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HSMS-282M-TR2 Datasheet(PDF) 9 Page - Agilent(Hewlett-Packard) |
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HSMS-282M-TR2 Datasheet(HTML) 9 Page - Agilent(Hewlett-Packard) |
9 / 14 page 9 Sampling Applications The six lead HSMS-282P can be used in a sampling circuit, as shown in Figure 25. As was the case with the six lead HSMS-282R in the mixer, the open bridge quad is closed with traces on the circuit board. The quad was not closed internally so that it could be used in other applications, such as illustrated in Figure 17. HSMS-282P sampling pulse sample point sampling circuit Figure 25. Sampling Circuit. Thermal Considerations The obvious advantage of the SOT-323 and SOT-363 over the SOT-23 and SOT-142 is combina- tion of smaller size and extra leads. However, the copper leadframe in the SOT-3x3 has a thermal conductivity four times higher than the Alloy 42 leadframe of the SOT-23 and SOT-143, which enables the smaller packages to dissipate more power. The maximum junction tempera- ture for these three families of Schottky diodes is 150 °C under all operating conditions. The following equation applies to the thermal analysis of diodes: Tj = (Vf If + PRF) θjc + Ta (1) where Tj = junction temperature Ta = diode case temperature θjc = thermal resistance Vf If = DC power dissipated PRF = RF power dissipated Note that θjc, the thermal resis- tance from diode junction to the foot of the leads, is the sum of two component resistances, θjc = θpkg + θchip (2) Package thermal resistance for the SOT-3x3 package is approxi- mately 100 °C/W, and the chip thermal resistance for the HSMS-282x family of diodes is approximately 40 °C/W. The designer will have to add in the thermal resistance from diode case to ambient — a poor choice of circuit board material or heat sink design can make this number very high. Equation (1) would be straightfor- ward to solve but for the fact that diode forward voltage is a func- tion of temperature as well as forward current. The equation for Vf is: 11600 (Vf – If Rs) nT (3) If = IS e – 1 where n = ideality factor T = temperature in °K Rs = diode series resistance and IS (diode saturation current) is given by 2 1 1 n – 4060 ( T – 298) Is = I0 ( T ) e 298 (4) Equation (4) is substituted into equation (3), and equations (1) and (3) are solved simultaneously to obtain the value of junction temperature for given values of diode case temperature, DC power dissipation and RF power dissipation. Diode Burnout Any Schottky junction, be it an RF diode or the gate of a MESFET, is relatively delicate and can be burned out with excessive RF power. Many crystal video receivers used in RFID (tag) applications find themselves in poorly controlled environments where high power sources may be present. Examples are the areas around airport and FAA radars, nearby ham radio operators, the vicinity of a broadcast band transmitter, etc. In such environments, the Schottky diodes of the receiver can be protected by a device known as a limiter diode.[5] Formerly available only in radar warning receivers and other high cost electronic warfare applications, these diodes have been adapted to commercial and consumer circuits. Agilent offers a complete line of surface mountable PIN limiter diodes. Most notably, our HSMP- 4820 (SOT-23) can act as a very fast (nanosecond) power-sensitive switch when placed between the antenna and the Schottky diode, shorting out the RF circuit temporarily and reflecting the excessive RF energy back out the antenna. [5] Agilent Application Note 1050, “Low Cost, Surface Mount Power Limiters.” 88759/02-5.PM6.0J 2001.04.25, 6:42 PM Page 9 Adobe PageMaker 6.0J/PPC |
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