ST7294 datasheet(54/69 Pages) STMICROELECTRONICS

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ST72E94 - ST72T94

2.2 POWER CONSIDERATIONS

TJ, the average chip-junction temperature in Celsi-

us can be calculated from the following equation:

TJ =TA +(PD x θJA) (1)

Where:

–TA is the Ambient Temperature in °C,

–

θJ

A is the Package Thermal Resistance,

Junction-to-Ambient in

°C/W,

–PD the sum of PINT and PI/O,

–PINT equals ICC time VCC,

Watts-Chip Internal Power

–PI/O the Power Dissipation on Input and Output

Pins, User Determined.

For most applications PI/O <PINT and can be ne-

glected.

PI/O may be significant if the device is configured

to drive Darlington bases or sink LED Loads.

An approximate relationship between PD and TJ

(if PI/O is neglected) is:

PD =K÷ (TJ + 273°C)

(2)

Therefore:

K= PD x(TA + 273°C) + θJA xPD

2 (3)

Where K is constant pertaining to the particular

part, K can be determined from equation (3) by

measuring PD (at equilibrium) for a known TA. Us-

ing this value of K, the values of PD and TJ can be

obtained by solving equations (1) and (2) iterative-

ly for any value of TA.

Thermal Characteristics

Symbol

Characteristics

Value

Unit

θJA

Thermal Resistance

PDIP28

PSO28

°C/W

Equivalent Test Load

VDD =3V

Pins

PA0-PA7

PB0-PB7

PC0-PC5

10.91k

Ω

6.32k

Ω

50pF

VDD = 4.5V

Pins

PA0-PA7

PB0-PB7

PC0-PC5

3.26k

Ω

2.38k

Ω

50pF

Test Diagram

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