LM117/LM317A/LM317
Application Hints
(Continued)
when
either
the input or output is shorted. Internal to the
LM117 is a 50Ω resistor which limits the peak discharge
current. No protection is needed for output voltages of 25V
or less and 10µF capacitance.
shows an LM117
with protection diodes included for use with outputs greater
than 25V and high values of output capacitance.
00906355
FIGURE 4.
θ
(J−A)
vs Copper (1 ounce) Area for the
TO-263 Package
00906307
D1 protects against C1
D2 protects against C2
As shown in the figure, increasing the copper area beyond 1
square inch produces very little improvement. It should also
be observed that the minimum value of
θ
(J−A)
for the TO-263
package mounted to a PCB is 32˚C/W.
As a design aid,
shows the maximum allowable
power dissipation compared to ambient temperature for the
TO-263 device (assuming
θ
(J−A)
is 35˚C/W and the maxi-
mum junction temperature is 125˚C).
FIGURE 3. Regulator with Protection Diodes
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.
θ
(H−A)
is specified numerically by the heatsink manufacturer
in the catalog, or shown in a curve that plots temperature rise
vs power dissipation for the heatsink.
HEATSINKING TO-263, SOT-223 AND TO-252 PACKAGE
PARTS
The TO-263 (“S”), SOT-223 (“MP”) and TO-252 (”DT”) 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.
shows for the TO-263 the measured values of
θ
(J−A)
for different copper area sizes using a typical PCB with 1
ounce copper
and no solder mask over the copper area used
for heatsinking.
00906356
FIGURE 5. Maximum Power Dissipation vs T
AMB
for
the TO-263 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
junction temperature of 125˚C.
9
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