LM138/LM338 5-Amp Adjustable Regulators
May 1998
LM138/LM338
5-Amp Adjustable Regulators
General Description
The LM138 series of adjustable 3-terminal positive voltage
regulators is capable of supplying in excess of 5A over a
1.2V to 32V output range. They are exceptionally easy to
use and require only 2 resistors to set the output voltage.
Careful circuit design has resulted in outstanding load and
line regulation — comparable to many commercial power
supplies. The LM138 family is supplied in a standard 3-lead
transistor package.
A unique feature of the LM138 family is time-dependent cur-
rent limiting. The current limit circuitry allows peak currents
of up to 12A to be drawn from the regulator for short periods
of time. This allows the LM138 to be used with heavy tran-
sient loads and speeds start-up under full-load conditions.
Under sustained loading conditions, the current limit de-
creases to a safe value protecting the regulator. Also in-
cluded on the chip are thermal overload protection and safe
area protection for the power transistor. Overload protection
remains functional even if the adjustment pin is accidentally
disconnected.
Normally, no capacitors are needed unless the device is situ-
ated more than 6 inches from the input filter capacitors in
which case an input bypass is needed. An output capacitor
can be added to improve transient response, while bypass-
ing the adjustment pin will increase the regulator’s ripple re-
jection.
Besides replacing fixed regulators or discrete designs, the
LM138 is useful in a wide variety of other applications. Since
the regulator is “floating” and sees only the input-to-output
differential voltage, supplies of several hundred volts can be
regulated as long as the maximum input to output differential
is not exceeded, i.e., do not short-circuit output to ground.
The part numbers in the LM138 series which have a K suffix
are packaged in a standard Steel TO-3 package, while those
with a T suffix are packaged in a TO-220 plastic package.
The LM138 is rated for −55˚C
≤
T
J
≤
+150˚C, and the LM338
is rated for 0˚C
≤
T
J
≤
+125˚C.
Features
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n
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Guaranteed 7A peak output current
Guaranteed 5A output current
Adjustable output down to 1.2V
Guaranteed thermal regulation
Current limit constant with temperature
P
+
Product Enhancement tested
Output is short-circuit protected
Applications
n
Adjustable power supplies
n
Constant current regulators
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Battery chargers
Connection Diagrams
(See Physical Dimension section for further information)
(TO-220)
Plastic Package
(TO-3 STEEL)
Metal Can Package
DS009060-31
Front View
Order Number LM338T
See NS Package Number T03B
DS009060-30
Bottom View
Order Number LM138K STEEL or LM338K STEEL
See NS Package Number K02A
© 1999 National Semiconductor Corporation
DS009060
www.national.com
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