Datasheet

Application Note (Continued)
The LMS8117A regulators have internal thermal shutdown to
protect the device from over-heating. Under all possible
operating conditions, the junction temperature of the
LMS8117A 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 appli-
cation. 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
IN
I
G
Figure 6 shows the voltages and currents which are present
in the circuit.
The next parameter which must be calculated is the maxi-
mum 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 tem-
perature 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
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 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
JA
s 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.
Figure 9 and Figure 10 shows the maximum allowable power
dissipation vs. ambient temperature for the SOT-223 and
TO-252 device. Figure 11 and Figure 12 shows the maxi-
mum allowable power dissipation vs. copper area (in
2
) for
the SOT-223 and TO-252 devices. Please see AN–1028 for
power enhancement techniques to be used with SOT-223
and TO-252 packages.
10119637
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
10119616
FIGURE 6. Power Dissipation Diagram
LMS8117A
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