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CM3121 Datasheet(PDF) 8 Page - California Micro Devices Corp |
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CM3121 Datasheet(HTML) 8 Page - California Micro Devices Corp |
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8 / 11 page  © 2004 California Micro Devices Corp. All rights reserved. 8 430 N. McCarthy Blvd., Milpitas, CA 95035-5112 ● Tel: 408.263.3214 ● Fax: 408.263.7846 ● www.calmicro.com 11/12/04 CM3121 PRELIMINARY Performance Information (cont’d) Typical Thermal Characteristics The overall junction to ambient thermal resistance ( θJA) for device power dissipation (PD) consists prima- 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: TJUNC = TAMB + PD ( θJC ) + PD ( θCA ) = TAMB + PD ( θJA) 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: PD = = 2W PCB Layout Considerations 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. Figure 3. Recommended Heat Sink PCB Layout 150 ° C70° C – 40 ° C/ W --------------------------------------- |
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