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MGA83563BLK Datasheet(PDF) 10 Page - Agilent(Hewlett-Packard) |
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MGA83563BLK Datasheet(HTML) 10 Page - Agilent(Hewlett-Packard) |
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10 / 24 page  10 Thermal Considerations The DC power dissipation of the MGA-83563, which can be on the order of 0.5 watt, is approaching the thermal limits of subminiature packaging such as the SOT-363. As a result, particular care should be taken to adequately heatsink the MGA-83563. The primary heat path from the MMIC chip to the system heatsink is by means of conduction through the package leads and ground vias to the groundplane on the backside of the PCB. As previously mentioned in the “PCB Layout” section, the use of multiple vias near all of the ground pins is desirable for low inductance. The use of multiple vias is also an especially impor- tant part of the heatsinking function. For heatsinking purposes, a thinner PCB with more vias, thicker clad metal, and heavier plating in the vias all result in lower thermal resistance and better heat conduction. Circuit boards thicker than 0.031 inches are not recommended for both thermal and electrical reasons. The importance of good thermal design on reliability is discussed in the next section. Thermal Design for Reliability Good thermal design is an important consideration in the reliable use of medium power devices such as the MGA-83563 because the Mean Time To Failure (MTTF) of semiconductor devices is inversely proportional to the operating temperature. The following examples show the thermal prerequisites for using the MGA-83563 reliably in both saturated and linear modes. Saturated Mode Thermal Example Less heat is dissipated in the MGA-83563 when operated in the saturated mode because a significant amount of power is removed from the RFIC as RF signal power. It is for this reason that the saturated mode allows the device to be used reliably at higher circuit board temperatures than for full power, linear applications. As an illustration of a thermal/ reliability calculation, consider the case of an MGA-83563 biased at 3.0 volts for use in a saturated mode application with a MTTF reliability goal of 106 hours (114 years). Reliability calcula- tions will first be presented for nominal conditions, followed by the conservative approach of using worst-case conditions. The first step is to calculate the power dissipated by the MGA-86353 as heat. Power flow for the MGA-83563 is represented in Figure 22. Σ Pn = 0 PDC Pin Pout Pdiss HEAT Figure 22. Thermal Representation of MGA-83563. From Figure 22, Pin + PDC = Pout + Pdiss where Pin and Pout are the RF input and output power, PDC is the DC input power, and Pdiss is the power dissipated as heat. For the saturated mode, Pout = Psat , and, Pdiss = Pin + PDC – Psat From the table of Electrical Specifications, the device current (typical) is 152 mA with a power supply voltage of 3 volts. Refer- ring to Figure 10, it can be seen that the current will decrease approximately 8% at elevated temperatures. The device DC power consumption is then: PDC = 3.0 volts * 152 mA * 0.92 PDC = 420 mW For a saturated amplifier, the RF input power level is +4 dBm (2.51 mW) and the saturated output power is +22 dBm (158 mW). The power dissipated as heat is then: Pdiss = 2.51 + 420 – 158 mW Pdiss = 264 mW The channel-to-case thermal resistance ( θ ch-c) from the table of Absolute Maximum Ratings is 175 °C/watt. Note that the mean- ing of “case” for packages such as the SOT-363 is defined as the interface between the package pins and the mounting surface, i.e., at the PCB pads. The tem- perature rise from the mounting surface to the MMIC channel is then calculated as 0.264 watt * 175 °C/watt, or 46°C. |
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