10
Thermal Considerations
The DC power dissipation of the
MGA-83563, which can be on the
order of 0.5 watt, is approaching
the thermal limits of subminiature
packaging such as the SOT-363.
As a result, particular care should
be taken to adequately heatsink
the MGA-83563.
The primary heat path from the
MMIC chip to the system heatsink
is by means of conduction
through the package leads and
ground vias to the groundplane
on the backside of the PCB. As
previously mentioned in the
“PCB Layout” section, the use of
multiple vias near all of the
ground pins is desirable for low
inductance. The use of multiple
vias is also an especially impor-
tant part of the heatsinking
function.
For heatsinking purposes, a
thinner PCB with more vias,
thicker clad metal, and heavier
plating in the vias all result in
lower thermal resistance and
better heat conduction. Circuit
boards thicker than 0.031 inches
are not recommended for both
thermal and electrical reasons.
The importance of good thermal
design on reliability is discussed
in the next section.
The following examples show the
thermal prerequisites for using
the MGA-83563 reliably in both
saturated and linear modes.
From Figure 22,
P
in
+ P
DC
= P
out
+ P
diss
where P
in
and P
out
are the RF
input and output power, P
DC
is
the DC input power, and P
diss
is
the power dissipated as heat. For
the saturated mode, P
out
= P
sat
,
and,
P
diss
= P
in
+ P
DC
– P
sat
From the table of Electrical
Specifications, the device current
(typical) is 152 mA with a power
supply voltage of 3 volts. Refer-
ring to Figure 10, it can be seen
that the current will decrease
approximately 8% at elevated
temperatures. The device DC
power consumption is then:
P
DC
= 3.0 volts * 152 mA * 0.92
P
DC
= 420 mW
For a saturated amplifier, the RF
input power level is +4 dBm
(2.51 mW) and the saturated
output power is +22 dBm
(158 mW).
The power dissipated as heat is
then:
P
diss
= 2.51 + 420 – 158 mW
P
diss
= 264 mW
The channel-to-case thermal
resistance (θ
ch-c
) from the table
of Absolute Maximum Ratings is
175°C/watt. Note that the mean-
ing of “case” for packages such as
the SOT-363 is defined as the
interface between the package
pins and the mounting surface,
i.e., at the PCB pads. The tem-
perature rise from the mounting
surface to the MMIC channel is
then calculated as
0.264 watt
*
175°C/watt, or 46°C.
Saturated Mode Thermal
Example
Less heat is dissipated in the
MGA-83563 when operated in the
saturated mode because a
significant amount of power is
removed from the RFIC as RF
signal power. It is for this reason
that the saturated mode allows
the device to be used reliably at
higher circuit board temperatures
than for full power, linear
applications.
As an illustration of a thermal/
reliability calculation, consider
the case of an MGA-83563 biased
at 3.0 volts for use in a saturated
mode application with a MTTF
reliability goal of 10
6
hours
(114 years). Reliability calcula-
tions will first be presented for
nominal
conditions, followed by
the conservative approach of
using
worst-case
conditions.
The first step is to calculate the
power dissipated by the
MGA-86353 as heat. Power flow
for the MGA-83563 is represented
in Figure 22.
P
DC
Thermal Design for
Reliability
Good thermal design is an
important consideration in the
reliable use of medium power
devices such as the MGA-83563
because the Mean Time To
Failure (MTTF) of semiconductor
devices is inversely proportional
to the operating temperature.
P
in
Σ
P
n
= 0
HEAT
P
diss
P
out
Figure 22. Thermal Representation
of MGA-83563.
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
