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ADT7490ARQZ-R7 Datasheet(PDF) 41 Page - ON Semiconductor |
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ADT7490ARQZ-R7 Datasheet(HTML) 41 Page - ON Semiconductor |
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41 / 75 page  ADT7490 http://onsemi.com 41 Step 8: THYST for Temperature Channels THYST is the amount of extra cooling a fan provides after the temperature measured has dropped back below TMIN before the fan turns off. The premise for temperature hysteresis (THYST) is that, without it, the fan would merely chatter, or cycle on and off regularly, whenever the temperature is hovering at about the TMIN setting. The THYST value chosen determines the amount of time needed for the system to cool down or heat up as the fan is turning on and off. Values of hysteresis are programmable in the range of 1 °C to 15°C. Larger values of THYST prevent the fans from chattering on and off. The THYST default value is set at 4 °C. The THYST setting applies not only to the temperature hysteresis for fan on/off, but the same setting is used for the TTHERM hysteresis value, described in the Step 7: TTHERM for Temperature Channels section. Therefore, programming Register 0x6D and Register 0x6E sets the hysteresis for both fan on/off and the THERM function. In some applications, it is required that fans not turn off below TMIN, but remain running at PWMMIN. Bits [7:5] of Enhanced Acoustics Register 1 (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. Figure 60. The THYST Value Applies to Fan On/Off Hysteresis and THERM Hysteresis REAR CHASSIS FRONT CHASSIS CPU FAN SINK LOCAL = VRM TEMP PWM1 PWM2 TACH1 TACH2 TACH3 PWM3 REMOTE 1 = AMBIENT TEMP REMOTE 2 = CPU TEMP MUX THERMAL CALIBRATION 0% TMIN TRANGE THERMAL CALIBRATION 100% 0% TMIN TRANGE THERMAL CALIBRATION 100% 0% TMIN TRANGE PWM MIN PWM MIN PWM MIN 100% TMIN 0% 100% TRANGE TTHERM RAMP CONTROL (ACOUSTIC ENHANCEMENT) RAMP CONTROL (ACOUSTIC ENHANCEMENT) RAMP CONTROL (ACOUSTIC ENHANCEMENT) PWM GENERATOR PWM GENERATOR TACHOMETER 3 AND 4 MEASUREMENT TACHOMETER 1 MEASUREMENT TACHOMETER 2 MEASUREMENT PWM GENERATOR PWM CONFIG PWM CONFIG PWM CONFIG THERM Hysteresis Any hysteresis programmed via Register 0x6D and Register 0x6E also applies hysteresis on the appropriate THERM channel. Enhanced Acoustics Register 1 (Register 0x62) Bit 7 (MIN3) = 0, PWM3 is off (0% PWM duty cycle) when temperature 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 the temperature is below TMIN − THYST. Bit 6 (MIN2) = 1, PWM2 runs at PWM2 minimum duty cycle below TMIN − THYST. |
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