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LM1085IT-ADJ 데이터시트(PDF) 9 Page - National Semiconductor (TI) |
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LM1085IT-ADJ 데이터시트(HTML) 9 Page - National Semiconductor (TI) |
9 / 13 page Application Note (Continued) must be added, one for case to heat-sink ( θ CH) and one for heatsink to ambient ( θ HA). The junction temperature can be predicted as follows: T J =TA +PD ( θ JC + θ CH + θ HA)= TA +PD θ JA T J is junction temperature, TA is ambient temperature, and P D is the power consumption of the device. Device power consumption is calculated as follows: I IN =IL +IG P D =(VIN−VOUT)IL +VINIG Figure 6 shows the voltages and currents which are present in the circuit. Once the devices power is determined, the maximum allow- able ( θ JA(max)) is calculated as: θ JA(max) =TR(max)/PD =TJ(max −TA(max))/PD The LM1085 has different temperature specifications for two different sections of the IC: the control section and the output section. The Electrical Characteristics table shows the junc- tion to case thermal resistances for each of these sections, while the maximum junction temperatures (T J(max)) for each section is listed in the Absolute Maximum section of the datasheet. T J(max) is 125˚C for the control section, while T J(max) is 150˚C for the output section. θ JA(max) should be calculated separately for each section as follows: θ JA (max, CONTROL SECTION) = (125˚C - TA(max))/PD θ JA(max, OUTPUT SECTION) = (150˚C - TA(max))/PD The required heat sink is determined by calculating its re- quired thermal resistance ( θ HA(max)). θ HA(max) = θ JA(max) −( θ JC + θ CH) θ HA(max) should also be calculated twice as follows: θ HA(max) = θ JA (max, CONTROL SECTION) - ( θ JC (CON- TROL SECTION) + θ CH) θ HA(max)= θ JA(max, OUTPUT SECTION) - ( θ JC(OUTPUT SECTION) + θ CH) If thermal compound is used, θ CH can be estimated at 0.2 C/W. If the case is soldered to the heat sink, then a θ CH can be estimated as 0 C/W. After, θ HA(max) is calculated for each section, choose the lower of the two θ HA(max) values to determine the appropriate heat sink. If PC board copper is going to be used as a heat sink, then Figure 7 can be used to determine the appropriate area (size) of copper foil required. 10094716 FIGURE 6. Power Dissipation Diagram 10094764 FIGURE 7. Heat sink thermal Resistance vs Area www.national.com 9 |
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