Thermal
Characteristics
Table 4.
Package
Symbol
θJA
Parameter
Value
33
4
Rating
°C/W
°C/W
°C/W
°C/W
Thermal Resistance - Ceramic Junction To Ambient
Thermal Resistance - Ceramic Junction To Case
Thermal Resistance - Ceramic Junction To Ambient
Thermal Resistance - Ceramic Junction To Case
PGA 68
θJC
θJA
33
3
CQFP
θJC
Power
The average chip-junction temperature, TJ, in °C can be obtained from:
Considerations
TJ = TA + (PD + θJA)
(1)
TA = Ambient Temperature, °C
θJA = Package Thermal Resistance, Junction-to-Ambient, °C/W
PD = PINT + PI/O
PINT = ICC x VCC, Watts - Chip Internal Power
PI/O = Power Dissipation on Input and Output Pins - User Determined
For most applications PI/O < PINT and can be neglected.
An Approximate relationship between PD and TJ (if PI/O is neglected) is:
PD = K: (TJ + 273)
(2)
Solving equations (1) and (2) for K gives
2
K = PD. (TA + 273) + θJA · PD
(3)
where K is constant pertaining to the particular part K can be determined from the equa-
tion (3) by measuring PD (at equilibrium) for a known TA. Using this value of K, the
values of PD and TJ can be obtained by solving equations (1) and (2) iteratively for any
value of TA.
The total thermal resistance of a package (θJA) can be separated into two components,
θ
JC and θCA, representing the barrier to heat flow from the semiconductor junction to the
package (case), surface (θJC) and from the case to the outside ambient (θCA). These
terms are related by the equation:
θJA = θJC + θCA
(4)
θJA is device related and cannot be influenced by the user. However, θCA is user depen-
dent and can be minimized by such thermal management techniques as heat sinks,
ambient air cooling and thermal convection. Thus, good thermal management on the
part of the user can significantly reduce θCA so that θJA approximately equals θJC. Substi-
tution of θJC for θJA in equation (1) will result in a lower semiconductor junction
temperature.
8
TS68882
2119A–HIREL–04/02
ATMEL [ ATMEL ]
