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ADM1031ARQZ-R7 数据表(PDF) 17 Page - ON Semiconductor |
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ADM1031ARQZ-R7 数据表(HTML) 17 Page - ON Semiconductor |
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17 / 30 page  ADM1031 http://onsemi.com 17 Figure 28. Filtered Mode Algorithm READ TEMPERATURE CALCULATE NEW PWM DUTY CYCLE INCREMENT PREVIOUS PWM VALUE BY RAMP RATE DECREMENT PREVIOUS PWM VALUE BY RAMP RATE IS NEW PWM VALUE > PREVIOUS VALUE? YES NO The filtered 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, 4, or 8 time slots (depending on the setting of bits <6:5> of the fan filter register). If the new PWM duty cycle value is less than the previous PWM value, the previous PWM duty cycle is decremented by 1, 2, 4, or 8 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. What does an increase of 1, 2, 4, or 8 time slots actually mean in terms of PWM duty cycle? A ramp rate of 1 corresponds to one time slot, which is 1/240 of the PWM period. In filtered auto fan speed control mode, incrementing or decrementing by 1 changes the PWM output duty cycle by 0.416%. Table 8. Effect of Ramp Rates on PWM_OUT Ramp Rate PWM Duty Cycle Change 1 0.416% 2 0.833% 4 1.66% 8 3.33% So programming a ramp rate of 1, 2, 4, or 8 simply increases or decreases the PWM duty cycle by the amounts shown in Table 8, depending on whether the temperature is increasing or decreasing. Figure 29 shows remote temperature plotted against PWM duty cycle for filtered mode. The ADC sample rate is the highest sample rate; 11.25 kHz. The ramp rate is set to 8, which would correspond to the fastest ramp rate. With these settings, it took approximately 12 seconds to go from 0% duty cycle to 100% duty cycle (full−speed). The TMIN value = 32 °C and the TRANGE = 80°C. Even though the temperature increased very rapidly, the fan gradually ramps up to full speed. Figure 29. Filtered Mode with Ramp Rate = 8 140 PWM DUTY CYCLE 0 012 TIME (s) 120 100 80 60 40 20 120 0 100 80 60 40 20 RTEMP Figure 30 shows how changing the ramp rate from 8 to 4 affects the control loop. The overall response of the fan is slower. Because the ramp rate is reduced, it takes longer for the fan to achieve full running speed. In this case, it took approximately 22 seconds for the fan to reach full speed. Figure 30. Filtered Mode with Ramp Rate = 4 120 PWM DUTY CYCLE 0 0 22 TIME (s) 110 80 60 40 20 120 140 0 100 80 60 40 20 RTEMP Figure 31 shows the PWM output response for a ramp rate of 2. In this instance the fan took about 54 seconds to reach full running speed. Figure 31. Filtered Mode with Ramp Rate = 2 140 PWM DUTY CYCLE 0 0 54 TIME (s) 120 100 80 60 40 20 120 0 100 80 60 40 20 RTEMP |
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