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TC647EUA 데이터시트(PDF) 10 Page - Microchip Technology |
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TC647EUA 데이터시트(HTML) 10 Page - Microchip Technology |
10 / 28 page TC647 DS21447C-page 10 2002 Microchip Technology Inc. 5.1 Temperature Sensor Design The temperature signal connected to VIN must output a voltage in the range of 1.25V to 2.65V (typical) for 0% to 100% of the temperature range of interest. The circuit in Figure 5-2 illustrates a convenient way to provide this signal. FIGURE 5-2: Temperature Sensing Circuit. Figure 5-2 illustrates a simple temperature dependent voltage divider circuit. RT1 is a conventional 100 kΩ @ 25°C NTC thermistor, while R1 and R2 are standard resistors. The supply voltage, VDD, is divided between R2 and the parallel combination of RT1 and R1 (for con- venience, the parallel combination of RT1 and R1 will be referred to as RTEMP). The resistance of the ther- mistor at various temperatures is obtained from the manufacturer’s specifications. Thermistors are often referred to in terms of their resistance at 25°C. Gener- ally, the thermistor shown in Figure 5-2 is a non-linear device with a negative temperature coefficient (also called an NTC thermistor). In Figure 5-2, R1 is used to linearize the thermistor temperature response and R2 is used to produce a positive temperature coefficient at the VIN node. As an added benefit, this configuration produces an output voltage delta of 1.4V, which is well within the range of the VC(SPAN) specification of the TC647. A 100 k Ω NTC thermistor is selected for this application in order to keep IDIV at a minimum. For the voltage range at VIN to be equal to 1.25V to 2.65V, the temperature range of this configuration is 0°C to 50°C. If a different temperature range is required from this circuit, R1 should be chosen to equal the resistance value of the thermistor at the center of this new temperature range. It is suggested that a maxi- mum temperature range of 50°C be used with this cir- cuit due to thermistor linearity limitations. With this change, R2 is adjusted according to the following equations: EQUATION These two equations facilitate solving for the two unknown variables, R1 and R2. More information about Thermistors may be obtained from AN679, “Tempera- ture Sensing Technologies”, and AN685, “Thermistors in Single Supply Temperature Sensing Circuits”, which can be downloaded from Microchip’s website at www.microchip.com. 5.2 Minimum Fan Speed A voltage divider on VMIN sets the minimum PWM duty cycle and, thus, the minimum fan speed. As with the VIN input, 1.25V to 2.65V corresponds to 0% to 100% duty cycle. Assuming that fan speed is linearly related to duty cycle, the minimum speed voltage is given by the equation: EQUATION For example, if 2500 RPM equates to 100% fan speed, and a minimum speed of 1000 RPM is desired, then the VMIN voltage is: EQUATION The VMIN voltage may be set using a simple resistor divider as shown in Figure 5-3. Per Section 1.0, “Electrical Characteristics”, the leakage current at the VMIN pin is no more than 1 µA. It would be very conservative to design for a divider current, IDIV, of 100 µA. If VDD = 5.0V then; EQUATION R2 = 23.2 kΩ R1 = 100 kΩ RT1 NTC Thermistor 100 k Ω @ 25ºC IDIV VIN VDD VDD x R2 RTEMP (T1) + R2 = V(T1) RTEMP (T2) + R2 = V(T2) VDD x R2 Where T1 and T2 are the chosen temperatures and RTEMP is the parallel combination of the thermistor and R1. Minimum Speed Full Speed VMIN = x (1.4V) + 1.25V 1000 2500 VMIN = x (1.4V) + 1.25V = 1.81V R1 + R2 IDIV = 1e–4A = , therefore 5.0V R1 + R2 = = 50,000 Ω = 50 kΩ 1e–4A 5.0V |
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