Datasheet HSMS-282x (Broadcom) - 9

Sampling Applications
Equation (1) would be straightforward to solve but for the fact that diode forward voltage is a function of tempera‑ The six lead HSMS‑282P can be used in a sampling circuit, ture as well as forward current. The equation for V is: as shown in Figure 25. As was the case with the six lead f HSMS‑282R in the mixer, the open bridge quad is closed with traces on the circuit board. The quad was not closed 11600 (Vf – If R s ) internally so that it could be used in other applications, nT I f = I S e – 1 such as illustrated in Figure 17. where n = ideality factor
sample point HSMS-282P
T = temperature in °K R = diode series resistance s and I (diode saturation current) is given by S 2 1 1 ) n – 4060 ( T – 298 I
sampling
s = I 0 ( T ) e 298
pulse sampling circuit Figure 25. Sampling Circuit.
Equation (4) is substituted into equation (3), and equa‑ tions (1) and (3) are solved simultaneously to obtain the
Thermal Considerations
value of junction temperature for given values of diode case temperature, DC power dissipation and RF power The obvious advantage of the SOT‑323 and SOT‑363 over dissipation. the SOT‑23 and SOT‑142 is combination 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 temperature for these three fami‑ lies of Schottky diodes is 150°C under all operating condi‑ tions. The following equation applies to the thermal anal‑ ysis of diodes: Tj = (V I + P ) θ + T (1) f f RF jc a where T = junction temperature j T = diode case temperature a θ = thermal resistance jc V I = DC power dissipated f f P = RF power dissipated RF Note that θ , the thermal resistance from diode junction jc to the foot of the leads, is the sum of two component re‑ sistances, θ = θ + θ (2) jc pkg chip Package thermal resistance for the SOT‑3x3 package is ap‑ proximately 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. Note 5. Avago Application Note 1050, “Low Cost, Surface Mount Power Limiters.” 9