STR71XF datasheet(46/49 Pages) STMICROELECTRONICS

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STR71xF - PACKAGE CHARACTERISTICS

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3.2 Thermal Characteristics

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.

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

(3)

where:

– K is a constant for the particular part, which may be determined from equation (3) by meas-

uring 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 24. Thermal characteristics

Symbol

Parameter

Value

Unit

Thermal Resistance Junction-Ambient

TQFP 144 - 20 x 20 mm / 0.5 mm pitch

°C/W

Thermal Resistance Junction-Ambient

TQFP 64 - 10 x 10 mm / 0.5 mm pitch

°C/W

Thermal Resistance Junction-Ambient

LFBGA 64 - 8 x 8 x 1.7mm

TBD

°C/W

Thermal Resistance Junction-Ambient

LFBGA 144 - 10 x 10 x 1.7mm

TBD

°C/W

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