A Look Back by Ashok Bindra Three-Terminal Linear Regulator Evolution Continues Unabated espite severe competition alongside Widlar in the development of was not supportive because he was from switching regulators for linear regulators. As a young engineer, convinced that with the floating n-p-n D decades, the three-terminal he had many ideas about improving the pass transistor at the output, it was not linear regulator has not been given fixed-output linear device. Therefore, possible to build an adjustable-output up. It continues to hold its niche space he suggested that the fixed-output three-terminal linear regulator with because backers like Linear Technol- regulator must be redesigned as an ad- good performance. However, young ogy, National Semiconductor/Texas justable-output, three-terminal linear and dynamic Dobkin was undeterred Instruments, Fairchild Semi, and a part. But National’s linear guru Widlar and continued to pursue his vision. few others continue to improve the device and serve the applications whose performance requirements are Input stringent and cannot be met by the Q18 switching alternatives. Q16 R15 Q19 First introduced in 1969 by Nation- 10 K R13 al Semiconductor, the three-terminal D1 Q17 R11 D3 2 K linear regulator has survived for over D2 3.1 K 6.3 V 45 years and continues to make prog- 6.3 V Q15 R10 R12 ress. The first three-terminal buck Q14 200 130 R14 linear regulator, LM309, was designed 0.3 by the late Bob Widlar at National Semiconductor in 1969 (Figure 1). A Q2 Output fixed +5-V output with 200-mA or 1-A Q13 output current, this bipolar device Q1 was housed in a TO-5 or TO-3 Can 4 X package (Figure 2). Despite efforts at Q3 Q12 that time to make them simple to use, R1 3 K C1 there were several limitations. First, R8 30 pF it required bypass filtering capacitors R2 24 K + R9 D4 2.4 K 4 K at the input and output and a silicon 6.3 V diode at the output to keep the posi- Q4 Q11 R3 tive output from being pulled too far Q9 25 negative by the high current supply. Q6 Q10 Second, the output voltage was not ad- R7 Q5 Q7 justable. Third, these regulators were 4 K not suitable for paralleling. R4 R5 R6 In early 1969, Bob Dobkin joined 1.2 K 12.1 K 1 K Q8 National Semiconductor to work Ground Digital Object Identifier 10.1109/MPEL.2014.2361596 FIG 1 The LM309 was the first three-terminal linear regulator introduced by National Date of publication: 18 December 2014 Semiconductor in 1969. (Figure courtesy of Texas Instruments.) 12 IEEE POWER ELECTRONICS MAGAZINe z December 2014 LM317 Designer Dobkin, there were several barriers LT317A he had to overcome to real- VIn 5 V VIn VOut ize the first adjustable three- ADJ terminal linear regulator. + 121 X “Because the n-p-n pass tran- 1 nF sistor at the output was floating and there was no 121 X ground pin, I had to ensure 2.5 V FIG 2 LM309 in a TO-3 can package. that any feedback did not LT1009 (Figure courtesy of Texas Instruments.) produce oscillations to make the regulator unstable,” he Adjustable Output said. In addition, he added, Based on Dobkin’s design and develop- to make the device robust, ment work, National Semi released the the output transistors had to FIG 3 The adjustable three-terminal linear regulator first adjustable three-terminal positive- be protected using foldback LT317 uses only two external resistors to adjust positive voltage linear regulator, the LM317, in current limiting. output voltage. (Figure courtesy of Linear Technology.) 1976. Implemented in a 7-µm bipolar Both the line and load process, LM317 offered a die size of regulation were better than standard adjustable output voltage range was 8,000–9,000 mil2 and could handle fixed regulators, and it implemented −1.2 to −37 V with an output current of about 1.5 A of output current. The out- on-chip current limiting, thermal −1.5 A. Another analog guru working put was adjustable from 1.25 V, which overload protection, and safe oper- for National Semiconductor, Carl Nel- was the reference voltage, to 37 V ating-area protection. Soon after, son, introduced the fixed output pow- using only two external resistors (Fig- legendary analog guru the late Bob er negative regulator series LM320. It ure 3). It was housed in the metal TO-3 Pease, who was working for National was widely adopted in the industry. can, and the minimum low-output volt- Semiconductor, designed the adjust- Dobkin left National Semiconduc- age was limited by the on-chip band- able three-terminal linear regulator tor in July 1981 to start his own lin- gap reference voltage. According to with negative output, the LM337. Its ear IC company, Linear Technology PROGRAMMABLE DC POWER Magna-Power’s high frequency IGBT-based programmable DC power supply line spans 1.5 kW to 2000 kW+ with hundreds of models to target a variety of different applications. Using a Magna-Power supply is as simple or sophisticated as the application demands with front panel control, 37-pin isolated analog-digital I/O and a computer interface. 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Available worldwide. www.magna-power.com SL Series XR Series TS Series MS Series MT Series Power Levels 1.5 kW, 2.6 kW, 4 kW 2 kW, 4 kW, 6 kW, 8 kW, 10 kW 5 kW to 45 kW 30 kW, 45 kW, 60 kW, 75 kW 100 kW to 2,000 kW+ Package 1U Rack-mount 2U Rack-mount 3U to 9U Rack-mount Floor Standing Floor Standing No. of Models 54 70 80 80 65 Voltage Range 0-5 Vdc to 0-1,000 Vdc 0-5 Vdc to 0-10,000 Vdc 0-5 Vdc to 0-4,000 Vdc 0-5 Vdc to 0-4,000 Vdc 0-16 Vdc to 0-4,000 Vdc Current Range 0-1.5 Adc to 0-250 Adc 0-0.2 Adc to 0-600 Adc 0-1.2 Adc to 0-2,700 Adc 0-7.2 Adc to 0-4,500 Adc 0-24 Adc to 0-24,000 Adc VIn In LT3080-1 IRef + Vcontrol R1 - Output + VOut = IRef : R1 - 25 mX Out* AN142 FD1b Set V In FIG 4 The new architecture replaces In LT3080-1 bandgap reference with a current 4.8 V–28 V Vcontrol source and uses a voltage follower for the output amplifier. (Figure courtesy + of Linear Technology.) 1 nF - 25 mX VOut Out* 3.3 V Corporation, which he cofounded with 2.2 A Set Bob Swanson. A few years later, Dob- 10 nF 165 k kin’s linear team at Linear Technology created another milestone in this area. 30801 TA01 Around 1986 or 1987, the newly found- ed precision analog company unveiled FIG 5 The internal ballast resistor eases paralleling of LT3080-1 for higher output cur- high-output-current positive adjust- rent. (Figure courtesy of Linear Technology.) able regulators with very low dropout and pin compatible with older three- through another major boost. The rent, a 250-mA version (LT3013) was terminal regulators [1]. The LT1083 se- n-p-n pass transistor in the output added to this line in 2006. ries was designed to provide up to 7.5 A stage of the design was replaced by with higher efficiency and a maximum its p-n-p counterpart. The result was Transition to Current Source dropout of only 1.5 V at maximum out- very low dropout and high robust- Ever since the introduction of the first put current, which was substantially ness with low quiescent current and three-terminal adjustable linear regu- lower than the previous generation and a ground pin. The low power dis- lator in 1976, the architecture has more continued to use only two external re- sipation also enabled the company or less remained the same. Therefore, sistors to set the output voltage. to offer these regulators in surface- the reference-voltage-dependent ad- During this period, the switching mount packages. As a result, using justable output could not go below regulators were encroaching the linear bipolar technology, Linear intro- the 1.25-V reference voltage, making turf. According to Dobkin, “Switching duced the first-generation p-n-p mi- it unsuitable for powering low-voltage regulators began to grow significantly cropower low-dropout linear regula- integrated circuits that were emerging in the late 1980s and early 1990s, driv- tors with adjustable output voltages in the market based on low-geometry en by the portable PCs and portable in 1992. The first member in this line complementary metal–oxide–semi- electronics markets, which required was LT1121, with 150-mA output cur- conductor processes. A new architec- low voltage and high current.” Switch- rent, adjustable and fixed outputs, ture was needed to end the low-voltage ing regulators were initially used pri- 0.4-V dropout, and 30-µA quiescent limitation of three-terminal linear reg- marily in offline applications and later current. In addition, it did not require ulators. Linear Technology answered moved to point-of-load applications. protection diodes and was available the call by replacing the bandgap volt- Linear regulator applications were in multiple package choices. The age reference with a current source limited to 5–25 W.