DSP5685XUM datasheet(60/64 Pages) FREESCALE

60 / 64 page

56858 Technical Data, Rev. 6

Freescale Semiconductor

Part 6 Design Considerations

6.1 Thermal Design Considerations

An estimation of the chip junction temperature, TJ, in °C can be obtained from the equation:

Equation 1: TJ = TA + (PD x RθJA)

Where:

TA = ambient temperature °C

RθJA = package junction-to-ambient thermal resistance °C/W

PD = power dissipation in package

Historically, thermal resistance has been expressed as the sum of a junction-to-case thermal resistance and

a case-to-ambient thermal resistance:

Equation 2: RθJA = RθJC + RθCA

Where:

RθJA = package junction-to-ambient thermal resistance °C/W

RθJC = package junction-to-case thermal resistance °C/W

RθCA = package case-to-ambient thermal resistance °C/W

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

change the case-to-ambient thermal resistance, RθCA. 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 RθJA do not satisfactorily answer whether

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

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.

DSP5685XUM Datasheet(PDF) 60 Page - Freescale Semiconductor, Inc