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

is rated for 0˚C

≤ T

Features

n Guaranteed 7A peak output current

n Guaranteed 5A output current

n Adjustable output down to 1.2V

n Guaranteed thermal regulation

n Current limit constant with temperature

n P

n Output is short-circuit protected

Applications

n Adjustable power supplies

n Constant current regulators

n Battery chargers

(TO-3 STEEL)

Metal Can Package

DS009060-30

Bottom View

Order Number LM138K STEEL or LM338K STEEL

See NS Package Number K02A

(TO-220)

Plastic Package

DS009060-31

Front View

Order Number LM338T

See NS Package Number T03B

May 1998

© 1999 National Semiconductor Corporation

DS009060

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