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參數(shù)資料
| 型號： | LM394N |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 小信號晶體管 |
| 英文描述： | LM194/LM394 Supermatch Pair |
| 中文描述： | 20 mA, 35 V, 2 CHANNEL, NPN, Si, SMALL SIGNAL TRANSISTOR |
| 封裝： | PLASTIC, DIP-8 |
| 文件頁數(shù)： | 6/11頁 |
| 文件大小： | 291K |
| 代理商： | LM394N |
Application Hints
(Continued)
If a manufactured heatsink is to be selected, the value of
heatsink-to-ambient thermal resistance,
θ
(HA)
, must first be
calculated:
θ
(HA)
=
θ
(JA)
θ
(CH)
θ
(JC)
Where:
θ
(JC)
is defined as the thermal resistance from
the junction to the surface of the case. A
value of 4C/W can be assumed for
θ
(JC)
for this calculation.
θ
(CH)
is defined as the thermal resistance be-
tween the case and the surface of the heat-
sink. The value of
θ
will vary from
about 1.5C/W to about 2.5C/W (depend-
ing on method of attachment, insulator,
etc.). If the exact value is unknown, 2C/W
should be assumed for
θ
(CH)
.
When a value for
θ
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
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 AND SOT-223 PACKAGE PARTS
Both the TO-263 (“S”) and SOT-223 (“MP”) packages 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 3shows for the TO-263 the measured values of
θ
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
(JA)
for the TO-263
package mounted to a PCB is 32C/W.
As a design aid, Figure 4 shows the maximum allowable
power dissipation compared to ambient temperature for the
TO-263 device (assuming
θ
is 35C/W and the maxi-
mum junction temperature is 125C).
Figure 5 and Figure 6 show the information for the SOT-223
package. Figure 6 assumes a
θ
of 74C/W for 1 ounce
copper and 51C/W for 2 ounce copper and a maximum
junction temperature of 125C.
Please see AN1028 for power enhancement techniques to
be used with the SOT-223 package.
DS012080-7
FIGURE 3.
θ
(JA)
vs Copper (1 ounce) Area for the
TO-263 Package
DS012080-8
FIGURE 4. Maximum Power Dissipation vs T
AMB
for
the TO-263 Package
DS012080-11
FIGURE 5.
θ
(JA)
vs Copper (2 ounce) Area for the
SOT-223 Package
DS012080-12
FIGURE 6. Maximum Power Dissipation vs T
AMB
for
the SOT-223 Package
www.national.com
6
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