LM117/LM317A/LM317
Application Hints
(Continued)
mine if a heatsink is needed, the power dissipated by the
regulator, P
D
, must be calculated:
I
IN
= I
L
+ I
G
P
D
= (V
IN
− V
OUT
) I
L
+ V
IN
I
G
shows the voltage and currents which are present in
the circuit.
00906360
00906357
FIGURE 6.
θ
(J−A)
vs Copper (2 ounce) Area for the
SOT-223 Package
FIGURE 8. Power Dissipation Diagram
The next parameter which must be calculated is the maxi-
mum allowable temperature rise, T
R
(max):
T
R
(max) = T
J
(max) − T
A
(max)
where T
J
(max) is the maximum allowable junction tempera-
ture (125˚C), and T
A
(max) is the maximum ambient tem-
perature which will be encountered in the application.
Using the calculated values for T
R
(max) and P
D
, the maxi-
mum allowable value for the junction-to-ambient thermal
resistance (θ
JA
) can be calculated:
θ
JA
= T
R
(max)/P
D
If the maximum allowable value for
θ
JA
is found to be
≥92˚C/W
(Typical Rated Value) for TO-252 package, no
heatsink is needed since the package alone will dissipate
enough heat to satisfy these requirements. If the calculated
value for
θ
JA
falls below these limits, a heatsink is required.
As a design aid,
shows the value of the
θ
JA
of
TO-252 for different heatsink area. The copper patterns that
we used to measure these
θ
JA
s are shown at the end of the
Application Notes Section.
reflects the same test
results as what are in the
shows the maximum allowable power dissipation
vs. ambient temperature for the TO-252 device.
shows the maximum allowable power dissipation vs. copper
area (in
2
) for the TO-252 device. Please see AN1028 for
power enhancement techniques to be used with SOT-223
and TO-252 packages.
00906358
FIGURE 7. Maximum Power Dissipation vs T
AMB
for
the SOT-223 Package
The LM317 regulators have internal thermal shutdown to
protect the device from over-heating. Under all possible
operating conditions, the junction temperature of the LM317
must be within the range of 0˚C to 125˚C. A heatsink may be
required depending on the maximum power dissipation and
maximum ambient temperature of the application. To deter-
TABLE 1.
θ
JA
Different Heatsink Area
Layout
Top Side (in )*
1
2
3
4
5
6
7
8
9
www.national.com
2
Copper Area
Bottom Side (in )
0
0
0
0
0
0
0.2
0.4
0.6
10
2
Thermal Resistance
(θ
JA
˚C/W) TO-252
103
87
60
54
52
47
84
70
63
0.0123
0.066
0.3
0.53
0.76
1
0
0
0
NSC [ NATIONAL SEMICONDUCTOR ]
