DSP56166ROM datasheet(13/63 Pages) MOTOROLA

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PRELIMINARY - 6/15/93

DSP56166 Technical Data Sheet

MOTOROLA

Power Considerations

The average chip junction temperature, TJ, in °C can be obtained from:

TJ = TA + (PD*ΘJA)

(1)

Where:

TA = Ambient Temperature, °C

JA = Package Thermal Resistance, Junction-to-Ambient, °C/W

PD = PINT + PI/O

PINT = ICC*Vdd, Watts — Chip Internal Power

PI/O = Power Dissipation on Input and Output Pins — User Determined

For most applications PI/O < PINT and can be neglected. An appropriate relationship between PD and TJ (if

PI/O is neglected) is:

PD = K/(TJ + 273° C)

(2)

Solving equations (1) and (2) for K gives:

K = PD*(TA + 273° C) + ΘJA*PD

(3)

Where K is a constant pertaining to the particular part. K can be determined from equation (2) by measuring

PD (at equilibrium) for a known TA. Using this value of K, the values of PD and TJ can be obtained by solving

equations (1) and (2) iteratively for any value of TA. The total thermal resistance of a package (ΘJA) can be

separated into two components,

JA and CA, representing the barrier to heat flow from the semiconductor

junction to the package (case) surface (

JC) and from the case to the outside ambient (CA). These terms

are related by the equation:

JA = ΘJC + CA

(4)

JC is device related and cannot be influenced by the user. However, CA is user dependent and can be

minimized by such thermal management techniques as heat sinks, ambient air cooling, and thermal con-

vection. Thus, good thermal management on the part of the user can significantly reduce CA so thatΘJA ap-

proximately equals

JC. Substitution of ΘJC for ΘJA in equation (1) will result in a lower semiconductor junc-

tion temperature. Values for thermal resistance presented in this document, unless estimated, were derived

using the procedure described in Motorola Reliability Report 7843, “Thermal Resistance Measurement

Method for MC68XX Microcomponent Devices”, and are provided for design purposes only. Thermal mea-

surements are complex and dependent on procedure and setup. User—derived values for thermal resis-

tance may differ.

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