STR720 datasheet(378/401 Pages) STMICROELECTRONICS

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STR720 - ELECTRICAL CHARACTERISTICS

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28.5 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,

–

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

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

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

–PPORT represents the Power Dissipation on Input and Output Pins; User Determined.

PPORT is going to be significant when the device is configured to drive continuously external

memories and/or modules, otherwise PPORT< PINT and it might be neglected.

An approximate relationship between PD and TJ (if PPORT 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 mea-

suring 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.

The power dissipation is obtained from the formula PD=PINT+PPORT where:

PINT is the chip internal power (IDDxVDD)

PPORT is the port power dissipation determined by the user.

The average chip-junction temperature can be obtained from the formula TJ = TA + PD x ΘJA.

Table 91. Thermal Characteristics

Symbol

Description

Value

Unit

Min

Max

Thermal Resistance Junction-Ambient

PQFP 208 - 28 x 28 x 3.4mm / 0.5 mm pitch

-48

°C/W

Power dissipation(1)

PQFP208 - 28 x 28 x 3.4mm / 0.5 mm pitch

Junction temperature(2)

°C

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