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.
sample
point
HSMS-282P
Note that
θ
jc
, the thermal resistance from diode junction
to the foot of the leads, is the sum of two component re‑
sistances,
θ
jc
=
θ
pkg
+
θ
chip
(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.
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
f
is:
11600 (V
f
– I
f
R
s
)
nT
e 11600 (V
f
– I
f
R
s
) – 1
nT
e
–1
sampling
pulse
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 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 con‑
ditions. The following equation applies to the thermal
analysis of diodes:
Tj = (V
f
I
f
+ P
RF
)
θ
jc
+ T
a
where
T
j
= junction temperature
T
a
= diode case temperature
θ
jc
= thermal resistance
V
f
I
f
= DC power dissipated
P
RF
= RF power dissipated
(1)
I
s
= I
0
I
s
= I
0
I
f
= I
S
I
f
= I
S
where
(3)
n = ideality factor
T = temperature in °K
R
s
= diode series resistance
and I
S
(diode saturation current) is given by
2
n – 4060
T
2
e
298
n – 4060
T
( )
(
298
)
e
(
1
T
1
(
T
)
–
)
298
–
1
298
1
(4)
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.
9
AVAGO [ AVAGO TECHNOLOGIES LIMITED ]
