POWER CONSIDERATIONS
The average
|
| Tj = TA + (PD ° 03A) | (1 |
where:. |
| .. |
|
TA | = | Ambient Temperature, °C |
|
0jA | = | Package Thermal Resistance, |
|
|
|
| |
PD | = | PINT+ PI/O |
|
'PINT | = | I c c x V c c , W a t t s - - Chip lnternaI Power |
|
PI/O . | - | Power,Dissipationon Input and Output Pins |
|
|
| - - User Determined |
For most applications PI/O<PINT and can be neglected. '
The foliowing is an approximate relationship between PD and Tj (if PI/O is
neglected):. .. . |
|
|
|
|
|
. . . . . . | . | PD= . K+(Tj+273°C) | , , , . . | . | (2) |
Solving equations (1)and | (2) f o r K g i v e s : | ' ' " " | ": |
| |
| K = PD" (TA + 273°C) + eJA ° PD2 |
|
| (3) | |
where K is a constant pertaining to the particular part. K can be determined from equation (3) by measuring PD (at equilibrium) for a known TA. Using this value of K, the values (~f PD and Tj can be obtained by solving'equ"ations
(1) and (2) iteratively for any value of TA.
The total thermal resistance of a package (0jA)can be separated into two components, 0jC and 0CA, representing the barrier to heatflow from the semiconductor junction to the package (case) surface (eJC) and from the case to the outside ambient (0CA). These terms are related by the equation:
0 j A = 0 j C + 0 C A | (4) |
ejC is device related and cannot be influenced by the user. However, 0CA is user dependent and can be minimized by such thermal management tech- niques as heat sinks, ambient air cooling, and thermal convection. Thus, good thermal management on the part of the user can significantly reduce eCA so that 0jA approximately equals 0jC. Substitution of ejC for 0jA in equation (1) will result in a lower semiconductor junction temperature.
Values for thermal resistance presented in this document, unless.estimated, were derived using the procedure described in Motorola Reliability Report 7843, "Thermal Resistance Measurement Method for MC68XX Microcom- ponent Devices," and are provided for design purposes only. Thermal meas- urements are complex and dependent on procedure and setup. User derived values for thermal resistance may differ.
