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LM317T Datasheet(数据表) 9 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
部件型号  LM317T
说明  3-Terminal Adjustable Regulator
下载  25 Pages
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制造商  NSC [National Semiconductor (TI)]
网页  http://www.national.com
标志 

 9 page
background image
Application Hints (Continued)
when either the input or output is shorted. Internal to the
LM117 is a 50
Ω resistor which limits the peak discharge
current. No protection is needed for output voltages of 25V
or less and 10µF capacitance. Figure 3 shows an LM117
with protection diodes included for use with outputs greater
than 25V and high values of output capacitance.
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.
Figure 4 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.
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, Figure 5 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 6 and Figure 7 show the information for the SOT-223
package. Figure 7 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.
00906307
D1 protects against C1
D2 protects against C2
FIGURE 3. Regulator with Protection Diodes
00906355
FIGURE 4.
θ
(J−A) vs Copper (1 ounce) Area for the
TO-263 Package
00906356
FIGURE 5. Maximum Power Dissipation vs T
AMB for
the TO-263 Package
www.national.com
9




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