The line marked “RF diode, V
out
” is the transfer curve for
the detector diode — both the HSMS‑2825 and the HSMS‑
282K exhibited the same output voltage. The data were
taken over the 50 dB dynamic range shown. To the right
is the output voltage (transfer) curve for the reference
diode of the HSMS‑2825, showing 37 dB of isolation. To
the right of that is the output voltage due to RF leakage
for the reference diode of the HSMS‑282K, demonstrating
10 dB higher isolation than the conventional part.
Such differential detector circuits generally use single
diode detectors, either series or shunt mounted diodes.
The voltage doubler offers the advantage of twice
the output voltage for a given input power. The two
concepts can be combined into the differential voltage
doubler, as shown in Figure 32.
bias
P
RF
= RF power dissipated
T
j
that
I
,
P
RF
)
θ
jc
+ T
a
Note
= (V
f
θ
f
jc
+
the thermal resistance from diode junction
Equation (1).
to the foot of the leads, is the sum of two component
resistances,
θ
jc
=
θ
pkg
+
θ
chip
Equation (2).
Package thermal resistance for the SOT‑323 and SOT‑363
package is approximately 100°C/W, and the chip thermal
11600 (V
f
- I
f
R
s
)
Equation (3).
resistance for these three families of diodes is approxi‑
nT
designer will have to add in the
mately
e
- 1
I
f
= I
S
40°C/W. The
T
j
= (V
resistance from diode case to ambient — a poor
I
f
+ P
RF
)
θ
jc
+ T
a
Equation (1).
thermal
f
choice of circuit board material or heat sink design can
make this number very high.
Equation (1)
θ
chip
would be straightforward to solve but
θ
jc
=
θ
pkg
+
2 - 4060 1 - 1
Equation
n
diode forward voltage is a
(2).
T
298
for the fact that
function of
T
Equation (4).
I
s
= I
0
e
temperature as well as
T
T
j
= (V 298+ P
RF
)
θ
jc
+
forward current. The equation,
Equation (1).
f
I
f
a
equation 3, for V
f
is:
( )
(
)
differential
amplifier
matching
network
11600 (V
f
- I
f
R
s
)
θ
jc
=
θ
pkg
+
θ
chip
nT
- 1
I
f
= I
S
e
where
1
n = ideality
2
factor
I
f
R
s
1
11600 (V
f
-
)
n - 4060 T - 298
T
T
enT
I
f
= I
= temperature in °K
- 1
S
e
s
0
298
R
s
= diode series resistance
Equation (3).
Equation (2).
Figure 32. Differential Voltage Doubler, HSMS-286P.
Here, all four diodes of the HSMS‑286P are matched in
their V
f
characteristics, because they came from adjacent
sites on the wafer. A similar circuit can be realized using
the HSMS‑286R ring quad.
Other configurations of six lead Schottky products can
be used to solve circuit design problems while saving
space and cost.
( )
( )
T
298
(
)
Equation (3).
Equation (4).
and I
S
(diode saturation current) is given by
2
n
- 4060
Thermal Considerations
The obvious advantage of the SOT‑363 over the SOT‑
143 is combination of smaller size and two extra leads.
However, the copper leadframe in the SOT‑323 and SOT‑
363 has a thermal conductivity four times higher than
the Alloy 42 leadframe of the SOT‑23 and SOT‑143, which
enables it to dissipate more power.
The maximum junction temperature for these three
families of Schottky diodes is 150°C under all operating
conditions. The following equation, equation 1, applies
to the thermal analysis of diodes:
I
s
= I
0
e
(
1T
-
1
298
)
Equation (4).
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.
T
j
= (V
f
I
f
+ P
RF
)
θ
jc
+ T
a
where
T
j
= junction temperature
θ
jc
=
θ
pkg
+
θ
chip
T
a
= diode case temperature
θ
jc
= thermal resistance
V
f
I
f
= DC power dissipated
12
Equation (1).
Equation (2).
I
f
= I
S
11600 (V
f
- I
f
R
s
)
nT
e
- 1
Equation (3).
AVAGO [ AVAGO TECHNOLOGIES LIMITED ]
