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LM317 Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM317 Datasheet(HTML) 10 Page - National Semiconductor (TI) |
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10 / 25 page  Application Hints (Continued) The LM317 regulators have internal thermal shutdown to protect the device from over-heating. Under all possible operating conditions, the junction temperature of the LM317 must be within the range of 0˚C to 125˚C. A heatsink may be required depending on the maximum power dissipation and maximum ambient temperature of the application. To deter- mine if a heatsink is needed, the power dissipated by the regulator, P D, must be calculated: I IN =IL +IG P D =(VIN −VOUT)IL +VINIG Figure 8 shows the voltage and currents which are present in the circuit. The next parameter which must be calculated is the maxi- mum allowable temperature rise, T R(max): T R(max) = TJ(max) − TA(max) where T J(max) is the maximum allowable junction tempera- ture (125˚C), and T A(max) is the maximum ambient tem- perature which will be encountered in the application. Using the calculated values for T R(max) and PD, the maxi- mum allowable value for the junction-to-ambient thermal resistance ( θ JA) can be calculated: θ JA =TR(max)/PD If the maximum allowable value for θ JA is found to be ≥92˚C/W (Typical Rated Value) for TO-252 package, no heatsink is needed since the package alone will dissipate enough heat to satisfy these requirements. If the calculated value for θ JA falls below these limits, a heatsink is required. As a design aid, Table 1 shows the value of the θ JA of TO-252 for different heatsink area. The copper patterns that we used to measure these θ JAs are shown at the end of the Application Notes Section. Figure 9 reflects the same test results as what are in the Table 1 Figure 10 shows the maximum allowable power dissipation vs. ambient temperature for the TO-252 device. Figure 11 shows the maximum allowable power dissipation vs. copper area (in 2) for the TO-252 device. Please see AN1028 for power enhancement techniques to be used with SOT-223 and TO-252 packages. TABLE 1. θ JA Different Heatsink Area Layout Copper Area Thermal Resistance Top Side (in 2)* Bottom Side (in 2)( θ JA˚C/W) TO-252 1 0.0123 0 103 2 0.066 0 87 3 0.3 0 60 4 0.53 0 54 5 0.76 0 52 61 0 47 7 0 0.2 84 8 0 0.4 70 9 0 0.6 63 00906357 FIGURE 6. θ (J−A) vs Copper (2 ounce) Area for the SOT-223 Package 00906358 FIGURE 7. Maximum Power Dissipation vs T AMB for the SOT-223 Package 00906360 FIGURE 8. Power Dissipation Diagram www.national.com 10 |
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Similar Description - LM317 |
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