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TC664 Datasheet(PDF) 11 Page - Microchip Technology |
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TC664 Datasheet(HTML) 11 Page - Microchip Technology |
11 / 36 page 2002-2013 Microchip Technology Inc. DS21737B-page 11 TC664/TC665 4.3 Fan Startup Often overlooked in fan speed control is the actual startup control period. When starting a fan from a non- operating condition (fan speed is zero RPM), the desired PWM duty cycle or average fan voltage can not be applied immediately. Since the fan is at a rest posi- tion, the fan’s inertia must be overcome to get it started. The best way to accomplish this is to apply the full rated voltage to the fan for one second. This will ensure that in all operating environments, the fan will start and operate properly. The TC664/TC665 devices implement this fan control feature without any user programming. During a power up or release from shutdown condition, the TC664/ TC665 devices force the VOUT output to a 100% duty cycle, turning the fan full on for one second (CF = 1.0 µF). Once the one second period is over, the TC664/TC665 devices will look to see if SMBus or VIN control has been selected in the Configuration Register (DUTYC bit 5<0>). Based on this register, the device will choose which input will control the VOUT duty cycle. Duty cycle control based on VIN is the default state. If VIN control is selected and the VIN pin is open (nothing is connected to the VIN pin), then the TC664/TC665 will default to a duty cycle of 39.33%. This sequence is shown in Figure 4-4. This integrated one second startup feature will ensure the fan starts up every time. FIGURE 4-4: Power-up Flow Chart. 4.4 PWM Drive Frequency (CF) As previously discussed, the TC664/TC665 devices operate with a fixed PWM frequency. The frequency of the PWM drive output (VOUT) is set by a capacitor at the CF pin. With a 1 µF capacitor at the CF pin, the typical drive frequency is 30 Hz. This frequency can be raised, by decreasing the capacitor value, or lowered, by increasing the capacitor value. The relationship between the capacitor value and the PWM frequency is linear. If a frequency of 15 Hz is desired, a capacitor value of 2.0 µF should be used. The frequency should be kept in the range of 15 Hz to 35 Hz. See Section 7.2 for more details. 4.5 Duty Cycle Control (VIN and Duty- Cycle Register) The duty cycle of the VOUT PWM drive signal can be controlled by either the VIN analog input pin or by the Duty-Cycle Register, which is accessible via the SMBus interface. The control method is selectable via DUTYC (bit 5<0>) of the Configuration Register. The default state is for VIN control. If VIN control is selected and the VIN pin is open, the PWM duty cycle will default to 39.33%. The duty cycle control method can be changed at any time via the SMBus interface. VIN is an analog input pin. A voltage in the range of 1.62 V to 2.6 V (typical) at this pin commands a 30% to 100% duty cycle on the VOUT output, respectively. If the voltage at VIN falls below the 1.62 V level, the duty cycle will not go below 30%. The relationship between the voltage at VIN and the PWM duty cycle is shown in Figure 4-5. FIGURE 4-5: PWM Duty Cycle vs. Input Voltage, VIN (Typical). For the TC665 device, if the voltage at VIN exceeds the 2.6 V (typical) level, an over temperature fault indica- tion will be given by asserting a low at the FAULT output and setting OTF (bit 5<X>) in the Status Register to a ‘1’. A thermistor network or any other voltage output ther- mal sensor can be used to provide the voltage to the VIN input. The voltage supplied to the VIN pin can actu- ally be thought of as a temperature. For example, the circuit shown in Figure 4-6 represents a typical solution for a thermistor based temperature sensing network. See Section 7.3 for more details. Power Up or Release from SHDN One Second Pulse Select SMBus NO VIN Open? NO YES YES Default PWM: 39.33% SMBus PWM Duty Cycle Control VIN PWM Duty Cycle Control 0 10 20 30 40 50 60 70 80 90 100 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 Input Voltage (VIN) |
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