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TC648EPA Folha de dados(PDF) 10 Page - Microchip Technology |
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TC648EPA Folha de dados(HTML) 10 Page - Microchip Technology |
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10 / 28 page  TC648 DS21448C-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 pro- vide this signal using a temperature dependent voltage divider circuit. FIGURE 5-2: Temperature Sensing Circuit. RT1 is a conventional NTC thermistor and R1 and R2 are standard resistors. The supply voltage (VDD) is divided between R2 and the parallel combination of RT1 and R1. For convenience, the parallel combination of RT1 and R1 will be referred to as RTEMP. The resis- tance of the thermistor at various temperatures is obtained from the manufacturer’s specifications. Ther- mistors are often referred to in terms of their resistance at 25°C. 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 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 TC648. A 100 k Ω NTC thermistor is selected for this application in order to keep IDIV to 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 web site at www.microchip.com. 5.2 Minimum Speed Mode The TC648 is configured for minimum speed mode by grounding VAS and designing the temperature sensor network such that VIN operates the fan at relatively con- stant, minimum speed when the thermistor is at minimum temperature. Figure 5-3 shows operation in minimum speed mode. The 0% and 100% fan speeds correspond to VIN values of 1.25V and 2.65V, typical. Minimum system temperature (TMIN) is defined as the lowest measured temperature at which proportional fan speed control is required by the system. The fan operates at minimum speed for all temperatures below TMIN and at speeds proportional to the measured temperature between TMIN and TMAX. FIGURE 5-3: Minimum Fan Speed Mode Operation. Temperature sensor design consists of a two-point calculation: one at TMIN and one at TMAX. At TMIN, the ohmic value of the thermistor must be much higher than that of R1 so that minimum speed is determined primarily by the values of R1 and R2. At TMAX, the ohmic value of the thermistor must result in a VIN of 2.65V nominal. The design procedure consists of ini- tially choosing R1 to be 10 times smaller than the ther- R2 = 23.2 kΩ R1 = 100 kΩ IDIV VIN VDD NTC Thermistor 100 k Ω @25˚C RT1 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. Fan Speed 100% Minimum Speed 0% TMIN TMAX |
Nº de peça semelhante - TC648EPA |
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Descrição semelhante - TC648EPA |
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