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 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 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 R2 24 K + R9 D4 2.4 K 4 K at the input and output and a silicon 6.3 V 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

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 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 had to fig 3 The adjustable three-terminal linear regulator first adjustable three-terminal positive- be protected using foldback LT317 uses only two external to adjust positive voltage linear regulator, the LM317, in . 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

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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 . (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 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 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.” - 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 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. “In the 1980s and company continued to add members and using a voltage follower for the 1990s, linear regulators were focused with higher output current capabili- output amplifier [2], [3] (Figure 4). It on general-purpose applications. Now, ties. In 1995, a 3-A version, the LT1529, was implemented in LT3080, which linear regulators are used when de- was introduced. was introduced in 2007. Per Dobkin’s signers want reduced complexity, low The second-generation parts of- explanation in [3], the architecture noise, low power, and lower solution fered lower noise and faster transient delivers two key benefits: the abil- cost,” notes Dobkin. The market for response. Concurrently, the company ity to operate down to 0 V and to al- linear regulators has grown as has the also improved the input voltage capa- low paralleling of regulators for more market for switching regulators. “A lot bility. A good example is the LT3010, output current. Furthermore, because comes down to a designer’s individual a part that was released in February the output amplifier always operates preference,” he asserts. 2003, and handles a wide input volt- at unity gain, both the bandwidth and By the early 1990s, the three-ter- age range of 3–80 V. Though LT3010 the regulation are constant, explains minal linear regulators were going supported only 50-mA output cur- Dobkin. Also, transient response is

14 IEEE Power Electronics Magazine z December 2014 independent of output voltage, thus protection, reverse-current protection, Electronics Magazine and a veteran allowing regulation to be specified in internal current limiting, and thermal writer and editor with more than 30 millivolts rather than the traditional shutdown. Other features include an years of editorial experience covering percentage of output. extended safe operating area, 1.5-A power electronics, analog/RF technolo- The LT3080 specifications from maximum output current, stable with gies, and semiconductors. He has the data sheet show that this 1.1-A ad- or without input/output capacitors, and worked for leading electronics trade justable single-resistor, low-dropout a wide input voltage range of 1.2–36 V. publications in the United States, includ- linear regulator incorporates an inter- In summary, three-terminal linear ing EETimes, Electronic Design, Power nal ballast resistor that eases direct regulators have come a long way since Electronics Technology, and RF Design. paralleling of these devices for higher they were first introduced in 1969. output current, as shown in Figure 5. They will continue to play an impor- References Since these devices can be paralleled tant role in applications where noise, [1] J. Williams, “Performance enhancement tech- directly on a surface mount board, a cost, and simplicity are important. As niques for three-terminal regulators,” Application board layout is simple and easy. As can a result, they continue to be used in Note 2, Linear Technol. Corp., Milpitas, CA, Tech. be seen, any output voltage can be ob- general-purpose applications of a wide Rep., Aug. 1984. tained from zero up to the maximum variety. Bipolar has enough juice left [2] H. J. Zhang, “Basic concepts of linear regula- defined by the input power supply. to keep pushing the performance of tor and switching mode power supplies,” Appli- Last June, the company started these devices at lower cost and adding cation Note 140, Linear Technol. Corp., Milpitas, adding monitoring functions to the new bells and whistles as needed by CA, Tech. Rep., Oct. 2013. adjustable single-resistor, low-dropout the market. The evolution continues. [3] B. Dobkin. (2013, Aug.). New linear regula- linear regulator. The LT3081 was in- tors solve old problems. Application Note 142. troduced with current and tempera- About the Author Linear Technol. Corp. Milpitas, CA. [Online]. ture monitoring functions and built-in Ashok Bindra ([email protected]) Available: http://cds.linear.com/docs/en/appli- protection circuitry for reverse input is the editor-in-chief of IEEE Power cation-note/an142f.pdf

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December 2014 z IEEE Power Electronics Magazine 15