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參數(shù)資料
| 型號： | LM78L00_SERIES |
| 廠商： | National Semiconductor Corporation |
| 文件頁數(shù)： | 7/10頁 |
| 文件大?。?/td> | 164K |
| 代理商： | LM78L00_SERIES |
Design Considerations
The LM78L series regulators have thermal overload protec-
tion from excessive power, internal short-circuit protection
which limits each circuit’s maximum current, and output
transistor safe-area protection for reducing the output cur-
rent as the voltage across each pass transistor is increased.
Although the internal power dissipation is limited, the junc-
tion temperature must be kept below the maximum speci-
fied temperature (125
§
C) in order to meet data sheet specifi-
cations. To calculate the maximum junction temperature or
heat sink required, the following thermal resistance values
should be used:
Package
Typ
i
JC
Max
i
JC
Typ
i
JA
Max
i
JA
TO-92
160
160
Thermal Considerations
The TO-92 molded package is capable of unusually high
power dissipation due to the lead frame design. However, its
thermal capabilities are generally overlooked because of a
lack of understanding of the thermal paths from the semi-
conductor junction to ambient temperature. While thermal
resistance is normally specified for the device mounted
1 cm above an infinite heat sink, very little has been men-
tioned of the options available to improve on the conserva-
tively rated thermal capability.
An explanation of the thermal paths of the TO-92 will allow
the designer to determine the thermal stress he is applying
in any given application.
The TO-92 Package
The TO-92 package thermal paths are complex. In addition
to the path through the molding compound to ambient tem-
perature, there is another path through the leads, in parallel
with the case path, to ambient temperature, as shown in
Figure 1.
The total thermal resistance in this model is then:
i
JA
e
(
i
JC
a
i
CA
) (
i
JL
a
i
LA
)
i
JC
a
i
CA
a
i
JL
a
i
LA
Where:
i
JC
e
thermal resistance of the case between the regu-
lator die and a point on the case directly above
the die location.
i
CA
e
thermal resistance between the case and air at
ambient temperature.
i
JL
e
thermal resistance from regulator die through the
input lead to a point
(/16
inch below the regulator
case.
i
LA
e
total thermal resistance of the input/output
ground leads to ambient temperature.
i
JA
e
junction to ambient thermal resistance.
(1)
TL/H/10051–4
FIGURE 1. TO-92 Thermal Equivalent Circuit
Methods of Heat Sinking
With two external thermal resistances in each leg of a paral-
lel network available to the circuit designer as variables, he
can choose the method of heat sinking most applicable to
his particular situation. To demonstrate, consider the effect
of placing a small 72
§
C/W flag type heat sink, such as the
Staver F1-7D-2, on the LM78L00 molded case. The heat
sink effectively replaces the
i
CA
(Figure 2) and the new
thermal resistance,
i
ê
JA
, equals 145
§
C/W (assuming, 0.125
inch lead length).
The net change of 15
§
C/W increases the allowable power
dissipation to 0.86W with a minimal inserted cost. A still
further decrease in
i
JA
could be achieved by using a heat
sink rated at 46
§
C/W, such as the Staver FS-7A. Also, if the
case sinking does not provide an adequate reduction in total
i
JA
, the other external thermal resistance,
i
LA
, may be re-
duced by shortening the lead length from package base to
mounting medium. However, one point must be kept in
mind. The lead thermal path includes a thermal resistance,
i
SA
, from the leads at the mounting point to ambient, that is,
the mounting medium.
i
LA
is then equal to
i
LS
a
i
SA
. The
new model is shown in Figure 2.
In the case of a socket,
i
SA
could be as high as 270
§
C/W,
thus causing a net increase in
i
JA
and a consequent de-
crease in the maximum dissipation capability. Shortening
the lead length may return the net
i
JA
to the original value,
but lead sinking would not be accomplished.
In those cases where the regulator is inserted into a copper
clad printed circuit board, it is advantageous to have a maxi-
mum area of copper at the entry points of the leads. While it
would be desirable to rigorously define the effect of PC
board copper, the real world variables are too great to allow
anything more than a few general observations.
7
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