The LM138 series of adjustable 3-terminal positive
voltage regulators is capable of supplying in excess
of 5 A over a 1.2-V to 32-V 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
A unique feature of the LM138 family is timedependent
current limiting. The current limit circuitry
allows peak currents of up to 12 A 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 (ADJ) pin is accidentally
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 increases the ripple rejection of the regulator.
Besides replacing fixed regulators or discrete
designs, the LM138 is useful in a wide variety of
other applications. Because the regulator is floating
and receives 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; 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-CAN package, while those with a T suffix
are packaged in a TO-220 plastic package. The
LM138 is rated for TJ = –55°C to 150°C, and the
LM338 is rated for TJ = 0°C to 125°C
5 x LM338T LM338 Adjustable Regulator 5A 1.2V To 32V TO-220
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