Application Notes: continued
Once the value of PD(max) is known, the maximum permis-
Heat Sinks
sible value of RQJA can be calculated:
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.
150¡C - TA
(2)
RQJA
=
PD
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
The value of RQJA 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
to determine the value of RQJA
.
the die temperature below 150¡C.
R
QJA = RQJC + RQCS + RQSA
(3)
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.
where:
R
R
R
QJC = the junctionÐtoÐcase thermal resistance,
QCS = the caseÐtoÐheatsink thermal resistance, and
QSA = the heatsinkÐtoÐambient thermal resistance.
I
IN
R
R
QJC appears in the package section of the data sheet. Like
QJA, it is a function of package type. RQCS and RQSA are
I
OUT
V
IN
Smart
Regulator
V
OUT
functions of the package type, heatsink and the interface
between them. These values appear in heat sink data
sheets of heat sink manufacturers.
Control
Features
}
I
Q
Figure 1. Single output regulator with key performance parameters
labeled.
7
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
