TMAX = 30C + (255 - 128) 40C/170 TMAX = 60C (effective TRANGE = 30C) SELECTING A T RANGE SLOPE The 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 Bits <7:4>* 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111 TRANGE 2C 2.5C 3.33C 4C 5C 6.67C 8C 10C 13.33C 16C 20C 26.67C 32C (default) 40C 53.33C 80C % fan speed = PWM duty cycle x 10 100 90 80 2C 2.5 C 3.33 C 4C 5C 6.67 C 8C 10 C 13.3 C 16 C 20 C 26.6 C 32 C 40 C 53.3 C 0 20 40 60 80 TEMPERATURE ABOVE TMIN 100 120 80 C PWM DUTY CYCLE - % 70 60 50 40 30 20 10 0 100 90 80 2C 2.5 C 3.33 C 4C 5C 6.67 C 8C 10 C 13.3 C 16 C 20 C 26.6 C 32 C 40 C 53.3 C 0 20 40 60 80 TEMPERATURE ABOVE TMIN 100 120 80 C FAN SPEED - % OF MAX 70 60 50 40 30 20 10 0 * Register 0x5F configures Remote 1 T RANGE Register 0x60 configures Local T RANGE Register 0x61 configures Remote 2 T RANGE SUMMARY OF T RANGE FUNCTION When using the automatic fan control function, the temperature at which the fan reaches full speed can be calculated by 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. TMAX = TMIN + TRANGE (1) Equation 1 only holds true when PWMMIN = 33% PWM duty cycle. Increasing or decreasing PWMMIN will change the effective TRANGE, although the fan control will still follow the same PWM duty cycle to temperature slope. The effective TRANGE for different PWMMIN values can be calculated using Equation 2. TMAX = TMIN + (Max D. C. - Min D. C.) where: (Max D. C. - Min D. C.) value. TRANGE /170 (2) TRANGE /170 = effective TRANGE -10- REV. 0 AN-613 100 90 80 2C 2.5 C 3.33 C 4C 5C 6.67 C 8C 10 C 13.3 C 16 C 20 C 26.6 C 32 C 40 C 53.3 C 0 20 40 60 80 TEMPERATURE ABOVE TMIN 100 120 80 C 100 90 80 PWM DUTY CYCLE - % 70 60 50 40 30 20 10 0 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 temperature 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%. PWM DUTY CYCLE - % 100 90 80 2C 2.5 C 3.33 C 4C 5C 6.67 C 8C 10 C 13.3 C 16 C 20 C 26.6 C 32 C 40 C 53.3 C 0 20 40 60 80 TEMPERATURE ABOVE TMIN 100 120 80 C 70 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 FAN SPEED - % OF MAX 70 60 50 40 30 20 10 0 TEMPERATURE ABOVE TMIN 100 90 80 FAN SPEED - % MAX RPM Figure 14. TRANGE, % Fan Speed Slopes with PWMMIN = 20% EXAMPLE: DETERMINING T RANGE FOR EACH TEMPERATURE CHANNEL The 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 = 80C for Ambient Temperature TRANGE = 53.3C for CPU Temperature TRANGE = 40C for VRM Temperature 70 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 TEMPERATURE ABOVE TMIN Figure 15. TRANGE, % Fan Speed Slopes for VRM, Ambient, and CPU Temperature Channels REV. 0 -11- AN-613 STEP 6: DETERMINING T THERM FOR EACH TEMPERATURE CHANNEL TTHERM 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 4C. 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 settings are configured. This allows some flexibility since a TRANGE value can be selected based on its slope, while a "hard limit," e.g., 70C, can be programmed as TMAX (the temperature at which the fan reaches full speed) by setting TTHERM to 70C. THERM REGISTERS Reg. 0x6A Remote 1 THERM limit = 0x64 (100C default) Reg. 0x6B Local Temp THERM limit = 0x64 (100C default) Reg. 0x6C Remote 2 THERM limit = 0x64 (100C default) HYSTERESIS REGISTERS Reg. 0x6D Remote 1, Local Hysteresis Register <7:4> = Remote 1 Temp Hysteresis (4C default) <3:0> = Local Temp Hysteresis (4C default) Reg. 0x6E Remote 2 Temp Hysteresis Register <7:4> = Remote 2 Temp Hysteresis (4C default) Since each hysteresis setting is four bits, hysteresis values are programmable from 1C to 15C. It is not recommended that hysteresis values ever be programmed to 0C, as this actually disables hysteresis. In effect, this would cause the fans to cycle between normal speed and 100% speed, creating unsettling acoustic noise. TRANGE 100% PWM DUTY CYCLE 0% TMIN TTHERM PWM MIN RAMP CONTROL (ACOUSTIC ENHANCEMENT) PWM CONFIG PWM GENERATOR THERMAL CALIBRATION 100% PWM1 CPU FAN SINK TACH1 REMOTE 2 = CPU TEMP TMIN TRANGE 0% TACHOMETER 1 MEASUREMENT PWM MIN PWM CONFIG RAMP CONTROL (ACOUSTIC ENHANCEMENT) TACHOMETER 2 MEASUREMENT PWM MIN RAMP CONTROL (ACOUSTIC ENHANCEMENT) PWM CONFIG PWM GENERATOR PWM GENERATOR THERMAL CALIBRATION 100% MUX LOCAL = VRM TEMP PWM2 FRONT CHASSIS TACH2 TMIN TRANGE 0% THERMAL CALIBRATION 100% PWM3 REAR CHASSIS REMOTE 1 = AMBIENT TEMP TMIN TRANGE 0% TACHOMETER 3 AND 4 MEASUREMENT TACH3 Figure 16. Understanding How TTHERM Relates to Automatic Fan Control -12- REV. 0 AN-613 STEP 7: DETERMINING T HYST FOR EACH TEMPERATURE CHANNEL 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 temperature 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 1C to 15C. Larger values of THYST prevent the fans from chattering on and off as previously described. The THYST default value is set at 4C. 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 (4C default) <3:0> = Local Temp Hysteresis (4C default) Reg. 0x6E Remote 2 Temp Hysteresis Register <7:4> = Remote 2 Temp Hysteresis (4C 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 spinning below TMIN. If the fans are always on, the THYST value has no effect on the fan when the temperature drops below TMIN. TRANGE 100% PWM DUTY CYCLE THYST 0% TMIN TTHERM PWM MIN THERMAL CALIBRATION 100% PWM CONFIG RAMP CONTROL (ACOUSTIC ENHANCEMENT) PWM GENERATOR PWM1 CPU FAN SINK TACH1 REMOTE 2 = CPU TEMP TMIN TRANGE 0% TACHOMETER 1 MEASUREMENT PWM MIN THERMAL CALIBRATION 100% PWM CONFIG RAMP CONTROL (ACOUSTIC ENHANCEMENT) MUX LOCAL = VRM TEMP PWM GENERATOR PWM2 FRONT CHASSIS TACH2 TMIN TRANGE 0% TACHOMETER 2 MEASUREMENT PWM MIN THERMAL CALIBRATION 100% PWM CONFIG RAMP CONTROL (ACOUSTIC ENHANCEMENT) PWM GENERATOR PWM3 REAR CHASSIS REMOTE 1 = AMBIENT TEMP TMIN TRANGE 0% TACHOMETER 3 AND 4 MEASUREMENT TACH3 Figure 17. The THYST Value Applies to Fan On/Off Hysteresis and THERM Hysteresis REV. 0 -13- 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 duty cycle 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 duty cycle 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 duty cycle below TMIN - THYST. DYNAMIC T MIN CONTROL MODE In addition to the automatic fan speed control mode described in the previous section, the ADT7460/ADT7463 have a mode that extends the basic automatic fan speed control loop. Dynamic T MIN 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 SECTION This section has two primary goals: 1. To show how dynamic TMIN control alleviates the need for designing for worst-case conditions. 2. To illustrate how the dynamic TMIN 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 40C air temperature at 10,000 ft to 20C 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. 2. Worst-Case Fan. Due to manufacturing tolerances, fan speeds in RPM are normally quoted with a tolerance 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 temperature. Note that fans 20% out of tolerance will negatively impact system acoustics since they run faster and generate more noise. 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 FAN I/O CARDS I/O CARDS POWER SUPPLY VENTS POWER SUPPLY FAN GOOD CPU AIRFLOW CPU POOR CPU AIRFLOW CPU FAN DRIVE BAYS DRIVE BAYS VENTS GOOD VENTING = GOOD AIR EXCHANGE POOR VENTING = POOR AIR EXCHANGE Figure 18. Chassis Airflow Issues 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. TA HEAT SINK THERMAL INTERFACE MATERIAL INTEGRATED HEAT SPREADER SUBSTRATE SA TIMS TS CA TTIM TC CS JA CTIM TIMC PROCESSOR EPOXY JTIM TTIM THERMAL INTERFACE MATERIAL TJ Figure 19. Thermal Model -14- REV. 0 AN-613 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 TMIN control function. DYNAMIC T MIN CONTROL--OVERVIEW Dynamic TMIN Control mode builds upon the basic automatic fan control loop by adjusting the TMIN 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 ADT7460/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 50C. If the temperature is below 50C, the fans may not need to run or may run very slowly. If the temperature is higher than 50C, the fans need to throttle up. How is this different from the automatic fan control mode? 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 TMIN and TRANGE settings for the control loop, and the best PWMMIN value for the quietest fan speed setting. Using the ADT7460/ADT7463's dynamic TMIN control mode shortens the characterization time and alleviates tweaking the control loop settings because the device can self-adjust during system operation. DYNAMIC T MIN CONTROL--THE SPECIFICS The dynamic TMIN 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 = 70C, VRM operating temperature = 80C, ambient operating temperature = 50C. The ADT7460/ADT7463 will dynamically alter the control solution to maintain each zone temperature as closely as possible to their target operating points. OPERATING POINT REGISTERS Reg. 0x33 Remote 1 Operating Point = 0x64 (100C) Reg. 0x34 Local Temp Operating Point = 0x64 (100C) Reg. 0x35 Remote 2 Operating Point = 0x64 (100C) PWM DUTY CYCLE TEMPERATURE TLOW TMIN OPERATING THIGH TTHERM TRANGE POINT Figure 20. Dynamic TMIN Control Loop Figure 20 shows an overview of the parameters that affect the operation of the dynamic TMIN control loop. A brief description of each parameter follows: 1. TLOW. If temperature drops below the TLOW limit, an error flag is set in a status register and an SMBALERT interrupt can be generated. 2. THIGH. If temperature exceeds the THIGH limit, an error flag gets set in a status register and an SMBALERT interrupt can be generated. 3. TMIN. 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 ADT7460/ADT7463 attempt to maintain system temperature at about the operating point by adjusting the TMIN parameter of the control loop. 5. TTHERM. If temperature exceeds this critical limit, the fans can be run at 100% for maximum cooling. 6. TRANGE. This programs the PWM duty cycle versus temperature control slope. DYNAMIC T MIN CONTROL PROGRAMMING Since the dynamic TMIN 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 ADT7460/ADT7463 data sheet before proceeding with dynamic TMIN control mode programming. REV. 0 -15- AN-613 STEP 8: DETERMINING 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 100% all of the time. The ADT7460/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 TMIN value can be selected in the system characterization. If the TMIN value is too low, the fans will run sooner than required, and the temperature will be below the operating point. In response, the ADT7460/ADT7463 will increase TMIN to keep the fans off for longer and allow the temperature zone to get closer to the operating point. Likewise, too high a TMIN value will cause the operating point to be exceeded, and in turn, the ADT7460/ADT7463 will reduce TMIN to turn the fans on earlier to cool the system. PROGRAMMING 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 1C resolution. OPERATING POINT REGISTERS Reg. 0x33 Remote 1 Operating Point = 0x64 (100C) Reg. 0x34 Local Temp Operating Point = 0x64 (100C) Reg. 0x35 Remote 2 Operating Point = 0x64 (100C) THERMAL CALIBRATION OPERATING PWM POINT MIN 100% PWM CONFIG RAMP CONTROL (ACOUSTIC ENHANCEMENT) PWM GENERATOR PWM1 CPU FAN SINK TACH1 REMOTE 2 = CPU TEMP 0% TMIN TRANGE PWM MIN TACHOMETER 1 MEASUREMENT PWM CONFIG RAMP CONTROL (ACOUSTIC ENHANCEMENT) TACHOMETER 2 MEASUREMENT PWM MIN PWM CONFIG RAMP CONTROL (ACOUSTIC ENHANCEMENT) PWM GENERATOR PWM GENERATOR THERMAL CALIBRATION 100% MUX PWM2 FRONT CHASSIS TACH2 LOCAL = VRM TEMP TMIN TRANGE 0% THERMAL CALIBRATION 100% PWM3 REAR CHASSIS TACH3 REMOTE 1 = AMBIENT TEMP TMIN TRANGE 0% TACHOMETER 3 AND 4 MEASUREMENT Figure 21. Operating Point Value Dynamically Adjusts Automatic Fan Control Settings -16- REV. 0 AN-613 STEP 9: DETERMINING THE HIGH AND LOW LIMITS FOR EACH TEMPERATURE CHANNEL The low limit defines the temperature at which the TMIN 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 TMIN 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. PROGRAMMING 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 1C resolution. TEMPERATURE LIMIT REGISTERS Reg. 0x4E Remote 1 Temp Low Limit = 0x81 Reg. 0x4F Remote 1 Temp High Limit = 0x7F Reg. 0x50 Local Temp Low Limit = 0x81 Reg. 0x51 Local Temp High Limit = 0x7F Reg. 0x52 Remote 2 Temp Low Limit = 0x81 Reg. 0x53 Remote 2 Temp High Limit = 0x7F Figure 22. Dynamic TMIN Control in Operation REV. 0 -17- AN-613 HOW DOES DYNAMIC T MIN 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, TMIN is reduced and the fan speed increases. 3. As the temperature drops below the operating point temperature, TMIN 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 TMIN can increase or decrease. SHORT CYCLE AND LONG CYCLE The ADT7460/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: Remote 1 = CYR1 = Bits <2:0> of Calibration Control Register 2 (Addr = 0x37) Local = CYL = Bits <5:3> of Calibration Control Register 2 (Addr = 0x37) Remote 2 = CYR2 = Bits <7:6> of Calibration Control Register 2 and Bit 0 of Calibration Control Register 1 (Addr = 0x36) Table II. Cycle Bit Assignments CODE 000 001 010 011 100 101 110 111 Short Cycle 8 cycles (1 s) 16 cycles (2 s) 32 cycles (4 s) 64 cycles (8 s) 128 cycles (16 s) 256 cycles (32 s) 512 cycles (64 s) 1024 cycles (128 s) Long Cycle 16 cycles (2 s) 32 cycles (4 s) 64 cycles (8 s) 128 cycles (16 s) 256 cycles (32 s) 512 cycles (64 s) 1024 cycles (128 s) 2048 cycles (256 s) Care should be taken in choosing the cycle time. A long cycle time means that the TMIN is not updated very often; if your system has very fast temperature transients, the dynamic TMIN control loop will always be lagging. If you choose a cycle time that is too fast, the full benefit of changing TMIN 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. -18- REV. 0 AN-613 SHORT CYCLE Figure 23 displays the steps taken during the short cycle. WAIT n MONITORING CYCLES CURRENT TEMPERATURE MEASUREMENT T1(n) OPERATING POINT TEMPERATURE OP1 PREVIOUS TEMPERATURE MEASUREMENT T1 (n-1) IS T1(n) > (OP1 - HYS) NO DO NOTHING YES YES DO NOTHING (i.e., SYSTEM IS COOLING OFF OR CONSTANT.) IS T1(n) - T1(n-1) 0.25 C NO IS T1(n) - T1(n-1) = 0.5 - 0.75 C IS T1(n) - T1(n-1) = 1.0 - 1.75 C IS T1(n) - T1(n-1) > 2.0 C DECREASE TMIN by 1 C DECREASE TMIN by 2 C DECREASE TMIN by 4 C Figure 23. Short Cycle LONG CYCLE Figure 24 displays the steps taken during the long cycle. WAIT 2n MONITORING CYCLES CURRENT TEMPERATURE MEASUREMENT T1(n) OPERATING POINT TEMPERATURE OP1 IS T1(n) OP1 YES DECREASE TMIN by 1 C NO IS T1(n) < LOW TEMP LIMIT AND TMIN < HIGH TEMP LIMIT YES AND TMIN < OP1 AND T1(n) > TMIN NO INCREASE TMIN by 1 C DO NOT CHANGE Figure 24. Long Cycle REV. 0 -19- AN-613 EXAMPLES The following are examples of some circumstances that may cause TMIN to increase or decrease or stay the same. NORMAL OPERATION--NO T MIN ADJUSTMENT 1. If measured temperature never exceeds the programmed operating point-hysteresis temperature, then TMIN is not adjusted, i.e., remains at its current setting. 2. If measured temperature never drops below the low temperature limit, then TMIN is not adjusted. THERM LIMIT HIGH TEMP LIMIT OPERATING POINT Once the temperature exceeds the operating temperature less the hysteresis (OP - Hys), the TMIN starts to decrease. This occurs during the short cycle; see Figure 23. The rate with which TMIN 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 1C, then TMIN is reduced by 2C. Decreasing TMIN 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., 0.25C per short monitoring cycle, then TMIN does not decrease. This allows small changes in temperature in the desired operating zone without changing TMIN. The long cycle makes no change to TMIN 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 TMIN to reduce by 1C every long cycle while the temperature remains above the operating temperature. This takes place in addition to the decrease in TMIN 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 0.25C per short cycle, no reduction in TMIN takes place during the short cycle. Once the temperature has fallen below the operating temperature, TMIN stays the same. Even when the temperature starts to increase slowly, TMIN stays the same because the temperature increases at a rate 0.25C per cycle. HYSTERESIS ACTUAL TEMP LOW TEMP LIMIT TMIN 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 TMIN value is not adjusted and the fan runs at a speed determined by the fixed TMIN and TRANGE values defined in the automatic fan speed control mode. OPERATING POINT EXCEEDED--T MIN REDUCED When the measured temperature is below the operating point temperature less the hysteresis, TMIN remains the same. THERM LIMIT HIGH TEMP LIMIT OPERATING POINT HYSTERESIS ACTUAL TEMP TMIN LOW TEMP LIMIT NO CHANGE IN TMIN HERE DUE TO ANY CYCLE SINCE T1(n) - T1 (n-1) 0.25 C AND T1(n) < OP = > TMIN STAYS THE SAME DECREASE HERE DUE TO SHORT CYCLE ONLY T1(n) - T1 (n-1) = 0.5 C OR 0.75 C = > TMIN DECREASES BY 1 C EVERY SHORT CYCLE DECREASE HERE DUE TO LONG CYCLE ONLY T1(n) - T1 (n-1) 0.25 C AND T1(n) > OP = > TMIN DECREASES BY 1 C EVERY LONG CYCLE Figure 26. Effect of Exceeding Operating Point - Hysteresis Temperature -20- REV. 0 AN-613 INCREASE T MIN CYCLE When the temperature drops below the low temperature limit, TMIN can increase in the long cycle. Increasing TMIN 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 TMIN to increase. Here is a quick summary of those conditions and the reasons they need to be true. TMIN 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 TMIN would never increase and the fans would run faster than necessary. AND 2. TMIN is below the high temperature limit. TMIN 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 TMIN can go. AND 3. TMIN is below the operating point temperature. TMIN 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 4. The temperature is above TMIN. The dynamic TMIN control is turned off below TMIN. THERM LIMIT HIGH TEMP LIMIT OPERATING POINT THERM LIMIT OPERATING POINT Figure 27 shows how TMIN increases when the current temperature is above TMIN and below the low temperature limit, and TMIN is below the high temperature limit and below the operating point. Once the temperature rises above the low temperature limit, TMIN stays the same. WHAT PREVENTS T MIN FROM REACHING FULL SCALE? Since TMIN is dynamically adjusted, it is undesirable for TMIN to reach full scale (127C) because the fan would never switch on. As a result, TMIN is allowed to vary only within a specified range: 1. The lowest possible value to TMIN is -127C. 2. TMIN cannot exceed the high temperature limit. 3. If the temperature is below TMIN, the fan is switched off or is running at minimum speed and dynamic TMIN control is disabled. HYSTERESIS LOW TEMP LIMIT HIGH TEMP LIMIT TMIN ACTUAL TEMP TMIN PREVENTED FROM INCREASING Figure 28. TMIN Adjustments Limited by the High Temperature Limit HYSTERESIS LOW TEMP LIMIT ACTUAL TEMP TMIN Figure 27. Increasing TMIN for Quieter Operation REV. 0 -21- AN-613 STEP 10: DETERMINING WHETHER TO MONITOR THERM Using the operating point limit ensures that the dynamic TMIN 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 TMIN 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(R) 4 processor. To do this, the PROCHOT output of the Pentium 4 is connected to the THERM input of the ADT7460/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 ADT7460/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 TMIN control Register 1 (Reg. 0x36) enable/disable THERM monitoring to program the operating point. DYNAMIC T MIN CONTROL REGISTER 1 (0x36) <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 = 0 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 = 0 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 = 0 ignores any THERM assertions. The Remote 1 operating point register will reflect its programmed value. ENABLING DYNAMIC T MIN CONTROL MODE Bits <7:5> of dynamic TMIN control Register 1 (Reg. 0x36) enable/disable dynamic TMIN control on the temperature channels. DYNAMIC T MIN CONTROL REGISTER 1 (0x36) <5> R2T = 1 enables dynamic TMIN control on the Remote 2 temperature channel. The chosen TMIN value will be dynamically adjusted based on the current temperature, operating point, and high and low limits for this zone. R2T = 0 disables dynamic TMIN control. The TMIN value chosen will not be adjusted and the channel will behave as described in the Automatic Fan Control section. <6> LT = 1 enables dynamic TMIN control on the local temperature channel. The chosen TMIN value will be dynamically adjusted based on the current temperature, operating point, and high and low limits for this zone. LT = 0 disables dynamic TMIN control. The TMIN value chosen will not be adjusted and the channel will behave as described in the Automatic Fan Control section. <7> R1T = 1 enables dynamic TMIN control on the Remote 1 temperature channel. The chosen TMIN value will be dynamically adjusted based on the current temperature, operating point, and high and low limits for this zone. R1T = 0 disables dynamic TMIN control. The TMIN value chosen will not be adjusted and the channel will behave as described in the Automatic Fan Control section. -22- REV. 0 AN-613 ENHANCING SYSTEM ACOUSTICS Automatic fan speed control mode reacts instantaneously to changes in temperature, i.e., the PWM duty cycle will respond immediately to temperature change. Any impulses in temperature can cause an impulse in fan noise. For psycho-acoustic reasons, the ADT7460/ ADT7463 can prevent the PWM output from reacting instantaneously to temperature changes. Enhanced acoustic mode will control the maximum change in PWM 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 ADT7460/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 PWM 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 PWM1) and a chassis fan (on PWM2) 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 PWM duty cycle driving the fan at a fixed rate, e.g., 6%. Each time the PWM duty cycle is updated, it is incremented by a fixed 6%. As a result, the fan ramps ACOUSTIC ENHANCEMENT THERMAL CALIBRATION 100% PWM MIN RAMP CONTROL (ACOUSTIC ENHANCEMENT) PWM CONFIG PWM GENERATOR PWM1 CPU FAN SINK TACH1 REMOTE 2 = CPU TEMP TMIN TRANGE 0% TACHOMETER 1 MEASUREMENT PWM MIN PWM CONFIG RAMP CONTROL (ACOUSTIC ENHANCEMENT) TACHOMETER 2 MEASUREMENT PWM MIN RAMP CONTROL (ACOUSTIC ENHANCEMENT) PWM CONFIG PWM GENERATOR PWM GENERATOR THERMAL CALIBRATION 100% MUX LOCAL = VRM TEMP PWM2 FRONT CHASSIS TACH2 TMIN TRANGE 0% THERMAL CALIBRATION 100% PWM3 REAR CHASSIS REMOTE 1 = AMBIENT TEMP TMIN TRANGE 0% TACHOMETER 3 AND 4 MEASUREMENT TACH3 Figure 29. Acoustic Enhancement Smooths Fan Speed Variations under Automatic Fan Speed Control REV. 0 -23- AN-613 smoothly to its newly calculated speed. If the temperature starts to drop, the PWM duty cycle immediately decreases by 6% every update. So the fan ramps smoothly up or down without inherent system delay. Consider controlling the same CPU cooler fan (on PWM1) and chassis fan (on PWM2) using Remote 1 temperature. The TMIN and TRANGE settings have already been defined in automatic fan speed control mode, i.e., thermal characterization of the control loop has been optimized. Now the chassis fan is noisier than the CPU cooling fan. So PWM2 can be placed into acoustic enhancement mode independently of PWM1. The acoustics of the chassis fan can therefore be adjusted without affecting the acoustic behavior of the CPU cooling fan, even though both fans are being controlled by Remote 1 temperature. This is exactly how acoustic enhancement works on the ADT7460/ADT7463. ENABLING ACOUSTIC ENHANCEMENT FOR EACH PWM OUTPUT ENHANCE ACOUSTICS REGISTER 1 (Reg. 0x62) <3> = 1 Enables acoustic enhancement on PWM1 output. ENHANCE ACOUSTICS REGISTER 2 (Reg. 0x63) <7> = 1 Enables acoustic enhancement on PWM2 output. <3> = 1 Enables acoustic enhancement on PWM3 output. EFFECT OF RAMP RATE ON ENHANCED ACOUSTICS MODE The PWM signal driving the fan will have a period, T, given by the PWM drive frequency, f, since T = 1/f. For a given PWM period, T, the PWM period is subdivided into 255 equal time slots. One time slot corresponds to the smallest possible increment in PWM duty cycle. A PWM signal of 33% duty cycle will thus be high for 1/3 255 time slots and low for 2/3 255 time slots. Therefore, 33% PWM duty cycle corresponds to a signal that is high for 85 time slots and low for 170 time slots. PWM_OUT 33% DUTY CYCLE 85 TIME SLOTS READ TEMPERATURE CALCULATE NEW PWM DUTY CYCLE IS NEW PWM VALUE > PREVIOUS VALUE? YES INCREMENT PREVIOUS PWM VALUE BY RAMP RATE NO DECREMENT PREVIOUS PWM VALUE BY RAMP RATE Figure 31. Enhanced Acoustics Algorithm The enhanced acoustics mode algorithm calculates a new PWM duty cycle based on the temperature measured. If the new PWM duty cycle value is greater than the previous PWM value, the previous PWM duty cycle value is incremented by either 1, 2, 3, 5, 8, 12, 24, or 48 time slots, depending on the settings of the enhance acoustics registers. If the new PWM duty cycle value is less than the previous PWM value, the previous PWM duty cycle is decremented by 1, 2, 3, 5, 8, 12, 24, or 48 time slots. Each time the PWM duty cycle is incremented or decremented, it is stored as the previous PWM duty cycle for the next comparison. A ramp rate of 1 corresponds to one time slot, which is 1/255 of the PWM period. In enhanced acoustics mode, incrementing or decrementing by 1 changes the PWM output by 1/255 100%. STEP 11: DETERMINING THE RAMP RATE FOR ACOUSTIC ENHANCEMENT The optimal ramp rate for acoustic enhancement can be found through system characterization after the thermal optimization has been finished. The effect of each ramp rate should be logged, if possible, to determine the best setting for a given solution. ENHANCE ACOUSTICS REGISTER 1 (Reg. 0x62) <2:0> ACOU Select the Ramp Rate for PWM1. 000 = 1 Time Slot = 35 seconds 001 = 2 Time Slots = 17.6 seconds 010 = 3 Time Slots = 11.8 seconds 011 = 5 Time Slots = 7 seconds 100 = 8 Time Slots = 4.4 seconds 101 = 12 Time Slots = 3 seconds 110 = 24 Time Slots = 1.6 seconds 111 = 48 Time Slots = 0.8 seconds 170 TIME SLOTS PWM OUTPUT (ONE PERIOD) = 255 TIME SLOTS Figure 30. 33% PWM Duty Cycle Represented in Time Slots The ramp rates in the enhanced acoustics mode are selectable from the values 1, 2, 3, 5, 8, 12, 24, and 48. The ramp rates are actually discrete time slots. For example, if the ramp rate = 8, then eight time slots will be added to the PWM high duty cycle each time the PWM duty cycle needs to be increased. If the PWM duty cycle value needs to be decreased, it will be decreased by eight time slots. Figure 31 shows how the enhanced acoustics mode algorithm operates. -24- REV. 0 AN-613 ENHANCE ACOUSTICS REGISTER 2 (Reg. 0x63) <2:0> ACOU3 Select the ramp rate for PWM3. 000 = 1 Time Slot = 35 seconds 001 = 2 Time Slots = 17.6 seconds 010 = 3 Time Slots = 11.8 seconds 011 = 5 Time Slots = 7 seconds 100 = 8 Time Slots = 4.4 seconds 101 = 12 Time Slots = 3 seconds 110 = 24 Time Slots = 1.6 seconds 111 = 48 Time Slots = 0.8 seconds <6:4> ACOU2 Select the ramp rate for PWM2. 000 = 1 Time Slot = 35 seconds 001 = 2 Time Slots = 17.6 seconds 010 = 3 Time Slots = 11.8 seconds 011 = 5 Time Slots = 7 seconds 100 = 8 Time Slots = 4.4 seconds 101 = 12 Time Slots = 3 seconds 110 = 24 Time Slots = 1.6 seconds 111 = 48 Time Slots = 0.8 seconds Another way to view the ramp rates is the time it takes for the PWM output to ramp from 0% to 100% duty cycle for an instantaneous change in temperature. This can be tested by putting the ADT7460/ADT7463 into manual mode and changing the PWM output from 0% to 100% PWM duty cycle. The PWM output takes 35 seconds to reach 100% with a ramp rate of 1 time slot selected. 140 RTEMP ( C) 120 100 100 80 Figure 33 shows how changing the ramp rate from 48 to 8 affects the control loop. The overall response of the fan is slower. Since the ramp rate is reduced, it takes longer for the fan to achieve full running speed. In this case, it took approximately 4.4 seconds for the fan to reach full speed. 120 RTEMP 100 120 140 100 80 PWM DUTY CYCLE (%) 60 60 40 40 20 20 0 80 0 0 TIME - s 4.4 Figure 33. Enhanced Acoustics Mode with Ramp Rate = 8 Figure 34 shows the PWM output response for a ramp rate of 2. In this instance, the fan took about 17.6 seconds to reach full running speed. 140 120 RTEMP ( C) 120 100 120 100 80 80 60 60 PWM CYCLE (%) 40 20 60 80 PWM DUTY CYCLE (%) 60 40 40 40 20 20 0 0 TIME - s 20 0 17.6 0 0 0.76 0 TIME - s Figure 32. Enhanced Acoustics Mode with Ramp Rate = 48 Figure 32 shows remote temperature plotted against PWM duty cycle for enhanced acoustics mode. The ramp rate is set to 48, which would correspond to the fastest ramp rate. Assume that a new temperature reading is available every 115 ms. With these settings, it took approximately 0.76 seconds to go from 33% duty cycle to 100% duty cycle (full speed). Even though the temperature increased very rapidly, the fan ramps up to full speed gradually. Figure 34. Enhanced Acoustics Mode with Ramp Rate = 2 REV. 0 -25- AN-613 Finally, Figure 35 shows how the control loop reacts to temperature with the slowest ramp rate. The ramp rate is set to 1, while all other control parameters remain the same. With the slowest ramp rate selected, it takes 35 seconds for the fan to reach full speed. 120 RTEMP ( C) 100 120 100 80 80 60 PWM DUTY CYCLE (%) 40 40 20 20 0 0 TIME - s 35 60 140 SLOWER RAMP RATES The ADT7460/ADT7463 can be programmed for much longer ramp times by slowing the ramp rates. Each ramp rate can be slowed by a factor of 4. PWM1 CONFIGURATION REGISTER (Reg. 0x5C) <3> SLOW = 1 slows the ramp rate for PWM1 by 4. PWM2 CONFIGURATION REGISTER (Reg. 0x5D) <3> SLOW = 1 slows the ramp rate for PWM2 by 4. PWM3 CONFIGURATION REGISTER (Reg. 0x5E) <3> SLOW = 1 slows the ramp rate for PWM3 by 4. The following shows the ramp-up times when the SLOW bit is set for each PWM output. ENHANCE ACOUSTICS REGISTER 1 (Reg. 0x62) <2:0> ACOU Select the ramp rate for PWM1. 000 = 140 seconds 001 = 70.4 seconds 010 = 47.2 seconds 011 = 28 seconds 100 = 17.6 seconds 101 = 12 seconds 110 = 6.4 seconds 111 = 3.2 seconds ENHANCE ACOUSTICS REGISTER 2 (Reg. 0x63) <2:0> ACOU3 Select the ramp rate for PWM3. 000 = 140 seconds 001 = 70.4 seconds 010 = 47.2 seconds 011 = 28 seconds 100 = 17.6 seconds 101 = 12 seconds 110 = 6.4 seconds 111 = 3.2 seconds <6:4> ACOU2 Select the ramp rate for PWM2. 000 = 140 seconds 001 = 70.4 seconds 010 = 47.2 seconds 011 = 28 seconds 100 = 17.6 seconds 101 = 12 seconds 110 = 6.4 seconds 111 = 3.2 seconds 0 Figure 35. Enhanced Acoustics Mode with Ramp Rate = 1 As Figures 32 to 35 show, the rate at which the fan will react to temperature change is dependent on the ramp rate selected in the enhance acoustics registers. The higher the ramp rate, the faster the fan will reach the newly calculated fan speed. Figure 36 shows the behavior of the PWM output as temperature varies. As the temperature is rising, the fan speed ramps up. Small drops in temperature will not affect the ramp-up function since the newly calculated fan speed will still be higher than the previous PWM value. The enhance acoustics mode allows the PWM output to be made less sensitive to temperature variations. This will be dependent on the ramp rate selected and programmed into the enhance acoustics registers. 90 80 70 60 50 40 30 20 10 0 RTEMP PWM DUTY CYCLE (%) Figure 36. How Fan Reacts to Temperature Variation in Enhanced Acoustics Mode (c) 2003 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective companies. -26- REV. 0 -27- -28- E03235-0-2/03(0) PRINTED IN U.S.A. |
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