Once the value of PD(max) is known, the maximum permis-
sible value of RQJA can be calculated:
The value of RQJA can then be compared with those in
the package section of the data sheet. Those packages with
RQJA'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
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 RQJA:
RQJA = RQJC + RQCS + RQSA
RQJC = the junctionÐtoÐcase thermal resistance,
RQCS = the caseÐtoÐheatsink thermal resistance, and
RQSA = the heatsinkÐtoÐambient thermal resistance.
RQJC appears in the package section of the data sheet. Like
RQJA, it too is a function of package type. RQCS and RQSA
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.
Figure 6. Single output regulator with key performance parameters