Application Note (Continued)
The next parameter which must be calculated is the maxi-
mum allowable temperature rise, TR(max):
TR(max) = TJ(max)-TA(max)
where TJ(max) is the maximum allowable junction tempera-
ture (125˚C), and TA(max) is the maximum ambient tem-
perature which will be encountered in the application.
Using the calculated values for TR(max) and PD, the maxi-
mum allowable value for the junction-to-ambient thermal
resistance (θJA) can be calculated:
θJA = TR(max)/PD
10091937
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.
FIGURE 5. Cross-sectional view of Integrated Circuit
Mounted on a printed circuit board. Note that the case
temperature is measured at the point where the leads
contact with the mounting pad surface
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
The LM1117 regulators have internal thermal shutdown to
protect the device from over-heating. Under all possible
operating conditions, the junction temperature of the LM1117
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, PD , must be calculated:
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.
IIN = IL + IG
PD = (VIN-VOUT)I + VIN G
I
L
Figure 6 shows the voltages and currents which are present
in the circuit.
*Application Note AN-1187 discusses improved thermal per-
formance and power dissipation for the LLP.
10091916
FIGURE 6. Power Dissipation Diagram
TABLE 1. θJA Different Heatsink Area
Copper Area
Layout
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
0.53
0.76
1
0
0
136
123
84
103
87
60
54
52
47
84
70
63
57
57
89
72
3
0
4
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
0
82
0
79
0.066
0.175
0.066
0.175
125
93
11
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