HCS08 datasheet(174/198 Pages) NXP

174 / 198 page

Appendix A Electrical Characteristics

MC9S08QD4 Series MCU Data Sheet, Rev. 6

174

Freescale Semiconductor

A.3

Thermal Characteristics

This section provides information about operating temperature range, power dissipation, and package

thermal resistance. Power dissipation on I/O pins is usually small compared to the power dissipation in

on-chip logic and voltage regulator circuits and it is user-determined rather than being controlled by the

MCU design. In order to take PI/O into account in power calculations, determine the difference between

actual pin voltage and VSS or VDD and multiply by the pin current for each I/O pin. Except in cases of

unusually high pin current (heavy loads), the difference between pin voltage and VSS or VDD will be very

small.

The average chip-junction temperature (TJ) in °C can be obtained from:

TJ = TA + (PD × θJA)

Eqn. A-1

where:

TA = Ambient temperature, °C

θJA = Package thermal resistance, junction-to-ambient, °C/W

PD = Pint + PI/O

Pint = IDD × 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 approximate relationship between PD and TJ

(if PI/O is neglected) is:

PD = K ÷ (TJ + 273°C)

Eqn. A-2

Solving Equation A-1 and Equation A-2 for K gives:

K = PD × (TA + 273°C) + θJA × (PD)

Eqn. A-3

where K is a constant pertaining to the particular part. K can be determined from equation 3 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 Equation A-1 and Equation A-2 iteratively for any value of TA.

Table A-2. Thermal Characteristics

Rating

Symbol

Value

Unit

Operating temperature range

(packaged)

TL to TH

–40 to 125

°C

Maximum junction temperature

TJMax

135

°C

Thermal resistance (single-layer board)

8-pin PDIP

8-pin NB SOIC

113

150

°C/W

Thermal resistance (four-layer board)

8-pin PDIP

8-pin NB SOIC

°C/W

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