LM341 Datasheet, PDF (7/11 Pages) National Semiconductor (TI) – 3-Terminal Positive Voltage Regulators

Design Considerations (Continued)
DS010484-23
FIGURE 1. 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
The LM78MXX/LM341XX regulators have internal thermal
shutdown to protect the device from over-heating. Under all
possible operating conditions, the junction temperature of
the LM78MXX/LM341XX must be within the range of 0˚C to
125˚C. A heatsink may be required depending on the maxi-
mum power dissipation and maximum ambient temperature
of the application. To determine 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
shows the voltages and currents which are present in the
circuit.
DS010484-24
FIGURE 2. Power Dissipation Diagram
The next parameter which must be calculated is the maxi-
mum allowable temperature rise, T_R(max):
θ_JA = TR (max)/P_D
If the maximum allowable value for θ_JA˚C/w is found to be
≥60˚C/W for TO-220 package or ≥92˚C/W for TO-252 pack-
age, no heatsink is needed since the package alone will dis-
sipate enough heat to satisfy these requirements. If the cal-
culated value for θ_JA fall below these limits, a heatsink is
required.
As a design aid, Table 1 shows the value of the θ_JA of
TO-252 for different heatsink area. The copper patterns that
we used to measure these θ_JA are shown at the end of the
Application Note Section. reflects the same test results as
what are in the Table 1
shows the maximum allowable power dissipation vs. ambi-
ent temperature for theTO-252 device. shows the maximum
allowable power dissipation vs. copper area (in²) for the
TO-252 device. Please see AN1028 for power enhancement
techniques to be used with TO-252 package.
Layout
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TABLE 1. θ_JA Different Heatsink Area
Copper Area
Thermal Resistance
Top Sice (in²)*
0.0123
Bottom Side (in²)
0
(θ_JA, ˚C/W) TO-252
103
0.066
0
87
0.3
0
60
0.53
0
54
0.76
0
52
1
0
47
0
0.2
84
0
0.4
70
0
0.6
63
0
0.8
57
0
1
57
0.066
0.066
89
0.175
0.175
72
0.284
0.284
61
0.392
0.392
55
0.5
0.5
53
*Tab of device attached to topside copper
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