PRELIMINARY
CM3121
Performance Information (cont’d)
Typical Thermal Characteristics
PCB Layout Considerations
The overall junction to ambient thermal resistance
The CM3121-02SB/SH has a heat spreader attached
to the bottom of the PSOP-8 package in order for heat
to be transferred more easily from the package to the
PCB. The heat spreader is a copper pad of dimensions
just smaller than the package itself. By positioning the
matching pad on the PCB top layer to connect to the
spreader during manufacturing, the heat will be trans-
ferred between the two pads. The drawing below
shows the recommended PCB layout. Note that there
are six vias on either side to allow the heat to dissipate
into the ground and power planes on the inner layers of
the PCB. Vias can be placed underneath the chip, but
this can cause blockage of the solder. The ground and
power planes should be at least 2 sq in. of copper by
the vias. It also helps dissipation if the chip is posi-
tioned away from the edge of the PCB, and not near
other heat-dissipating devices. A good thermal link
from the PCB pad to the rest of the PCB will assure the
best heat transfer from the CM3121 package to ambi-
ent, θJA, of around 40°C/W.
(θJA) for device power dissipation (P ) consists prima-
D
rily of two paths in series. The first path is the junction
to the case (θJC) which is defined by the package style,
and the second path is case to ambient (θCA) thermal
resistance which is dependent on board layout. The
final operating junction temperature for any set of con-
ditions can be estimated by the following thermal equa-
tion:
T
= T
+ P ( θJC ) + P ( θCA )
AMB D D
JUNC
= T
+ P ( θJA)
D
AMB
When a CM3121-02SB/SH (PSOP-8) is mounted on a
double-sided printed circuit board with two square
inches of copper allocated for "heat spreading," the
resulting θJA is 40°C/W. Based on the over tempera-
ture limit of 150° C with an ambient of 70°C, the avail-
able power of this package will be:
150° C – 70° C
P = -------------------------------------- = 2W
D
40° C/ W
Figure 3. Recommended Heat Sink PCB Layout
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11/12/04
CALMIRCO [ CALIFORNIA MICRO DEVICES CORP ]
