LM1117-N
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SNOS412L –MAY 2004–REVISED JULY 2012
PD = (VIN-VOUT)I L + VINIG
Figure 13 shows the voltages and currents which are present in the circuit.
Figure 13. 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 APPLICATION INFORMATION.
Figure 14 and Figure 15 reflects the same test results as what are in the Table 1
Figure 16 and Figure 17 shows the maximum allowable power dissipation vs. ambient temperature for the SOT-
223 and TO-252 device. Figure 18 and Figure 19 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)(1)
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
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
0
82
0
79
0.066
0.175
0.284
0.066
0.175
0.284
125
93
83
(1) Tab of device attached to topside copper
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