поискавой системы для электроныых деталей |
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LM3940 датащи(PDF) 7 Page - National Semiconductor (TI) |
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LM3940 датащи(HTML) 7 Page - National Semiconductor (TI) |
7 / 11 page Application Hints (Continued) If a manufactured heatsink is to be selected, the value of heatsink-to-ambient thermal resistance, θ (H−A), must first be calculated: θ (H−A) = θ (JA) − θ (C−H) − θ (J−C) Where: θ (J−C) is defined as the thermal resistance from the junction to the surface of the case. A value of 4˚C/W can be assumed for θ (J−C) for this calculation. θ (C−H) is defined as the thermal resistance between the case and the surface of the heatsink. The value of θ (C−H) will vary from about 1.5˚C/W to about 2.5˚C/W (depending on method of at- tachment, insulator, etc.). If the exact value is unknown, 2˚C/W should be assumed for θ (C−H). When a value for θ (H−A) is found using the equation shown, a heatsink must be selected that has a value that is less than or equal to this number. θ (H−A) is specified numerically by the heatsink manufacturer in the catalog, or shown in a curve that plots temperature rise vs. power dissipation for the heatsink. HEATSINKING TO-263 AND SOT-223 PACKAGE PARTS Both the TO-263 (“S”) and SOT-223 (“MP”) packages use a copper plane on the PCB and the PCB itself as a heatsink. To optimize the heat sinking ability of the plane and PCB, solder the tab of the package to the plane. Figure 3 shows for the TO-263 the measured values of θ (JA) for different copper area sizes using a typical PCB with 1 ounce copper and no solder mask over the copper area used for heatsinking. As shown in the figure, increasing the copper area beyond 1 square inch produces very little improvement. It should also be observed that the minimum value of θ (JA) for the TO-263 package mounted to a PCB is 32˚C/W. As a design aid, Figure 4 shows the maximum allowable power dissipation compared to ambient temperature for the TO-263 device (assuming θ (JA) is 35˚C/W and the maximum junction temperature is 125˚C). Figure 5 and Figure 6 show the information for the SOT-223 package. Figure 6 assumes a θ (JA) of 74˚C/W for 1 ounce copper and 51˚C/W for 2 ounce copper and a maximum junction temperature of 125˚C. Please see AN1028 for power enhancement techniques to be used with the SOT-223 package. 01208007 FIGURE 3. θ (JA) vs. Copper (1 ounce) Area for the TO-263 Package 01208008 FIGURE 4. Maximum Power Dissipation vs. T AMB for the TO-263 Package 01208011 FIGURE 5. θ (JA) vs. Copper (2 ounce) Area for the SOT-223 Package 01208012 FIGURE 6. Maximum Power Dissipation vs. T AMB for the SOT-223 Package www.national.com 7 |
Аналогичный номер детали - LM3940_03 |
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Аналогичное описание - LM3940_03 |
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