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 current
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 transient
loads and speeds start-up under full-load conditions.
Under sustained loading conditions, the current limit decreases
to a safe value protecting the regulator. Also included
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 situated
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 bypassing
the adjustment pin will increase the regulator’s ripple rejection.
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 ￡ TJ ￡ +150°C, and the LM338
is rated for 0°C ￡ TJ ￡ +125°C.
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+ Product Enhancement tested
n Output is short-circuit protected
Applications
n Adjustable power supplies
n Constant current regulators
n Battery chargers

LM338 application circuit