34708 datasheet(151/159 Pages) FREESCALE

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Analog Integrated Circuit Device Data

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151

Freescale Semiconductor

MC34708

Typical Applications

8.4

Thermal Considerations

8.4.1

Rating Data

The thermal rating data of the packages has been simulated with the results listed in Table 5.

Junction to Ambient Thermal Resistance Nomenclature: the JEDEC specification reserves the symbol RθJA or θJA (Theta-JA)

strictly for junction-to-ambient thermal resistance on a 1s test board in natural convection environment. RθJMA or θJMA (Theta-

JMA) will be used for both junction-to-ambient on a 2s2p test board in natural convection and for junction-to-ambient with forced

convection on both 1s and 2s2p test boards. It is anticipated that the generic name, Theta-JA, will continue to be commonly used.

The JEDEC standards can be consulted at http://www.jedec.org/

8.4.2

Estimation of Junction Temperature

An estimation of the chip junction temperature TJ can be obtained from the equation

TJ = TA + (RθJA x PD)

with

TA = Ambient temperature for the package in °C

RJA = Junction to ambient thermal resistance in °C/W

PD = Power dissipation in the package in W

The junction to ambient thermal resistance is an industry standard value that provides a quick and easy estimation of thermal

performance. Unfortunately, there are two values in common usage: the value determined on a single layer board RθJA and the

value obtained on a four layer board RθJMA. Actual application PCBs show a performance close to the simulated four layer board

value although this may be somewhat degraded in case of significant power dissipated by other components placed close to the

device.

At a known board temperature, the junction temperature TJ is estimated using the following equation

TJ = TB + (RθJB x PD) with

TB = Board temperature at the package perimeter in °C

RθJB = Junction to board thermal resistance in °C/W

PD = Power dissipation in the package in W

When the heat loss from the package case to the air can be ignored, acceptable predictions of junction temperature can be made.

See Thermal Characteristics for more details on thermal management.

34708 Datasheet(PDF) 151 Page - Freescale Semiconductor, Inc