NCV8508B
http://onsemi.com
19
APPLICATION NOTES
Calculating Power Dissipation in a Single Output
Linear Regulator
The maximum power dissipation for a single output
regulator (Figure 32) is:
P
D(max)
+ [V
IN(max)
* V
OUT(min)
]I
OUT(max)
(1)
) V
IN(max)
I
Q
where:
V
IN(max)
is the maximum input voltage,
V
OUT(min)
is the minimum output voltage,
I
OUT(max)
is the maximum output current for the
application, and
I
Q
is the quiescent current the regulator consumes at
I
OUT(max)
.
SMART
REGULATOR
I
Q
Control
Features
I
OUT
I
IN
Figure 32. Single Output Regulator with Key
Performance Parameters Labeled
V
IN
V
OUT
}
Once the value of P
D(max)
is known, the maximum
permissible value of R
q
JA
can be calculated:
R
qJA
+
150° C *
T
A
P
D
(2)
The value of R
q
JA
can then be compared with those in the
package section of the data sheet. Those packages with
R
q
JA
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.
Heatsinks
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.
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
q
JA
:
R
qJA
+ R
qJC
) R
qCS
) R
qSA
(3)
where:
R
q
JC
= the junction−to−case thermal resistance,
R
q
CS
= the case−to−heatsink thermal resistance, and
R
q
SA
= the heatsink−to−ambient thermal resistance.
R
q
JC
appears in the package section of the data sheet. Like
R
q
JA
, it too is a function of package type. R
q
CS
and R
q
SA
are
functions of the package type, heatsink and the interface
between them. These values appear in data sheets of
heatsink manufacturers.
