CS8140, CS8141
http://onsemi.com
13
Figure 23. Single Output Regulator With Key
Performance Parameters Labeled
SMART
REGULATOR
®
Control
Features
I
OUT
I
IN
I
Q
V
IN
V
OUT
Once the value of P
D(max)
is known, the maximum
permissible value of R
q
JA
can be calculated:
R
QJA
+
150°C * T
A
P
D
(2)
The value of R
q
JA
can then be compared with those in the
package section of the data sheet. Those packages with
R
q
JA
’s less than the calculated value in equation 2 will keep
the die temperature below 150°C.
In some cases, none of the packages will be sufficient to
dissipate the heat generated by the IC, and an external
heatsink will be required.
HEAT SINKS
A heat sink effectively increases the surface area of the
package to improve the flow of heat away from the IC and
into the surrounding air.
Each material in the heat flow path between the IC and the
outside environment will have a thermal resistance. Like
series electrical resistances, these resistances are summed to
determine the value of R
q
JA
.
R
QJA
+ R
QJC
) R
QCS
) R
QSA
(3)
where:
R
Q
JC
= the junction−to−case thermal resistance,
R
q
CS
= the case−to−heatsink thermal resistance, and
R
q
SA
= the heatsink−to−ambient thermal resistance.
R
q
JC
appears in the package section of the data sheet. Like
R
q
JA
, it too is a function of package type. R
q
CS
and R
q
SA
are
functions of the package type, heatsink and the interface
between them. These values appear in heat sink data sheets
of heat sink manufacturers.
