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AN-0003 Dataheets PDF



Part Number AN-0003
Manufacturers ANADIGICS
Logo ANADIGICS
Description Thermal Considerations
Datasheet AN-0003 DatasheetAN-0003 Datasheet (PDF)

Thermal Considerations for PAs AN-0003 Thermal Considerations for Power Amplifiers Overview Proper heatsinking to control junction temperature is an extremely important consideration for use of all power amplifiers. Gallium Arsenide (GaAs) devices can tolerate considerably higher junction temperatures than Silicon (Si), but due to its lower thermal conductivity, it requires more consideration than Si to remove heat. The thermal conductivity of GaAs is only about one third that of Si, and it has.

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Thermal Considerations for PAs AN-0003 Thermal Considerations for Power Amplifiers Overview Proper heatsinking to control junction temperature is an extremely important consideration for use of all power amplifiers. Gallium Arsenide (GaAs) devices can tolerate considerably higher junction temperatures than Silicon (Si), but due to its lower thermal conductivity, it requires more consideration than Si to remove heat. The thermal conductivity of GaAs is only about one third that of Si, and it has a nonlinear relationship with temperature (the conductivity worsens with increasing temperature). Although many factors inside and outside the package influence the junction temperature, the end user of the power amplifier can control the implementation of the connection and layout on the printed circuit board (PCB). The choice of the board material and thickness, the number of thermal vias placed beneath the part and the design of the heatsink are all important factors in properly using the part under difficult thermal requirements. Application Requirements ANADIGICS power amplifiers are typically packaged in LPCC (Leadless Plastic Chip Carrier) packages. These packages offer excellent thermal characteristics due to the thin copper paddle used for mounting the chip. The part itself (bare die) is also very thin and enables very good heat dissipation. A cross section of the package is shown in Figure 1. Gold Wire Mold Compound DIE 0.9mm Lead Solder Plating Copper Lead Frame Die Attach Epoxy Figure 1. Cross section of an LPCC package. The thermal design of such a part involves maintaining the junction temperature below a certain level, defined as Tmax. The junction temperature depends directly on the case temperature, defined as the temperature at the bottom of the copper lead frame. Although each application is different, junction temperatures of 150°C are considered desirable with an almost infinite device life. Junction temperatures of 180°C are typical in many applications and yield device MTTFs (Median-Time-To-Failures) better than 1 million hours (114 years). Figure 2 indicates the MTTF curves1 for the fabrication processes used in the ANADIGICS MESFET power amplifiers. Various applications may require different maximum junction temperatures depending upon the MTTF requirements. 05/2003 http://www.Datasheet4U.com AN-0003 Figure 2. Typical MTTF data for TriQuint MESFET, HFET, and PHEMT. In some applications, the device performance needs to be de-rated to ensure that the maximum acceptable junction temperature (Tmax) is not exceeded. Figure 3 shows a generic de-rating curve. The device can dissipate the maximum power for case temperatures less than Td. For case temperatures greater than Td, the dissipated power is limited as indicated in Figure 3 in order to keep the junction temperature below Tmax. ANADIGICS performs detailed thermal analysis of the junction to case temperature using finite difference software. These analysis results have been verified through the use of infrared microscope measurements, and are used to develop the de-rating curves. Di ss ip at ed p ow er Pmax Td T ma x Case te mp era tur e Figure 3. Generic de-rating curve. 2 05/2003 AN-0003 ANADIGICS has developed a generic thermal analysis for power amplifiers. Although each design has unique implementation requirements, the typical dimensions of a 4x4 mm part are presented in Figure 4. 0.1 mm 0.01 mm 0.2 mm 0.9 mm PCB board Thermal vias 2.8 mm 4 mm 2.8 mm 4 mm Figure 4. Typical dimensions of a 4x4 mm part. PCB Thermal Capability In typical applications the user should place a large number of thermal vias beneath the mounting pad for the paddle. For a 4x4 mm part mounted on a 0.032" (0.8 mm) PC board, it is possible to place approximately 16 vias under the part with each fabricated using a 0.014" (0.35 mm) drill. This drill size is typically used for holes of 0.010" (0.25 mm) finished size. Typically the plating thickness will be 0.001" (0.025mm). Thicker copper plating will lower the thermal resistance, helping to reduce the junction temperature. The thermal resistance of a single copper via can be calculated as: RTH = L 2 2 RO − RI σ π () * 05/2003 3 AN-0003 Where RTH is the thermal resistance in °C/W, σ is the thermal conductivity in W/m*K, and the radii (R O and RI) and length (L) of each via are in meters. For a 0.032" thick board, with 0.001" plating, and using a thermal conductivity of 390 W/m*K, the thermal resistance of a single via is on the order of 80°C/W. The total thermal resistance is simply the result of Equation 1 divided by the total number of thermal vias beneath the part. For 16 thermal vias this yields approximately 5°C/W. Thermal simulations are also used in ANADIGICS IC design process. Figure 5 shows the results from an FR4 board simulation and the temperature distribution through the 16 thermal vias. The simulation included a 3.4W power amplifier die, which is not shown. The expected tem.


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