78xx
The 78xx series is a family of three-terminal fixed positive linear voltage regulator integrated circuits designed to convert unregulated input voltages into stable output voltages ranging from 5 V to 24 V, with a maximum output current of up to 1.5 A.[1][2] These devices, first introduced in the early 1970s by Fairchild Semiconductor as the μA78xx series, have become a cornerstone in electronic design due to their simplicity, reliability, and low cost.[3] Common variants include the 7805 (5 V output), 7812 (12 V), 7815 (15 V), and 7824 (24 V), each denoted by the "xx" suffix indicating the nominal output voltage.[2][1] The series features built-in protections such as thermal overload shutdown, internal current limiting, and safe-area compensation to prevent damage from short circuits or excessive heat, requiring minimal external components—often just input and output capacitors—for operation.[1][4] Widely used in power supplies for applications including on-card regulation, portable instruments, computing systems, telecommunications equipment, and hobbyist projects, the 78xx regulators excel in scenarios where low noise and ease of integration are prioritized over high efficiency.[1][2] They operate by dropping excess input voltage across an internal pass transistor, dissipating the difference as heat, which necessitates adequate heatsinking for loads exceeding 500 mA.[2] A complementary 79xx series provides negative output voltages for dual-rail supplies.[2] Despite the rise of switching regulators offering better efficiency, the 78xx series remains prevalent in low-power, cost-sensitive designs and legacy systems, with ongoing production by manufacturers like Texas Instruments and STMicroelectronics.[5]Overview
Description
The 78xx series consists of a family of three-terminal integrated circuit (IC) linear voltage regulators designed to provide fixed positive output voltages ranging from 5 V to 24 V, such as the 7805 for 5 V output and the 7812 for 12 V output.[1][6] These devices are widely used in electronic circuits to deliver stable, regulated DC power from an unregulated input source. The nomenclature follows a standard pattern where "78" signifies positive voltage regulation, and the two digits "xx" indicate the nominal output voltage in volts.[1][6] In operation, the 78xx regulators convert a higher unregulated input voltage—typically ranging from 7 V to 35 V depending on the specific model—into a precise, low-ripple output voltage, with a minimum dropout voltage of approximately 2 V required between input and output for proper regulation (for example, at least 7 V input for a 5 V output).[1][6] They achieve this through internal circuitry that maintains output stability against variations in input voltage, load current (up to 1.5 A with adequate heat sinking), and temperature, while incorporating protections like current limiting and thermal shutdown.[1][6] These regulators play a crucial role in power supply design by supplying reliable DC power to sensitive components, such as TTL logic families, microcontrollers, and operational amplifiers, often without the need for additional external components in simple setups.[1][6] Their simplicity and effectiveness make them a foundational element in on-card regulation for various applications, ensuring minimal noise and voltage fluctuations.[1]History
The origins of the 78xx series can be traced to pioneering work in linear voltage regulation during the late 1960s, beginning with the µA723 adjustable regulator designed by Bob Widlar at Fairchild Semiconductor in 1967, which established foundational principles for monolithic IC-based regulation.[5] Widlar, a key figure in analog IC design known for innovations like the Widlar current source, transitioned to National Semiconductor and developed the LM109/309, the first three-terminal fixed voltage regulator, introduced in 1969 as a milestone that directly paved the way for the fixed-output 78xx family.[7][5] The 78xx series itself was commercialized in the early 1970s by Fairchild Semiconductor as the μA78xx series, a line of fixed positive voltage regulators, valued for their simplicity in three-terminal configurations that simplified circuit design for both hobbyists and industrial applications.[8] By the 1980s, the series saw widespread adoption in emerging personal computers and consumer electronics, powering devices from early microcomputers to audio equipment due to its reliability and cost-effectiveness.[5] Texas Instruments acquired National Semiconductor in 2011, ensuring continued production and support for the 78xx series without major discontinuations into the 2020s, marking over 50 years of sustained use.[9] While remaining a staple, the series has been supplemented since the 1990s by low-dropout (LDO) regulators, which gained prominence with the rise of battery-powered portable devices requiring lower input-output differentials.[10] Widlar's broader contributions to analog design, including bandgap references, profoundly influenced the enduring architecture of the 78xx and its lineage.[7]Design and Operation
Internal Circuitry
The 78xx series voltage regulators are three-terminal devices featuring an input pin (Vin) for the unregulated supply voltage, a ground pin (GND) for the common reference, and an output pin (Vout) for the regulated voltage. Internally, a series pass transistor, typically configured as a Darlington pair of NPN transistors, serves as the primary current-handling element, capable of sourcing up to 1.5 A while dissipating excess voltage as heat to maintain the output stability.[11][12] The regulation mechanism relies on an internal error amplifier that compares a portion of the output voltage—derived from a resistive divider—to a stable reference voltage of approximately 1.25 V generated by a bandgap reference circuit. This comparison drives the error amplifier to adjust the base current of the pass transistor, ensuring the output voltage remains constant despite variations in input voltage or load current.[11][6] The bandgap reference provides temperature-stable operation, superior to older Zener diode-based designs, by combining proportional-to-absolute-temperature (PTAT) and complementary-to-absolute-temperature (CTAT) components to yield a near-constant 1.25 V.[12] Key internal components include the bandgap reference for voltage stability, an integrated current-limiting circuit that activates at approximately 1 A during overload conditions to prevent excessive current draw, and a thermal shutdown mechanism that disables the regulator when the junction temperature exceeds about 150–175 °C to avoid damage from overheating.[11][6][12] The feedback loop operates such that the pass element drops the difference between Vin and Vout as heat, with the fixed output voltage determined internally by the formula V_\text{out} = V_\text{ref} \times \left(1 + \frac{R_2}{R_1}\right), where V_\text{ref} is the 1.25 V reference and R_1, R_2 are preset internal resistors, eliminating the need for external components in fixed-output configurations.[11][12] Protection features enhance reliability, including short-circuit protection through foldback current limiting, which reduces output current as the load voltage approaches zero to minimize power dissipation during faults, and inherent safe-area compensation to handle transients within the absolute maximum ratings.[11][6][12]Key Specifications
The 78xx series of linear voltage regulators are three-terminal devices designed to provide fixed positive output voltages, with standard models offering outputs ranging from 5 V (7805) to 33 V (7833) in increments such as 6 V, 8 V, 9 V, 12 V, 15 V, 18 V, and 24 V. The input voltage must typically not exceed 35 V maximum for most variants to ensure safe operation, though the absolute maximum is 35 V, with recommended operating range starting from about 2 V above the output voltage up to 25-30 V depending on the specific output voltage.[1] These regulators support a continuous output current of up to 1 A, with short-duration peak currents reaching 1.5 A under certain conditions, while the internal power dissipation is limited to approximately 2 W without an external heatsink to prevent thermal runaway. The dropout voltage, defined as the minimum difference between input and output voltages required for proper regulation, is typically around 2 V; for example, an input of at least 7 V is needed for a 5 V output to maintain stability. Ripple rejection, which measures the device's ability to attenuate AC noise from the input supply, ranges from 60 dB to 80 dB at 120 Hz, effectively filtering common mains-frequency ripple in rectified power supplies. Line regulation, the change in output voltage due to input voltage variations (typically from 7 V to 35 V), and load regulation, the variation with output current from 5 mA to 1 A, are both specified at ±1% to ±4% across the series, ensuring stable performance under fluctuating conditions. Thermal specifications include a junction-to-case thermal resistance of approximately 5°C/W in the common TO-220 package, allowing heat dissipation to an external sink for higher power applications. The operating temperature range spans from 0°C to +125°C for commercial versions, with extended ranges available in some variants, and built-in thermal shutdown activating above 150°C to protect the device. Efficiency in 78xx regulators is inherently low due to their linear operation, calculated as \eta = \frac{V_{out}}{V_{in}} \times 100\%, where excess input power is dissipated as heat; for instance, converting 7 V input to 5 V output yields about 71% efficiency.Variants
Positive Voltage Regulators
The 78xx series encompasses a range of three-terminal linear voltage regulators designed to provide fixed positive output voltages, typically ranging from 5 V to 24 V, with a maximum output current of 1 A to 1.5 A depending on the manufacturer and thermal conditions.[6][13][14] These devices incorporate internal current limiting, thermal overload protection, and safe operating area compensation to ensure reliable operation in various electronic circuits.[6][13][14] Key models in the standard lineup include the 7805 for 5 V output, widely used in logic systems; the 7808 for 8 V; the 7812 for 12 V, suitable for analog circuits and instrumentation; the 7815 for 15 V; and the 7824 for 24 V.[13][6][14] The 7805, in particular, is a staple for providing stable 5 V supplies in digital electronics, such as those requiring compatibility with TTL logic levels.[14] Across manufacturers like ON Semiconductor, STMicroelectronics, and Texas Instruments, these models maintain consistent electrical characteristics, with output voltage tolerances varying by grade: standard versions offer ±4% to ±5%, while precision variants achieve ±1.5% to ±2%.[13][6][14] Current ratings are typically 1 A to 1.5 A continuous with adequate heatsinking, depending on the manufacturer, though peak currents up to 2.2 A are possible briefly.[13][6] The pinout is standardized across the series for ease of integration: pin 1 serves as the input, pin 2 as ground, and pin 3 as the output, with the device tab often connected to ground or output depending on the package.[13][6][14] For proper operation, the input voltage must exceed the output voltage by at least 2 V to 3 V (dropout voltage), and the maximum input is limited to 35 V for outputs up to 18 V or 40 V for 24 V models to prevent damage.[13][6][14] Selection of a specific 78xx model depends primarily on the required output voltage and the available input voltage range; for instance, the 7805 requires a minimum input of 7 V to 7.5 V for reliable 5 V regulation, making it suitable for applications with 9 V to 12 V sources.[13][6][14] The 7812, with its 12 V output, is often chosen for systems needing higher voltages, such as audio amplifiers or 12 V power distribution, provided the input does not exceed 40 V.[13][6][14] Tolerance grades should be considered for precision needs, with A-suffix or equivalent precision models preferred in applications sensitive to voltage variations.[13][6]| Model | Output Voltage (V) | Typical Tolerance (Standard/Precision) | Minimum Input Voltage (V) | Maximum Output Current (A) |
|---|---|---|---|---|
| 7805 | 5 | ±4% / ±2% | 7 | 1.5 |
| 7808 | 8 | ±4% / ±2% | 10 | 1.5 |
| 7812 | 12 | ±4% / ±2% | 14.5 | 1.5 |
| 7815 | 15 | ±4% / ±2% | 17.5 | 1.5 |
| 7824 | 24 | ±4% / ±2% | 27 | 1.5 |