SNVS774L – MAY 2004 – REVISED FEBRUARY 2011
D1 protects against C1
D2 protects against C2
Figure 27. Regulator with Protection Diodes
Heatsink Requirements
The LM317-N regulators have internal thermal shutdown to protect the device from over-heating. Under all
operating conditions, the junction temperature of the LM317-N should not exceed the rated maximum junction
temperature (T
J
) of 150°C for the LM117, or 125°C for the LM317A and LM317-N. A heatsink may be required
depending on the maximum device power dissipation and the maximum ambient temperature of the application.
To determine if a heatsink is needed, the power dissipated by the regulator, P
D
, must be calculated:
P
D
= ((V
IN
−
V
OUT
) × I
L
) + (V
IN
× I
G
)
(2)
shows the voltage and currents which are present in the circuit.
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)
(3)
where T
J(MAX)
is the maximum allowable junction temperature (150°C for the LM117, or 125°C for the
LM317A/LM317-N), 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
)
(4)
Figure 28. Power Dissipation Diagram
If the calculated maximum allowable thermal resistance is higher than the actual package rating, then no
additional work is needed. If the calculated maximum allowable thermal resistance is lower than the actual
package rating either the power dissipation (P
D
) needs to be reduced, the maximum ambient temperature T
A(MAX)
needs to be reduced, the thermal resistance (θ
JA
) must be lowered by adding a heatsink, or some combination of
these.
If a heatsink is needed, the value can be calculated from the formula:
θ
HA
≤
(θ
JA
- (θ
CH
+
θ
JC
))
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