Application Notes: continued
In some cases, none of the packages will be sufficient to
Heatsinks
dissipate the heat generated by the IC, and an external
heatsink will be required.
A heatsink effectively increases the surface area of the
package to improve the flow of heat away from the IC and
into the surrounding air.
I
IN
I
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
OUT
V
Smart
Regulator
IN
V
OUT
to determine the value of RQJA
:
Control
Features
R
QJA = RQJC + RQCS + RQSA
(3)
}
where:
R
R
R
QJC = the junctionÐtoÐcase thermal resistance,
QCS = the caseÐtoÐheatsink thermal resistance, and
QSA = the heatsinkÐtoÐambient thermal resistance.
I
Q
R
R
QJC appears in the package section of the data sheet. Like
QJA, it too is a function of package type. RQCS and RQSA
Figure 6: Single output regulator with key performance parameters
labeled.
are functions of the package type, heatsink and the inter-
face between them. These values appear in heatsink data
sheets of heatsink manufacturers.
6
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
