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TC648VOA Datasheet(PDF) 11 Page - Microchip Technology |
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TC648VOA Datasheet(HTML) 11 Page - Microchip Technology |
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11 / 28 page  2002 Microchip Technology Inc. DS21448C-page 11 TC648 mistor resistance at TMIN. R2 is then calculated to deliver the desired speed at TMIN. The values for R1, R2 and RT1 are then checked at TMAX for 2.65V nominal. It may be necessary to adjust the values of R1 and R2 after the initial calculation to obtain the desired results. The design equations are: EQUATION EQUATION EQUATION Because the thermistor characteristics are fixed, it may not be possible, in certain applications, to obtain the desired values of VMIN and VMAX using the above equations. In this case, the circuit in Figure 5-4 can be used. Diode D1 clamps VIN to the voltage required to sustain minimum speed. The calculations of R1 and R2 for the temperature sensor are identical to the equation on the previous page. FIGURE 5-4: Minimum Fan Speed Circuit. 5.3 Auto-Shutdown Temperature Design A voltage divider on VAS sets the temperature at which 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 previously 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 the VAS voltage is: EQUATION The VAS voltage may be set using a simple resistor divider, as shown in Figure 5-5. FIGURE 5-5: VAS Circuit. R1 = (0.1)(RT1MIN) Where: RT1 = Thermistor resistance at TMIN R2 = (RT1MIN)(R1)(VMIN) (RT1MIN + R1)(VDD - VMIN) Where VMIN = the value of VIN required for minimum fan speed. VDD = Power Supply Voltage VMAX = (RT1MIN)(R1)(VMIN) R2 (R1 + RT1MAX )(VDD) Where RT1MAX = thermistor resistance at TMAX, VMAX = the value of VIN required for maximum fan speed. R1 R2 R3 R4 D1 RT1 VDD VIN 2.65 - 1.25V T2 - T1 = VAS - 1.25 TAS - T1 VAS = 1.4V T2 - T1 (TAS - T1) + 1.25 ( ) 1.4V (125 - 0) VAS = (25 - 0) + 1.25 = 1.53V R2 R1 GND VDD VAS IDIV IIN |
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