PRELIMINARY
CM3202-02
Application Info (cont’d)
Typical Thermal Characteristics
PCB Layout Considerations
The overall junction to ambient thermal resistance
The CM3202-02 has a heat spreader (exposed pad)
attached to the bottom of the TDFN-8 package in order
for the heat to be transferred more easily from the
package to the PCB. The heat spreader is a copper
pad with slightly smaller dimensions 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 transferred between the two pads.
Figure 2 shows the CM3202-02 recommended PCB
layout. Please note there are four vias to allow the heat
to dissipate into the ground and power planes on the
inner layers of the PCB. Vias must be placed
underneath the chip but this can result in solder
blockage. The ground and power planes need to be at
least 2 square inches of copper by the vias. It also
helps dissipation if the chip is positioned away from the
edge of the PCB, and away from 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 CM3202-02 to ambient temperature.
(θJA) for device power dissipation (P ) primarily
D
consists of two paths in the 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
condition can be estimated by the following thermal
equation:
TJUNC = TAMB + PD × (θJC) + PD × (θCA
)
= TAMB + PD × (θCA
)
When a CM3202-02 using TDFN-8 package is
mounted on a double-sided printed circuit board with
four square inches of copper allocated for “heat
spreading,” the θ is approximately 55°C/W. Based on
JA
the over temperature limit of 170°C with an ambient
temperature of 85°C, the available power of the
package will be:
Top View
Bottom Layer
Ground Plane
170° C – 85° C
Top Layer Copper
Connects to Heat Spreader
PD = ------------------------------------ = 1.5W
55° C ⁄
W
Pin Solder Mask
Thermal PAD
Solder Mask
Vias (0.3mm Diameter)
Note: This drawing is not to scale
Figure 2. Thermal Layout for TDFN-8 package
© 2007 California Micro Devices Corp. All rights reserved.
10
490 N. McCarthy Blvd., Milpitas, CA 95035-5112
●
Tel: 408.263.3214
●
Fax: 408.263.7846
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www.cmd.com 05/25/07
CALMIRCO [ CALIFORNIA MICRO DEVICES CORP ]
