Datasheet

Analog Integrated Circuit Device Data
Freescale Semiconductor 150
MC34708
Typical Applications
8.4 Thermal Considerations
8.4.1 Rating Data
The thermal rating data of the packages has been simulated with the results listed in Table 5.
Junction to Ambient Thermal Resistance Nomenclature: the JEDEC specification reserves the symbol R
θJA
or θJA (Theta-JA)
strictly for junction-to-ambient thermal resistance on a 1s test board in natural convection environment. R
θJMA
or θJMA (Theta-
JMA) will be used for both junction-to-ambient on a 2s2p test board in natural convection and for junction-to-ambient with forced
convection on both 1s and 2s2p test boards. The generic name, Theta-JA, is expected to continue to be commonly used.
The JEDEC standards can be consulted at http://www.jedec.org/
8.4.2 Estimation of Junction Temperature
An estimation of the chip junction temperature TJ can be obtained from the equation
T
J
= T
A
+ (R
θJA
x P
D
)
with
T
A
= Ambient temperature for the package in °C
R

JA
= Junction to ambient thermal resistance in °C/W
P
D
= Power dissipation in the package in W
The junction to ambient thermal resistance is an industry standard value that provides a quick and easy estimation of thermal
performance. Unfortunately, there are two values in common usage: the value determined on a single layer board R
θJA
and the
value obtained on a four layer board R
θJMA
. Actual application PCBs show a performance close to the simulated four layer board
value although this may be somewhat degraded in case of significant power dissipated by other components placed close to the
device.
At a known board temperature, the junction temperature TJ is estimated using the following equation
T
J
= T
B
+ (R
θJB
x P
D
) with
T
B
= Board temperature at the package perimeter in °C
R
θJB
= Junction to board thermal resistance in °C/W
P
D
= Power dissipation in the package in W
When the heat loss from the package case to the air can be ignored, acceptable predictions of junction temperature can be made.
See Thermal Characteristics for more details on thermal management.