STR91XF_07 datasheet(69/73 Pages) STMICROELECTRONICS

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STR91xF

Package mechanical data

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7.1

Thermal characteristics

The average chip-junction temperature, TJ must never exceed 125° C.

The average chip-junction temperature, TJ, in degrees Celsius, may be calculated using the

following equation:

TJ = TA + (PD x ΘJA)(1)

Where:

–TA is the Ambient Temperature in °C,

–

ΘJA is the Package Junction-to-Ambient Thermal Resistance, in ° C/W,

–PD is the sum of PINT and PI/O (PD = PINT + PI/O),

–PINT is the product of IDD and VDD, expressed in Watts. This is the Chip Internal

Power.

PI/O represents the Power Dissipation on Input and Output Pins;

Most of the time for the applications PI/O< PINT and may be neglected. On the other hand,

PI/O may be significant if the device is configured to drive continuously external modules and/or

memories. The worst case PINT of the STR91xF is 500mW (IDD x VDD, or 250mA x 2.0V).

An approximate relationship between PD and TJ (if PI/O is neglected) is given by:

PD = K / (TJ + 273°C) (2)

Therefore (solving equations 1 and 2):

K = PD x (TA + 273°C) + ΘJA x PD

2(3)

where:

–

K is a constant for the particular part, which may 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 may be obtained by solving equations (1) and (2) iteratively for any value of TA.

Table 30.

Thermal characteristics

Symbol

Parameter

Value

Unit

ΘJA

Thermal Resistance Junction-Ambient

LQFP 80 - 12 x 12 mm / 0.5 mm pitch

41.5

°C/W

ΘJA

Thermal Resistance Junction-Ambient

LQFP128 - 14 x 14 mm / 0.4 mm pitch

°C/W

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