IEEE Power Electronics Magazine - December 2014 - 13

FIG 2 LM309 in a TO-3 can package.
(Figure courtesy of Texas Instruments.)

Adjustable Output
Based on Dobkin's design and development work, National Semi released the
first adjustable three-terminal positivevoltage linear regulator, the LM317, in
1976. Implemented in a 7-µm bipolar
process, LM317 offered a die size of
8,000-9,000 mil2 and could handle
about 1.5 A of output current. The output was adjustable from 1.25 V, which
was the reference voltage, to 37 V
using only two external resistors (Figure 3). It was housed in the metal TO-3
can, and the minimum low-output voltage was limited by the on-chip bandgap reference voltage. According to

LM317 Designer Dobkin,
there were several barriers
LT317A
VIn
he had to overcome to real5V
VIn
VOut
ize the first adjustable threeADJ
121 X
terminal linear regulator.
+
1 nF
"Because the n-p-n pass transistor at the output was
121 X
floating and there was no
2.5 V
ground pin, I had to ensure
that any feedback did not
LT1009
produce oscillations to make
the regulator unstable," he
said. In addition, he added,
to make the device robust,
the output transistors had to FIG 3 The adjustable three-terminal linear regulator
be protected using foldback LT317 uses only two external resistors to adjust positive
output voltage. (Figure courtesy of Linear Technology.)
current limiting.
Both the line and load
adjustable output voltage range was
regulation were better than standard
−1.2 to −37 V with an output current of
fixed regulators, and it implemented
−1.5 A. Another analog guru working
on-chip current limiting, thermal
for National Semiconductor, Carl Neloverload protection, and safe operson, introduced the fixed output powating-area protection. Soon after,
er negative regulator series LM320. It
legendary analog guru the late Bob
was widely adopted in the industry.
Pease, who was working for National
Dobkin left National SemiconducSemiconductor, designed the adjusttor in July 1981 to start his own linable three-terminal linear regulator
ear IC company, Linear Technology
with negative output, the LM337. Its

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Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - December 2014