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 = (V
f
I
f
+ P
RF
) θ
jc
+ T
a
(1)
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
Note that θ
jc
, the thermal resis-
tance from diode junction to the
foot of the leads, is the sum of
two component resistances,
θ
jc
= θ
pkg
+ θ
chip
(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
V
f
is:
11600 (V
f
– I
f
R
s
)
nT
(3)
I
f
= I
S
e – 1
where n = ideality factor
T = temperature in °K
R
s
= diode series resistance
and I
S
(diode saturation current)
is given by
2 1 1
n
– 4060
(
T
–
298
)
I
s
= I
0
(
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.”
