|
HSMS-2828 Datasheet, PDF (9/15 Pages) Broadcom Corporation. – Surface Mount RF Schottky Barrier Diodes | |||
|
◁ |
Sampling Applications
The six lead HSMSâ282P can be used in a sampling circuit,
as shown in Figure 25. As was the case with the six lead
HSMSâ282R 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
Equation (1) would be straightforward to solve but for the
fact that diode forward voltage is a function of temperaâ
ture as well as forward current. The equation for Vf is:
11600 (Vf â I f R s )
If = IS e
nT
â1
where n = ideality factor
T = temperature in °K
Rs = diode series resistance
and IS (diode saturation current) is given by
sampling
pulse
sampling circuit
( ) ( ) Is = I 0
2
T
n â 4060
e
298
11
T â 298
Figure 25. Sampling Circuit.
Thermal Considerations
The obvious advantage of the SOTâ323 and SOTâ363 over
the SOTâ23 and SOTâ142 is combination of smaller size
and extra leads. However, the copper leadframe in the
SOTâ3x3 has a thermal conductivity four times higher than
the Alloy 42 leadframe of the SOTâ23 and SOTâ143, which
enables the smaller packages to dissipate more power.
The maximum junction temperature for these three famiâ
lies of Schottky diodes is 150°C under all operating condiâ
tions. The following equation applies to the thermal analâ
ysis of diodes:
Tj = (Vf If + PRF) θjc + Ta
(1)
where
Tj = junction temperature
Ta = diode case temperature
θjc = thermal resistance
VfIf = DC power dissipated
PRF = RF power dissipated
Note that θjc, the thermal resistance from diode junction
to the foot of the leads, is the sum of two component reâ
sistances,
θjc = θ pkg + θchip
(2)
Equation (4) is substituted into equation (3), and equaâ
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.
Package thermal resistance for the SOTâ3x3 package is apâ
proximately 100°C/W, and the chip thermal resistance for
the HSMSâ282x family of diodes is approximately 40°C/W.
The designer will have to add in the thermal resistance
from diode case to ambient â a poor choice of circuit
board material or heat sink design can make this number
very high.
Note 5. Avago Application Note 1050, âLow Cost, Surface Mount Power Limiters.â
9
|
▷ |