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 transistor package.
A unique feature of the LM138 family is time-dependent 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 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 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 suffix are
packaged in a standard steel TO-CAN package, while those with a
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.