LM117/LM317A/LM317
diodes included for use with outputs greater than 25V and
high values of output capacitance.
906360
FIGURE 4. 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 re-
sistance 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 ther-
mal 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
))
(6)
906307
(2)
D1 protects against C1
D2 protects against C2
FIGURE 3. Regulator with Protection Diodes
HEATSINK REQUIREMENTS
The LM317 regulators have internal thermal shutdown to pro-
tect the device from over-heating. Under all operating condi-
tions, the junction temperature of the LM317 should not
exceed the rated maximum junction temperature (T
J
) of 150°
C for the LM117, or 125°C for the LM317A and LM317. 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
)
(3)
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)
(4)
where (θ
CH
is the thermal resistance of the contact area be-
tween the device case and the heatsink surface, and
θ
JC
is
thermal resistance from the junction of the die to surface of
the package case.
When a value for
θ
(H−A)
is found using the equation shown, a
heatsink must be selected that has a value that is less than,
or equal to, this number.
The
θ
(H−A)
rating is specified numerically by the heatsink man-
ufacturer in the catalog, or shown in a curve that plots tem-
perature rise vs power dissipation for the heatsink.
HEATSINKING SURFACE MOUNT PACKAGES
The TO-263 (S), SOT-223 (EMP) and TO-252 (MDT) pack-
ages use a copper plane on the PCB and the PCB itself as a
heatsink. To optimize the heat sinking ability of the plane and
PCB, solder the tab of the package to the plane.
HEATSINKING THE SOT-223 PACKAGE
and
show the information for the SOT-223
package.
assumes a
θ
(J−A)
of 74°C/W for 1 ounce
copper and 51°C/W for 2 ounce copper and a maximum junc-
tion temperature of 125°C. Please see AN-1028 for thermal
enhancement techniques to be used with SOT-223 and
TO-252 packages.
where T
J(MAX)
is the maximum allowable junction temperature
(150°C for the LM117, or 125°C for the LM317A/LM317), 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
)
(5)
9
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