SM74503 Datasheet, PDF (9/16 Pages) Texas Instruments – 800mA Low-Dropout Linear Regulator

must be within the range of –40°C to 125°C. A heatsink may
be required depending on the maximum power dissipation
and maximum ambient temperature of the application. To de-
termine 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_INI_G
Figure 4 shows the voltages and currents which are present
in the circuit.
30160616
FIGURE 4. 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 tempera-
ture (125°C), and T_A(max) is the maximum ambient temper-
ature which will be encountered in the application.
Using the calculated values for T_R(max) and P_D, the maxi-
mum allowable value for the junction-to-ambient thermal re-
sistance (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 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_JA of
SOT-223 and TO-252 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 5 and Figure 6
reflects the same test results as what are in the Table 1
Figure 7 and Figure 8 shows the maximum allowable power
dissipation vs. ambient temperature for the SOT-223 and
TO-252 device. Figure 9 and Figure 10 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.
TABLE 1. θ_JA Different Heatsink Area
Layout
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Copper Area
Top Side (in²)*
Bottom Side (in²)
0.0123
0
0.066
0
0.3
0
0.53
0
0.76
0
1
0
0
0.2
0
0.4
0
0.6
0
0.8
0
0.066
0.175
0.284
1
0.066
0.175
0.284
0.392
0.5
0.392
0.5
Thermal Resistance
(θ_JA,°C/W) SOT-223
136
(θ_JA,°C/W) TO-252
103
123
87
84
60
75
54
69
52
66
47
115
84
98
70
89
63
82
57
79
57
125
89
93
72
83
61
75
55
70
53
*Tab of device attached to topside copper
www.ti.com
8