9

Note 5. Avago Application Note 1050, “Low Cost, Surface Mount Power Limiters.”

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.

Figure 25. Sampling Circuit.

Equation (4) is substituted into equation (3), and equa‑

tions (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.

where n = ideality factor

T = temperature in °K

R

and I

Thermal Considerations

The obvious advantage of the SOT‑323 and SOT‑363 over

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:

Note that

θ

to the foot of the leads, is the sum of two component re‑

sistances,

θ

(2)

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.

Tj = (V

(1)

where

T

T

θ

V

P

Equation (1) would be straightforward to solve but for the

fact that diode forward voltage is a function of tempera‑

ture as well as forward current. The equation for V

HSMS-282P

sampling

pulse

sample

point

sampling circuit

nT

– 1

2

1

1

298