NCV4275C
http://onsemi.com
11
Calculating Power Dissipation
in a Single Output Linear Regulator
The maximum power dissipation for a single output
regulator (Figure 23) 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° 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.
SMART
REGULATOR®
Iq
Control
Features
I
Q
I
I
Figure 23. 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 heatsink
data sheets of heatsink manufacturers.
Thermal, mounting, and heatsinking considerations are
discussed in the ON Semiconductor application note
AN1040/D.
40
60
80
100
120
140
0 100 200 300 400 500 600 700 800
40
60
80
100
120
140
160
0 100 200 300 400 500 600 700 800
Figure 24. qJA vs. Copper Spreader Area,
DPAK 5−Lead
Figure 25. qJA vs. Copper Spreader Area,
D
2
PAK 5−Lead
COPPER AREA SPREADER AREA (mm
2
)
RqJA, THERMAL RESISTANCE (C°/W)
DPAK 2 oz
DPAK 1 oz
COPPER AREA SPREADER AREA (mm
2
)
RqJA, THERMAL RESISTANCE (C°/W)
D2PAK 2 oz
D2PAK 1 oz
