CS8151
Application Diagram
Once the value of P
D(max)
is known, the maximum permis-
sible value of R
QJA
can be calculated:
R
QJA
=
150¡C - T
A
P
D
(2)
Heat Sinks
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
QJA
:
R
QJA
= R
QJC
+ R
QCS
+ R
QSA
(3)
where:
R
QJC
= the junctionÐtoÐcase thermal resistance,
R
QCS
= the caseÐtoÐheatsink thermal resistance, and
R
QSA
= the heatsinkÐtoÐambient thermal resistance.
R
QJC
appears in the package section of the data sheet. Like
R
QJA
, it too is a function of package type. R
QCS
and R
QSA
are functions of the package type, heatsink and the inter-
face between them. These values appear in heat sink data
sheets of heat sink manufacturers.
The value of R
QJA
can then be compared with those in
the package section of the data sheet. Those packages with
R
QJA
'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.
A heat sink effectively increases the surface area of the
package to improve the flow of heat away from the IC and
I
IN
V
IN
Smart
Regulator
I
OUT
V
OUT
}
Control
Features
I
Q
Figure 6. Single output regulator with key performance parameters
labeled.
into the surrounding air.
Application Diagram
BATTERY
V
C
1
IN
V
OUT
C
2
V
CC
CS8151
WDI
______
RESET
WAKE UP
I/O
Microprocessor
C
Delay
Gnd
C
Delay
______
RESET
I/O
7
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
