CS5210-1
Application Notes: continued
from the die junction to ambient air. The maximum junc-
tion temperature can be determined by:
T
J(max)
= T
A(max)
+ P
D(max)
´
R
QJA
The maximum ambient temperature and the power dissi-
pation are determined by the design while the maximum
junction temperature and the thermal resistance depend on
the manufacturer and the package type. The maximum
power dissipation for a regulator is:
P
D(max)
= (V
IN(max)
-V
OUT(min)
)I
OUT(max)
+ V
IN(max)
´
I
IN(max)
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 has a
thermal resistance which is measured in degrees per watt.
Like series electrical resistances, these thermal resistances
are summed to determine the total thermal resistance
between the die junction and the surrounding air, R
QJA
.
This total thermal resistance is comprised of three compo-
nents. These resistive terms are measured from junction to
case (R
QJC
), case to heat sink (R
QCS
), and heat sink to ambi-
ent air (R
QSA
). The equation is:
R
QJA
= R
QJC
+ R
QCS
+ R
QSA
R
QJC
is rated @ 1.4¡C/W for the CS5210-1. For a high cur-
rent regulator such as the CS5210-1 the majority of heat is
generated in the power transistor section. The value for
R
QSA
depends on the heat sink type, while the R
QCS
depends on factors such as package type, heat sink inter-
face (is an insulator and thermal grease used?), and the
contact area between the heat sink and the package. Once
these calculations are complete, the maximum permissible
value of R
QJA
can be calculated and the proper heat sink
selected. For further discussion on heat sink selection, see
our Cherry application note ÒThermal Management for
Linear Regulators.Ó
7
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
