MC9S08AC128CFGE datasheet(13/40 Pages) FREESCALE

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Chapter 3 Electrical Characteristics and Timing Specifications

MC9S08AC128 MCU Series Data Sheet, Rev. 2

Freescale Semiconductor

3.4

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 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. 3-1

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. 3-2

Solving equations 1 and 2 for K gives:

Table 3-3. Thermal Characteristics

Rating

Symbol

Value

Unit

Operating temperature range (packaged)

TL to TH

–40 to 125

°C

Maximum junction temperature

150

°C

Thermal resistance 1,2,3,4

80-pin LQFP

2s2p

64-pin QFP

2s2p

44-pin LQFP

2s2p

1 Junction temperature is a function of die size, on-chip power dissipation, package thermal

resistance, mounting site (board) temperature, ambient temperature, air flow, power dissipation

of other components on the board, and board thermal resistance.

2 Junction to Ambient Natural Convection

3 1s - Single Layer Board, one signal layer

4 2s2p - Four Layer Board, 2 signal and 2 power layers

°C/W

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