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TC642 Folha de dados(PDF) 10 Page - Microchip Technology |
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TC642 Folha de dados(HTML) 10 Page - Microchip Technology |
10 / 28 page TC642 DS21444C-page 10 2002 Microchip Technology Inc. FIGURE 5-2: Temperature Sensing Circuit. Generally, 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, while 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 TC642. 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. With this change, R2 is adjusted according to the formulas below. It is suggested that a maximum temperature range of 50°C be used with this circuit due to thermistor linearity limitations. The following two equations permit solving for the two unknown variables, R1 and R2. More information regarding thermistors can be found in AN679, “Temper- ature Sensing Technologies”, and AN685, “Thermistors in Single Supply Temperature Sensing Circuits”, which can be downloaded from Microchip’s web site at: www.microchip.com. EQUATION 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 typically 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 FIGURE 5-3: VIN Circuit. R2 = 23.2 kΩ R1 = 100 kΩ NTC Thermistor 100 k Ω @ 25˚C RT1 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 (1.4) + 1.25V 1000 2500 VMIN = x (1.4) + 1.25V = 1.81V R1+ R2 IDIV = 100µA = , therefore 5.0V R1 + R2 = = 50,000 Ω = 50kΩ 100µA 5.0V R2 R1 GND VDD VMIN IDIV IIN |
Nº de peça semelhante - TC642 |
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Descrição semelhante - TC642 |
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