PC8245 datasheet(21/61 Pages) ATMEL

21 / 61 page

PC8245

2171D–HIREL–06/04

RθJC is device-related and cannot be influenced by the user. The user controls the ther-

mal environment to change the case-to-ambient thermal resistance, RθCA. For instance,

the user can change the size of the heat sink, the air flow around the device, the inter-

face material, the mounting arrangement on the printed-circuit board, or the thermal

dissipation on the printed-circuit board surrounding the device.

To determine the junction temperature of the device in the application when heat sinks

are not used, the thermal characterization parameter (

JT) can be used to determine the

junction temperature with a measurement of the temperature at the top center of the

package case using the following equation:

J = TT + (θJT x PD)

where:

T = thermocouple temperature atop the package (°C)

JT = thermal characterization parameter (°C/W)

D = power dissipation in package (W)

The thermal characterization parameter is measured per JESD51-2 specification using

a 40-gauge type T thermocouple epoxied to the top center of the package case. The

thermocouple should be positioned so that the thermocouple junction rests on the pack-

age. A small amount of epoxy is placed over the thermocouple junction and over about 1

mm of wire extending from the junction. The thermocouple wire is placed flat against the

package case to avoid measurement errors caused by cooling effects of the thermocou-

ple wire.

When a heat sink is used, the junction temperature is determined from a thermocouple

inserted at the interface between the case of the package and the interface material. A

clearance slot or hole is normally required in the heat sink. Minimizing the size of the

clearance is important to minimize the change in thermal performance caused by

removing part of the thermal interface to the heat sink. Because of the experimental dif-

ficulties with this technique, many engineers measure the heat sink temperature and

then back calculate the case temperature using a separate measurement of the thermal

resistance of the interface.

From this case temperature, the junction temperature is determined from the junction-to-

case thermal resistance.

In many cases, it is appropriate to simulate the system environment using a computa-

tional fluid dynamics thermal simulation tool. In such a tool, the simplest thermal model

of a package which has demonstrated reasonable accuracy (about 20%) is a two-resis-

tor model consisting of a junction-to-board and a junction-to-case thermal resistance.

The junction-to-case covers the situation where a heat sink will be used or where a sub-

stantial amount of heat is dissipated from the top of the package. The junction-to-board

thermal resistance describes the thermal performance when most of the heat is con-

ducted to the printed-circuit board.

References

Semiconductor Equipment and Materials International

805 East Middlefield Rd.

Mountain View, CA 94043

(415) 964-5111

MIL-SPEC and EIA/JESD (JEDEC) specifications are available from Global Engineering

Documents at 800-854-7179 or 303-397-7956.

JEDEC specifications are available on the WEB at http://www.jedec.org.

PC8245 Datasheet(PDF) 21 Page - ATMEL Corporation

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