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TC647VPA Fiches technique(PDF) 11 Page - Microchip Technology |
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TC647VPA Fiches technique(HTML) 11 Page - Microchip Technology |
11 / 28 page 2002 Microchip Technology Inc. DS21447C-page 11 TC647 FIGURE 5-3: VMIN Circuit. We can further specify R1 and R2 by the condition that the divider voltage is equal to our desired VMIN. This yields the following equation: EQUATION Solving for the relationship between R1 and R2 results in the following equation: EQUATION In this example, R1 = (1.762) R2. Substituting this rela- tionship back into the previous equation yields the resistor values: R2 = 18.1 kΩ R1 = 31.9 kΩ In this case, the standard values of 31.6 k Ω and 18.2 k Ω are very close to the calculated values and would be more than adequate. 5.3 Operations at Low Duty Cycle One boundary condition which may impact the selec- tion of the minimum fan speed is the irregular activation of the Diagnostic Timer due to the TC647 “missing” fan commutation pulses at low speeds. This is a natural consequence of low PWM duty cycles (typically 25% or less). Recall that the SENSE function detects commu- tation of the fan as disturbances in the current through RSENSE. These can only occur when the fan is ener- gized (i.e., VOUT is “on”). At very low duty cycles, the VOUT output is “off” most of the time. The fan may be rotating normally, but the commutation events are occurring during the PWM’s off-time. The phase relationship between the fan’s commutation and the PWM edges tends to “walk around” as the system operates. At certain points, the TC647 may fail to capture a pulse within the 32-cycle missing pulse detector window. When this happens, the 3-cycle Diagnostic Timer will be activated, the VOUT output will be active continuously for three cycles and, if the fan is operating normally, a pulse will be detected. If all is well, the system will return to normal operation. There is no harm in this behavior, but it may be audible to the user as the fan will accelerate briefly when the Diagnostic Timer fires. For this reason, it is recommended that VMIN be set no lower than 1.8V. 5.4 FanSense™ Network (RSENSE and CSENSE) The FanSense network, comprised of RSENSE and CSENSE, allows the TC647 to detect commutation of the fan motor (FanSense™ technology). This network can be thought of as a differentiator and threshold detector. The function of RSENSE is to convert the fan current into a voltage. CSENSE serves to AC-couple this voltage signal and provide a ground referenced input to the SENSE pin. Designing a proper SENSE network is simply a matter of scaling RSENSE to provide the necessary amount of gain (i.e., the current-to-voltage conversion ratio). A 0.1 µF ceramic capacitor is recom- mended for CSENSE. Smaller values require larger sense resistors, and higher value capacitors are bulkier and more expensive. Using a 0.1 µF results in reasonable values for RSENSE. Figure 5-4 illustrates a typical SENSE network. Figure 5-5 shows the waveforms observed using a typical SENSE network. FIGURE 5-4: SENSE Network. R2 R1 GND VDD VMIN IDIV IIN VDD x R2 R1 + R2 VMIN = VDD - VMIN VMIN R1 = R2 x Q1 GND VDD RSENSE SENSE RBASE CSENSE (0.1 µF Typ.) VOUT FAN |
Numéro de pièce similaire - TC647VPA |
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Description similaire - TC647VPA |
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