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LM1085IT-5.0 Datasheet(PDF) 10 Page - Texas Instruments |
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LM1085IT-5.0 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 22 page LM1085 SNVS038G – JULY 1999 – REVISED MARCH 2013 www.ti.com THERMAL CONSIDERATIONS FOR THE TO-220 PACKAGE ICs heats up when in operation, and power consumption is one factor in how hot it gets. The other factor is how well the heat is dissipated. Heat dissipation is predictable by knowing the thermal resistance between the IC and ambient ( θJA). Thermal resistance has units of temperature per power (°C/W). The higher the thermal resistance, the hotter the IC. The LM1085 specifies the thermal resistance for the TO-220 package as Junction to Case ( θJC). In order to get the total resistance to ambient ( θJA), two other thermal resistances must be added, one for case to heat-sink ( θCH) and one for heatsink to ambient (θHA). The junction temperature can be predicted as follows: TJ = TA + (PD x (θJC + θCH + θHA)) (1) TJ = TA + (PD x θJA) (2) where TJ is junction temperature, TA is ambient temperature, and PD is the power dissipation of the device. Device power dissipation is calculated as follows: PD = OUTPUT Section Dissipation + CONTROL Section Dissipation (3) PD = ( (VIN - VOUT) x ILOAD) + ( (VIN - VOUT) x IGND) (4) Figure 20 shows the voltages and currents which are present in the circuit. Figure 20. Power Dissipation Diagram Once the devices power is determined, the maximum allowable ( θJA(max)) is calculated as: θJA(MAX) = TR(MAX) / PD (5) θJA(MAX)= TJ(MAX) - TA(MAX)) / PD (6) The required heat sink is determined by calculating its required thermal resistance ( θHA(MAX)). θHA(MAX) = θJA(MAX) − (θJC + θCH) (7) If thermal compound is used, θCH can be estimated at 0.2 C/W. If the case is soldered to the heat sink, then a θCH can be estimated as 0 C/W. If PC board copper is going to be used as a heat sink, then Figure 21 can be used to determine the appropriate area (size) of copper foil required. 10 Submit Documentation Feedback Copyright © 1999–2013, Texas Instruments Incorporated Product Folder Links: LM1085 |
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