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參數(shù)資料
| 型號： | LM883E |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | 3-Terminal Adjustable Regulator |
| 中文描述： | 三端可調(diào)穩(wěn)壓器 |
| 文件頁數(shù)： | 10/25頁 |
| 文件大小： | 697K |
| 代理商： | LM883E |
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 0C to 125C. 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
= I
L
+ I
G
P
D
= (V
IN
V
OUT
) I
L
+ V
IN
I
G
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) = T
J
(max) T
A
(max)
where T
(max) is the maximum allowable junction tempera-
ture (125C), and T
(max) is the maximum ambient tem-
perature which will be encountered in the application.
Using the calculated values for T
(max) and P
, 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
is found to be
≥
92C/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
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.
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
(
θ
JA
C/W) TO-252
103
87
60
54
52
47
84
70
63
Top Side (in
2
)*
0.0123
0.066
0.3
0.53
0.76
1
0
0
0
Bottom Side (in
2
)
0
0
0
0
0
0
0.2
0.4
0.6
1
2
3
4
5
6
7
8
9
00906357
FIGURE 6.
θ
(JA)
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
L
www.national.com
10
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