56F803 datasheet(3/57 Pages) FREESCALE

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Addendum to Rev.16 of 56F803 Technical Data, Rev. 0, 10/2015

Freescale Semiconductor, Inc.

Update the incomplete Thermal Design Considerations section

where

RJA = package junction-to-ambient thermal resistance °C/W

RJC = package junction-to-case thermal resistance °C/W

RCA = package case-to-ambient thermal resistance °C/W

RJC is device-related and cannot be influenced by the user. The user controls the thermal environment to

change the case-to-ambient thermal resistance,RCA . For example, the user can change the air flow around

the device, add a heat sink, change the mounting arrangement on the Printed Circuit Board (PCB), or

otherwise change the thermal dissipation capability of the area surrounding the device on the PCB. This

model is most useful for ceramic packages with heat sinks; some 90% of the heat flow is dissipated through

the case to the heat sink and out to the ambient environment. For ceramic packages, in situations where

the heat flow is split between a path to the case and an alternate path through the PCB, analysis of the

device thermal performance may need the additional modeling capability of a system level thermal

simulation tool.

The thermal performance of plastic packages is more dependent on the temperature of the PCB to which

the package is mounted. Again, if the estimations obtained from RJA do not satisfactorily answer whether

the thermal performance is adequate, a system level model may be appropriate.

Definitions

A complicating factor is the existence of three common definitions for determining the junction-to-case

thermal resistance in plastic packages:

•

Measure the thermal resistance from the junction to the outside surface of the package (case)

closest to the chip mounting area when that surface has a proper heat sink. This is done to minimize

temperature variation across the surface.

•

Measure the thermal resistance from the junction to where the leads are attached to the case. This

definition is approximately equal to a junction to board thermal resistance.

•

Use the value obtained by the equation (TJ – TT)/PD where TT is the temperature of the package

case determined by a thermocouple.

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 package. A small amount of epoxy is placed over the

thermocouple junction and over about 1mm 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

thermocouple wire.

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

theinterface 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 difficulties 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

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