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TC646EOA датащи(PDF) 11 Page - Microchip Technology |
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TC646EOA датащи(HTML) 11 Page - Microchip Technology |
11 / 28 page 2002 Microchip Technology Inc. DS21446C-page 11 TC646 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 shows a simple temperature dependent voltage divider circuit. RT1 is a conventional NTC ther- mistor, while 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 resistance of the thermistor at various temperatures is obtained from the manufacturer’s specifications. Thermistors 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 TC646. 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 web site at www.microchip.com. 5.2 Auto-Shutdown Temperature Design A voltage divider on VAS sets the temperature where the part is automatically shut down if the sensed temperature at VIN drops below the set temperature at VAS (i.e., VIN < VAS). As with the VIN input, 1.25V to 2.65V corresponds to the temperature range of interest from T1 to T2, respectively. Assuming that the temperature sensor network designed above is linearly related to temperature, the shutdown temperature TAS is related to T2 and T1 by: EQUATION For example, if 1.25V and 2.65V at VIN corresponds to a temperature range of T1 = 0°C to T2 = 125°C, and the auto-shutdown temperature desired is 25°C, then VAS voltage is: EQUATION The VAS voltage may be set using a simple resistor divider as shown in Figure 5-3. R2 = 23.2kΩ R1 = 100 kΩ NTC Thermistor 100 k Ω@25˚C IDIV VIN VDD 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. 2.65V - 1.25V T2 - T1 = VAS - 1.25V TAS - T1 VAS = ( ( TAS - T1) + 1.25V 1.4V ) T2 - T1 VAS = (25 - 0) + 1.25V = 1.53V 1.4V (125 - 0) |
Аналогичный номер детали - TC646EOA |
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Аналогичное описание - TC646EOA |
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