Datasheet

Electrical and Thermal Characteristics
MPC5121E/MPC5123 Data Sheet, Rev. 5
Freescale Semiconductor 25
3.1.6.1 Heat Dissipation
An estimation of the chip-junction temperature, T
J
, can be obtained from the following equation:
T
J
= T
A
+ ( R
 JA
 P
D
) Eqn. 3
where:
T
A
= ambient temperature for the package ( º C )
R
JA
= junction to ambient thermal resistance ( º C / W )
P
D
= power dissipation in package ( W )
The junction to ambient thermal resistance is an industry standard value, which 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, and the value
obtained on a board with two planes. For packages such as the PBGA, these values can be different by a factor of two. Which
value is correct depends on the power dissipated by other components on the board. The value obtained on a single layer board
is appropriate for the tightly packed printed circuit board. The value obtained on the board with the internal planes is usually
appropriate if the board has low power dissipation and the components are well separated.
Historically, the thermal resistance has frequently been expressed as the sum of a junction to case thermal resistance and a case
to ambient thermal resistance:
R
 JA
= R
 JC
+ R
 CA
Eqn. 4
where:
R
JA
= junction to ambient thermal resistance ( º C / W )
R
JC
= junction to case thermal resistance ( º C / W )
R
CA
= case to ambient thermal resistance ( º C / W )
R
JC
is device related and cannot be influenced by the user. You control the thermal environment to change the case to ambient
thermal resistance, R
CA
. For instance, you can change the air flow around the device, add a heat sink, change the mounting
arrangement on printed circuit board, or change the thermal dissipation on the printed circuit board surrounding the device. This
description is most useful for ceramic packages with heat sinks where some 90% of the heat flow is through the case to the heat
sink to ambient. For most packages, a better model is required.
A more accurate thermal model can be constructed from the junction to board thermal resistance and the junction to case thermal
resistance. The junction to case covers the situation where a heat sink is used or where a substantial amount of heat is dissipated
from the top of the package. The junction to board thermal resistance describes the thermal performance when most of the heat
is conducted to the printed circuit board. This model can be used for hand estimations or for a computational fluid dynamics
(CFD) thermal model.
To determine the junction temperature of the device in the application after prototypes are available, the Thermal
Characterization Parameter (
JT
) can be used to determine the junction temperature with a measurement of the temperature at
the top center of the package case using the following equation:
T
J
= T
T
+ ( 
JT
 P
D
) Eqn. 5
where:
T
T
= thermocouple temperature on top of package ( º C )

JT
= thermal characterization parameter ( º C / W )
P
D
= power dissipation in package ( W )
The thermal characterization parameter is measured per JESD51-2 specification using a 40-gauge type T thermocouple epoxied
to the top center of the package case. The thermocouple should be positioned, so that the thermocouple junction rests on the
package. A small amount of epoxy is placed over the thermocouple junction and over approximately one
mm of wire extending