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TB3R2LD Datasheet(PDF) 8 Page - Texas Instruments |
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TB3R2LD Datasheet(HTML) 8 Page - Texas Instruments |
8 / 11 page www.ti.com APPLICATION INFORMATION Power Dissipation V Sn I Sn (1) (V Ln I Ln) (2) T J + TA ) PD q JA (3) T J + TA ) PD q JA(S) (4) q JA(S) + q JC )qCA q JB )qBA q JC )qCA)qJB)qBA (5) 40 60 80 100 120 140 0 100 200 300 400 500 D, Low−K DW, Low−K D, High−K DW, High−K Air Flow − LFM TB3R1, TB3R2 SLLS587B – NOVEMBER 2003 – REVISED MAY 2004 Note that θ JA is highly dependent on the PCB on which the device is mounted, and on the airflow over The power dissipation rating, often listed as the the device and PCB. JEDEC/EIA has defined package dissipation rating, is a function of the ambi- standardized test conditions for measuring θ JA. Two ent temperature, TA, and the airflow around the commonly used conditions are the low-K and the device. This rating correlates with the device's maxi- high-K boards, covered by EIA/JESD51-3 and mum junction temperature, sometimes listed in the EIA/JESD51-7 respectively. Figure 10 shows the absolute maximum ratings tables. The maximum low-K and high-K values of θ JA versus air flow for this junction temperature accounts for the processes and device and its package options. materials used to fabricate and package the device, in addition to the desired life expectancy. The standardized θ JA values may not accurately represent the conditions under which the device is There are two common approaches to estimating the used. This can be due to adjacent devices acting as internal die junction temperature, TJ. In both of these heat sources or heat sinks, to nonuniform airflow, or methods, the device internal power dissipation PD to the system PCB having significantly different ther- needs to be calculated This is done by totaling the mal characteristics than the standardized test PCBs. supply power(s) to arrive at the system power The second method of system thermal analysis is dissispation: more accurate. This calculation uses the power dissipation and ambient temperature, along with two device and two system-level parameters: and then subtracting the total power dissipation of the • θ JC, the junction-to-case thermal resistance, in external load(s): degrees Celsius per watt • θ JB, the junction-to-board thermal resistance, in degrees Celsius per watt The first TJ calculation uses the power dissipation • θ CA, the case-to-ambient thermal resistance, in and ambient temperature, along with one parameter: degrees Celsius per watt θ JA, the junction-to-ambient thermal resistance, in • θ BA, the board-to-ambient thermal resistance, in degrees Celsius per watt. degrees Celsius per watt. The product of PD and θJA is the junction temperature In this analysis, there are two parallel paths, one rise above the ambient temperature. Therefore: through the case (package) to the ambient, and another through the device to the PCB to the ambi- ent. The system-level junction-to-ambient thermal im- pedance, θ JA(S), is the equivalent parallel impedance of the two parallel paths: where The device parameters θ JC and θJB account for the internal structure of the device. The system-level parameters θ CA and θBA take into account details of the PCB construction, adjacent electrical and mech- anical components, and the environmental conditions including airflow. Finite element (FE), finite difference (FD), or computational fluid dynamics (CFD) pro- grams can determine θ CA and θBA. Details on using these programs are beyond the scope of this data sheet, but are available from the software manufac- turers. Figure 10. Thermal Impedance vs Air Flow 8 |
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