SM74503_15 Datasheet, PDF (8/14 Pages) Texas Instruments – SM74503 800mA Low-Dropout Linear Regulator

SM74503
SNVS722A – NOVEMBER 2011 – REVISED APRIL 2013
Figure 13 shows the voltages and currents which are present in the circuit.
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Figure 13. Power Dissipation Diagram
The next parameter which must be calculated is the maximum allowable temperature rise, T_R(max):
T_R(max) = T_J(max)-T_A(max)
where T_J(max) is the maximum allowable junction temperature (125°C), and T_A(max) is the maximum ambient
temperature which will be encountered in the application.
Using the calculated values for T_R(max) and P_D, the maximum 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_JA is found to be ≥136°C/W for SOT-223 package or ≥92°C/W for PFM
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 SOT-223 and PFM 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 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 PFM device. Figure 18 and Figure 19 shows the maximum allowable power dissipation vs. copper area
(in²) for the SOT-223 and PFM devices. Please see AN1028 for power enhancement techniques to be used with
SOT-223 and PFM packages.
Layout
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Table 1. θ_JA Different Heatsink Area
Copper Area
Top Side (in²) ⁽¹⁾
Bottom Side (in²)
0.0123
0
Thermal Resistance
(θ_JA,°C/W) SOT-223
136
(θ_JA,°C/W) PFM
103
0.066
0
123
87
0.3
0
84
60
0.53
0
75
54
0.76
0
69
52
1
0
66
47
0
0.2
115
84
0
0.4
98
70
0
0.6
89
63
0
0.8
82
57
0
1
79
57
0.066
0.066
125
89
0.175
0.175
93
72
0.284
0.284
83
61
0.392
0.392
75
55
0.5
0.5
70
53
(1) Tab of device attached to topside copper
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