Cirrus Logic AN Integrating CobraNet into Audio Products - Manual (Page 8)

Thermal P RELIMINARY
6
CS18101 (CM-2) AppNote1 - rev 1.1 Jan, 2004
w w w . c i r r u s . c o m
Thermal
While the CobraNet modules and the CS18101 are relatively low-power devices, heat
dissipation can not be ignored. If the system will be in a sealed case with no ventilation
and/or your host board is dissipating some heat then it is necessary to plan for system
cooling.
Thermal analysis of a complete system is complex and difficult. Like EMI, the theory
behind it is well understood and documented but the practical application of it is more of
an art than a science. This applications note shows the thermal analysis of the CM-1 and
CM-2 and what its practical effects are.
The Theory
There are many factors that effect the thermal performance of a circuit. The major ones
are power consumption, ambient temperature, and airflow/convection. Lesser factors
include humidity, air turbulence, PCB construction, and thermal conductivity of all parts.
But none of these describes how a circuit might fail due to high temperatures.
As a semiconductor heats up it slows down. If it slows down too much then it will start to
exceed the timing margins and the logic will fail. This failure is dependant on the
temperature of the semiconductor die, also called the "junction temperature". The junction
temperature is dependant on the power being dissipated in the die and how fast that heat
can be removed from the die through the chip package.
The basic formulas governing this are:
θ
ja
= (T
j
- T
a
) / P
d
θ
jc
= (T
j
- T
c
) / P
d
Where:
T
j
is the junction temperature, in °C.
T
a
is the ambient air temperature, in °C.
P
d
is the power being dissipated by the chip, in watts.
θ
ja
is the thermal coefficient between the junction and ambient temperature, in °C/Watt.
θ
ja
is the thermal coefficient between the junction and case temperature, in °C/Watt.
Normally, T
ja
, T
jc
, and P
d
are provided in the data sheets for the chip in question. So using
the above formulas we can calculate the junction temperature of each chip and determine
if it is above the maximum value stated in the data sheet. Easy, right? Well...