LM1117/LM1117I
10091916
FIGURE 6. Power Dissipation Diagram
The next parameter which must be calculated is the maximum allowable temperature rise, TR(max):
TR(max) = TJ(max)-TA(max)
where TJ(max) is the maximum allowable junction temperature (125°C), and TA(max) is the maximum ambient temperature which
will be encountered in the application.
Using the calculated values for TR(max) and PD, the maximum allowable value for the junction-to-ambient thermal resistance
(θJA) can be calculated:
ꢀθJA = TR(max)/PD
If the maximum allowable value for θJA is found to be ≥136°C/W for SOT-223 package or ≥79°C/W for TO-220 package or ≥92°
C/W 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 θJA falls below these limits, a heatsink is required.
As a design aid, Table 1 shows the value of the θJA of SOT-223 and TO-252 for different heatsink area. The copper patterns that
we used to measure these θJAs are shown at the end of the Application Notes Section. Figure 7 and Figure 8 reflects the same
test results as what are in the Table 1
Figure 9 and Figure 10 shows the maximum allowable power dissipation vs. ambient temperature for the SOT-223 and TO-252
device. Figures Figure 11 and Figure 12 shows the maximum allowable power dissipation vs. copper area (in2) for the SOT-223
and TO-252 devices. Please see AN1028 for power enhancement techniques to be used with SOT-223 and TO-252 packages.
*Application Note AN-1187 discusses improved thermal performance and power dissipation for the LLP.
TABLE 1. θJA Different Heatsink Area
Layout
Copper Area
Thermal Resistance
Top Side (in2)*
Bottom Side (in2)
(θJA,°C/W) SOT-223
(θJA,°C/W) TO-252
1
2
0.0123
0.066
0.3
0
0
136
123
84
103
87
60
54
52
47
84
70
63
57
57
89
72
61
55
53
3
0
4
0.53
0.76
1
0
75
5
0
69
6
0
66
7
0
0.2
0.4
0.6
0.8
1
115
98
8
0
9
0
89
10
11
12
13
14
15
16
0
82
0
79
0.066
0.175
0.284
0.392
0.5
0.066
0.175
0.284
0.392
0.5
125
93
83
75
70
*Tab of device attached to topside copper
14
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