HCS08 datasheet(262/290 Pages) MOTOROLA

262 / 290 page

Electrical Characteristics

MC9S08GB/GT

262

MOTOROLA

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:

Equation 1

TJ = TA + (PD ×θJA)

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:

Equation 2

PD = K ÷ (TJ + 273°C)

Solving equations 1 and 2 for K gives:

Equation 3

K = PD × (TA + 273°C) + θJA × (PD)

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 equations 1 and 2 iteratively for any value of TA.

Table A-2 Thermal Characteristics

Rating

Symbol

Value

Unit

Temp.

Code

Operating temperature range (packaged)

–40 to 85

°CC

Thermal resistance

64-pin LQFP (GB60)

42-pin SDIP (GT60)

44-pin QFP (GT60)

θJA

118

°C/W

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