STR73XF datasheet(36/40 Pages) STMICROELECTRONICS

36 / 40 page

Package characteristics

STR73xF

36/40

4.1.1

Power considerations

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 Watt. This is the Chip Internal

Power,

–PI/O represents the Power Dissipation on Input and Output Pins; User Determined.

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 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 16.

Thermal Characteristics

Symbol

Description

Package

Value (typical)

Unit

ΘJA

Thermal Resistance Junction-Ambient

LFBGA144

TBD

°C/W

TQFP144

TQFP100

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