IEEE Power Electronics Magazine - December 2014 - 12
A Look Back
by Ashok Bindra
Three-Terminal Linear Regulator
Evolution Continues Unabated
D
espite severe competition
from switching regulators for
decades, the three-terminal
linear regulator has not been given
up. It continues to hold its niche space
because backers like Linear Technology, National Semiconductor/Texas
Instruments, Fairchild Semi, and a
few others continue to improve the
device and serve the applications
whose performance requirements are
stringent and cannot be met by the
switching alternatives.
First introduced in 1969 by National Semiconductor, the three-terminal
linear regulator has survived for over
45 years and continues to make progress. The first three-terminal buck
linear regulator, LM309, was designed
by the late Bob Widlar at National
Semiconductor in 1969 (Figure 1). A
fixed +5-V output with 200-mA or 1-A
output current, this bipolar device
was housed in a TO-5 or TO-3 Can
package (Figure 2). Despite efforts at
that time to make them simple to use,
there were several limitations. First,
it required bypass filtering capacitors
at the input and output and a silicon
diode at the output to keep the positive output from being pulled too far
negative by the high current supply.
Second, the output voltage was not adjustable. Third, these regulators were
not suitable for paralleling.
In early 1969, Bob Dobkin joined
National Semiconductor to work
Digital Object Identifier 10.1109/MPEL.2014.2361596
Date of publication: 18 December 2014
12
IEEE POWER ELECTRONICS MAGAZINE
was not supportive because he was
convinced that with the floating n-p-n
pass transistor at the output, it was not
possible to build an adjustable-output
three-terminal linear regulator with
good performance. However, young
and dynamic Dobkin was undeterred
and continued to pursue his vision.
alongside Widlar in the development of
linear regulators. As a young engineer,
he had many ideas about improving the
fixed-output linear device. Therefore,
he suggested that the fixed-output
regulator must be redesigned as an adjustable-output, three-terminal linear
part. But National's linear guru Widlar
Input
Q18
R15
10 K
Q16
Q17
D1
D2
6.3 V
Q19
R13
2K
D3
6.3 V
R11
3.1 K
Q15
R10
200
Q14
R12
130
R14
0.3
Q2
Output
Q13
Q1
Q3
4X
Q12
R1
3K
R8
24 K
R2
2.4 K
C1
30 pF
+
Q4
Q11
R3
25
Q5
R5
12.1 K
D4
6.3 V
Q9
Q6
R4
1.2 K
R9
4K
R7
4K
Q7
R6
1K
Q10
Q8
Ground
FIG 1 The LM309 was the first three-terminal linear regulator introduced by National
Semiconductor in 1969. (Figure courtesy of Texas Instruments.)
December 2014
Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - December 2014
IEEE Power Electronics Magazine - December 2014 - Cover1
IEEE Power Electronics Magazine - December 2014 - Cover2
IEEE Power Electronics Magazine - December 2014 - 1
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