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LM78M12CH bảng dữ liệu(PDF) 7 Page - National Semiconductor (TI) |
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7 / 11 page Design Considerations (Continued) The LM78MXX/LM341XX regulators have internal thermal shutdown to protect the device from over-heating. Under all possible operating conditions, the junction temperature of the LM78MXX/LM341XX must be within the range of 0˚C to 125˚C. A heatsink may be required depending on the maxi- mum power dissipation and maximum ambient temperature of the application. To determine 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 shows the voltages 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): θ JA = TR (max)/PD If the maximum allowable value for θ JA˚C/w is found to be ≥60˚C/W for TO-220 package or ≥92˚C/W for TO-252 pack- age, no heatsink is needed since the package alone will dis- sipate enough heat to satisfy these requirements. If the cal- culated value for θ JA fall 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 JA are shown at the end of the Application Note Section. reflects the same test results as what are in the Table 1 shows the maximum allowable power dissipation vs. ambi- ent temperature for theTO-252 device. 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 TO-252 package. TABLE 1. θ JA Different Heatsink Area Layout Copper Area Thermal Resistance Top Sice (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 52 610 47 7 0 0.2 84 8 0 0.4 70 9 0 0.6 63 10 0 0.8 57 11 0 1 57 12 0.066 0.066 89 13 0.175 0.175 72 14 0.284 0.284 61 15 0.392 0.392 55 16 0.5 0.5 53 *Tab of device attached to topside copper DS010484-23 FIGURE 1. Cross-sectional view of Integrated Circuit Mounted on a printed circuit board. Note that the case temperature is measured at the point where the leads contact with the mounting pad surface DS010484-24 FIGURE 2. Power Dissipation Diagram www.national.com 7 |
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