NCV4276, NCV4276A
http://onsemi.com
17
Calculating Power Dissipation
in a Single Output Linear Regulator
The maximum power dissipation for a single output
regulator (Figure 50) is:
P
D(max)
+ [V
I(max)
* V
Q(min)
]I
Q(max)
(1)
) V
I(max)
I
q
where
V
I(max)
is the maximum input voltage,
V
Q(min)
is the minimum output voltage,
I
Q(max)
is the maximum output current for the
application,
I
q
is the quiescent current the regulator
consumes at I
Q(max)
.
Once the value of P
D(max)
is known, the maximum
permissible value of R
q
JA
can be calculated:
R
qJA
+
150
o
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
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.
SMART
REGULATOR®
Iq
Control
Features
I
Q
I
I
Figure 50. Single Output Regulator with Key
Performance Parameters Labeled
V
I
V
Q
}
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
is the junction−to−case thermal resistance,
R
q
CS
is the case−to−heatsink thermal resistance,
R
q
SA
is 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.
Thermal, mounting, and heatsinking considerations are
discussed in the ON Semiconductor application note
AN1040/D.
