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LM317AEMP Datasheet(Arkusz danych) 10 Page - National Semiconductor (TI)

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Numer części LM317AEMP
Szczegółowy opis  3-Terminal Adjustable Regulator
Pobierz  25 Pages
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Producent  NSC [National Semiconductor (TI)]
Strona internetowa  http://www.national.com
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 10 page
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Application Hints (Continued)
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-
mine if a heatsink is needed, the power dissipated by the
regulator, P
D, must be calculated:
I
IN =IL +IG
P
D =(VIN −VOUT)IL +VINIG
Figure 8 shows the voltage and currents which are present in
the circuit.
The next parameter which must be calculated is the maxi-
mum allowable temperature rise, T
R(max):
T
R(max) = TJ(max) − TA(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 PD, the maxi-
mum allowable value for the junction-to-ambient thermal
resistance (
θ
JA) can be calculated:
θ
JA =TR(max)/PD
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, Table 1 shows the value of the
θ
JA of
TO-252 for different heatsink area. The copper patterns that
we used to measure these
θ
JAs are shown at the end of the
Application Notes Section. Figure 9 reflects the same test
results as what are in the Table 1
Figure 10 shows the maximum allowable power dissipation
vs. ambient temperature for the TO-252 device. Figure 11
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.
TABLE 1.
θ
JA Different Heatsink Area
Layout
Copper Area
Thermal Resistance
Top Side (in
2)*
Bottom Side (in
2)(
θ
JA˚C/W) TO-252
1
0.0123
0
103
2
0.066
0
87
3
0.3
0
60
4
0.53
0
54
5
0.76
0
52
61
0
47
7
0
0.2
84
8
0
0.4
70
9
0
0.6
63
00906357
FIGURE 6.
θ
(J−A) vs Copper (2 ounce) Area for the
SOT-223 Package
00906358
FIGURE 7. Maximum Power Dissipation vs T
AMB for
the SOT-223 Package
00906360
FIGURE 8. Power Dissipation Diagram
www.national.com
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