Datasheet

LM341/LM78MXX
SNVS090E –MAY 2004–REVISED AUGUST 2005 www.ti.com
APPLICATION INFORMATION
Note that the case temperature is measured at the point where the leads contact with the mounting pad surface
Figure 14. Cross-sectional view of Integrated Circuit Mounted on a printed circuit board.
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 maximum 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
(1)
P
D
= (V
IN
−V
OUT
) I
L
+ V
IN
I
G
(2)
Figure 15 shows the voltages and currents which are present in the circuit.
Figure 15. Power Dissipation Diagram
The next parameter which must be calculated is the maximum allowable temperature rise, T
R
(max):
θ
JA
= TR (max)/P
D
(3)
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 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
fall below these limits, a heatsink is required.
As a design aid, Table 1 shows the value of the θ
JA
of PFM 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. Figure 16 reflects the same
test results as what are in the Table 1
Figure 17 shows the maximum allowable power dissipation vs. ambient temperature for the TO-252 device.
Figure 18 shows the maximum allowable power dissipation vs. copper area (in
2
) for the TO-252 device. Please
see AN-1028 (SNVA036) for power enhancement techniques to be used with TO-252 package.
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