Linear regulator

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In electronics, a linear regulator is a system used to maintain a steady voltage. The resistance of the regulator varies in accordance with the load resulting in a constant voltage output. The regulating device is made to act like a variable resistor, continuously adjusting a voltage divider network to maintain a constant output voltage and continually dissipating the difference between the input and regulated voltages as waste heat. By contrast, a switching regulator uses an active device that switches on and off to maintain an average value of output. Because the regulated voltage of a linear regulator must always be lower than input voltage, efficiency is limited and the input voltage must be high enough to always allow the active device to drop some voltage.

Electronics physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter

Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter.

Resistor Passive electrical component providing electrical resistance

A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines, among other uses. High-power resistors that can dissipate many watts of electrical power as heat, may be used as part of motor controls, in power distribution systems, or as test loads for generators. Fixed resistors have resistances that only change slightly with temperature, time or operating voltage. Variable resistors can be used to adjust circuit elements, or as sensing devices for heat, light, humidity, force, or chemical activity.

Voltage divider linear circuit that produces an output voltage that is a fraction of its input voltage

In electronics, a voltage divider is a passive linear circuit that produces an output voltage (Vout) that is a fraction of its input voltage (Vin). Voltage division is the result of distributing the input voltage among the components of the divider. A simple example of a voltage divider is two resistors connected in series, with the input voltage applied across the resistor pair and the output voltage emerging from the connection between them.

Contents

Linear regulators may place the regulating device in parallel with the load (shunt regulator) or may place the regulating device between the source and the regulated load (a series regulator). Simple linear regulators may only contain as little as a Zener diode and a series resistor; more complicated regulators include separate stages of voltage reference, error amplifier and power pass element. Because a linear voltage regulator is a common element of many devices, integrated circuit regulators are very common. Linear regulators may also be made up of assemblies of discrete solid-state or vacuum tube components.

In electronics, a shunt is a device which creates a low-resistance path for electric current, to allow it to pass around another point in the circuit. The origin of the term is in the verb 'to shunt' meaning to turn away or follow a different path.

Zener diode diode that allows current to flow in the reverse direction at a specific voltage

A Zener diode is a type of diode that allows current to flow not only from its anode to its cathode, but also in the reverse direction, when the Zener voltage is reached.

Voltage regulator regulator, designed to automatically maintain a constant voltage level;may: use a simple feed-forward design or include negative feedback, use an electromechanical mechanism or electronic components

A voltage regulator is a system designed to automatically maintain a constant voltage level. A voltage regulator may use a simple feed-forward design or may include negative feedback. It may use an electromechanical mechanism, or electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.

Overview

The transistor (or other device) is used as one half of a potential divider to establish the regulated output voltage. The output voltage is compared to a reference voltage to produce a control signal to the transistor which will drive its gate or base. With negative feedback and good choice of compensation, the output voltage is kept reasonably constant. Linear regulators are often inefficient: since the transistor is acting like a resistor, it will waste electrical energy by converting it to heat. In fact, the power loss due to heating in the transistor is the current multiplied by the voltage difference between input and output voltage. The same function can often be performed much more efficiently by a switched-mode power supply, but a linear regulator may be preferred for light loads or where the desired output voltage approaches the source voltage. In these cases, the linear regulator may dissipate less power than a switcher. The linear regulator also has the advantage of not requiring magnetic devices (inductors or transformers) which can be relatively expensive or bulky, being often of simpler design, and cause less electromagnetic interference. Some designs of linear regulators use only transistors, diodes and resistors, which are easier to fabricate into an integrated circuit, further reducing their weight, footprint on a PCB, and price.

In electronics engineering, frequency compensation is a technique used in amplifiers, and especially in amplifiers employing negative feedback. It usually has two primary goals: To avoid the unintentional creation of positive feedback, which will cause the amplifier to oscillate, and to control overshoot and ringing in the amplifier's step response. It is also used extensively to improve the bandwidth of single pole systems.

Switched-mode power supply electronic power supply that incorporates a switching regulator

A switched-mode power supply is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently. Like other power supplies, an SMPS transfers power from a DC or AC source to DC loads, such as a personal computer, while converting voltage and current characteristics. Unlike a linear power supply, the pass transistor of a switching-mode supply continually switches between low-dissipation, full-on and full-off states, and spends very little time in the high dissipation transitions, which minimizes wasted energy. A hypothetical ideal switched-mode power supply dissipates no power. Voltage regulation is achieved by varying the ratio of on-to-off time. In contrast, a linear power supply regulates the output voltage by continually dissipating power in the pass transistor. This higher power conversion efficiency is an important advantage of a switched-mode power supply. Switched-mode power supplies may also be substantially smaller and lighter than a linear supply due to the smaller transformer size and weight.

Electromagnetic interference Disturbance generated in an electrical circuit due to external fields or sources of radio-frequency electromagnetic radiation

Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrostatic coupling, or conduction. The disturbance may degrade the performance of the circuit or even stop it from functioning. In the case of a data path, these effects can range from an increase in error rate to a total loss of the data. Both man-made and natural sources generate changing electrical currents and voltages that can cause EMI: ignition systems, cellular network of mobile phones, lightning, solar flares, and auroras. EMI frequently affects AM radios. It can also affect mobile phones, FM radios, and televisions, as well as observations for radio astronomy and atmospheric science.

All linear regulators require an input voltage at least some minimum amount higher than the desired output voltage. That minimum amount is called the dropout voltage. For example, a common regulator such as the 7805 has an output voltage of 5 V, but can only maintain this if the input voltage remains above about 7 V, before the output voltage begins sagging below the rated output. Its dropout voltage is therefore 7 V − 5 V = 2 V. When the supply voltage is less than about 2 V above the desired output voltage, as is the case in low-voltage microprocessor power supplies, so-called low dropout regulators (LDOs) must be used.

Microprocessor Computer processor contained on an integrated-circuit chip

A microprocessor is a computer processor that incorporates the functions of a central processing unit on a single integrated circuit (IC), or at most a few integrated circuits. The microprocessor is a multipurpose, clock driven, register based, digital integrated circuit that accepts binary data as input, processes it according to instructions stored in its memory and provides results as output. Microprocessors contain both combinational logic and sequential digital logic. Microprocessors operate on numbers and symbols represented in the binary number system.

When the output regulated voltage must be higher than the available input voltage, no linear regulator will work (not even a Low dropout regulator). In this situation, something like a switched-mode power supply of the "boost" type or a charge pump must be used. Most linear regulators will continue to provide some output voltage approximately the dropout voltage below the input voltage for inputs below the nominal output voltage until the input voltage drops significantly.

Charge pump

A charge pump is a kind of DC to DC converter that uses capacitors for energetic charge storage to raise or lower voltage. Charge-pump circuits are capable of high efficiencies, sometimes as high as 90–95%, while being electrically simple circuits.

Linear regulators exist in two basic forms: shunt regulators and series regulators. Most linear regulators have a maximum rated output current. This is generally limited by either power dissipation capability, or by the current carrying capability of the output transistor.

Shunt regulators

The shunt regulator works by providing a path from the supply voltage to ground through a variable resistance (the main transistor is in the "bottom half" of the voltage divider). The current through the shunt regulator is diverted away from the load and flows uselessly to ground, making this form usually less efficient than the series regulator. It is, however, simpler, sometimes consisting of just a voltage-reference diode, and is used in very low-powered circuits where the wasted current is too small to be of concern. This form is very common for voltage reference circuits. A shunt regulator can usually only sink (absorb) current.

Diode abstract electronic component with two terminals that allows current to flow in one direction

A diode is a two-terminal electronic component that conducts current primarily in one direction ; it has low resistance in one direction, and high resistance in the other. A diode vacuum tube or thermionic diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons can flow in only one direction, from cathode to plate. A semiconductor diode, the most commonly used type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. Semiconductor diodes were the first semiconductor electronic devices. The discovery of asymmetric electrical conduction across the contact between a crystalline mineral and a metal was made by German physicist Ferdinand Braun in 1874. Today, most diodes are made of silicon, but other materials such as gallium arsenide and germanium are used.

Low-power electronics are electronics, such as notebook processors, that have been designed to use less electric power.

Series regulators

Series regulators are the more common form; they are more efficient than shunt designs. The series regulator works by providing a path from the supply voltage to the load through a variable resistance, usually a transistor (in this role it is usually termed the series pass transistor); it is in the "top half" of the voltage divider - the bottom half being the load. The power dissipated by the regulating device is equal to the power supply output current times the voltage drop in the regulating device. For efficiency and reduced stress on the pass transistor, designs try to minimize the voltage drop but not all circuits regulate well once the input (unregulated) voltage comes close to the required output voltage; those that do are termed Low Dropout regulators, A series regulator can usually only source (supply) current, unlike shunt regulators.

Simple shunt regulator

Simple shunt voltage regulator Zener reg.gif
Simple shunt voltage regulator

The image shows a simple shunt voltage regulator that operates by way of the Zener diode's action of maintaining a constant voltage across itself when the current through it is sufficient to take it into the Zener breakdown region. The resistor R1 supplies the Zener current as well as the load current IR2 (R2 is the load). R1 can be calculated as , where is the Zener voltage, and IR2 is the required load current.

This regulator is used for very simple low-power applications where the currents involved are very small and the load is permanently connected across the Zener diode (such as voltage reference or voltage source circuits). Once R1 has been calculated, removing R2 will allow the full load current (plus the Zener current) through the diode and may exceed the diode's maximum current rating, thereby damaging it. The regulation of this circuit is also not very good because the Zener current (and hence the Zener voltage) will vary depending on and inversely depending on the load current. In some designs, the Zener diode may be replaced with another similarly functioning device, especially in an ultra-low-voltage scenario, like (under forward bias) several normal diodes or LEDs in series. [1]

Simple series regulator

Simple series voltage regulator Series reg.gif
Simple series voltage regulator

Adding an emitter follower stage to the simple shunt regulator forms a simple series voltage regulator and substantially improves the regulation of the circuit. Here, the load current IR2 is supplied by the transistor whose base is now connected to the Zener diode. Thus the transistor's base current (IB) forms the load current for the Zener diode and is much smaller than the current through R2. This regulator is classified as "series" because the regulating element, viz., the transistor, appears in series with the load. R1 sets the Zener current (IZ) and is determined as where, VZ is the Zener voltage, IB is the transistor's base current, K = 1.2 to 2 (to ensure that R1 is low enough for adequate IB) and where, IR2 is the required load current and is also the transistor's emitter current (assumed to be equal to the collector current) and hFE(min) is the minimum acceptable DC current gain for the transistor.

This circuit has much better regulation than the simple shunt regulator, since the base current of the transistor forms a very light load on the Zener, thereby minimising variation in Zener voltage due to variation in the load. Note that the output voltage will always be about 0.65V less than the Zener due to the transistor's VBE drop. Although this circuit has good regulation, it is still sensitive to the load and supply variation. This can be resolved by incorporating negative feedback circuitry into it. This regulator is often used as a "pre-regulator" in more advanced series voltage regulator circuits.

The circuit is readily made adjustable by adding a potentiometer across the Zener, moving the transistor base connection from the top of the Zener to the pot wiper. It may be made step adjustable by switching in different Zeners. Finally it is occasionally made microadjustable by adding a low value pot in series with the Zener; this allows a little voltage adjustment, but degrades regulation (see also capacitance multiplier).

Fixed regulators

An assortment of 78xx series ICs 7800 IC regulatorsa.jpg
An assortment of 78xx series ICs

"Fixed" three-terminal linear regulators are commonly available to generate fixed voltages of +3.3 V, and plus or minus 5 V, 6V, 9 V, 12 V, or 15 V, when the load is less than 1.5 A.

The "78xx" series (7805, 7812, etc.) regulate positive voltages while the "79xx" series (7905, 7912, etc.) regulate negative voltages. Often, the last two digits of the device number are the output voltage (e.g., a 7805 is a +5 V regulator, while a 7915 is a −15 V regulator). There are variants on the 78xx series ICs, such as 78L and 78S, some of which can supply up to 2 Amps. [2]

Adjusting fixed regulators

By adding another circuit element to a fixed voltage IC regulator, it is possible to adjust the output voltage. Two example methods are:

  1. A Zener diode or resistor may be added between the IC's ground terminal and ground. Resistors are acceptable where ground current is constant, but are ill-suited to regulators with varying ground current. By switching in different Zener diodes, diodes or resistors, the output voltage can be adjusted in a step-wise fashion.
  2. potentiometer can be placed in series with the ground terminal to increase the output voltage variably. However, this method degrades regulation, and is not suitable for regulators with varying ground current.

Variable regulators

An adjustable regulator generates a fixed low nominal voltage between its output and its adjust terminal (equivalent to the ground terminal in a fixed regulator). This family of devices includes low power devices like LM723 and medium power devices like LM317 and L200. Some of the variable regulators are available in packages with more than three pins, including dual in-line packages. They offer the capability to adjust the output voltage by using external resistors of specific values.

Adjustable voltage regulator circuit showing 'adjust' terminal Low Drop Voltage Regulator.png
Adjustable voltage regulator circuit showing 'adjust' terminal

For output voltages not provided by standard fixed regulators and load currents of less than 7 A, commonly available adjustable three-terminal linear regulators may be used. The LM317 series (+1.25 V) regulates positive voltages while the LM337 series (−1.25 V) regulates negative voltages. The adjustment is performed by constructing a potential divider with its ends between the regulator output and ground, and its centre-tap connected to the 'adjust' terminal of the regulator. The ratio of resistances determines the output voltage using the same feedback mechanisms described earlier.

Single IC dual tracking adjustable regulators are available for applications such as op-amp circuits needing matched positive and negative DC supplies. Some have selectable current limiting as well. Some regulators require a minimum load.[ citation needed ]

Protection

Linear IC voltage regulators may include a variety of protection methods:

Sometimes external protection is used, such as crowbar protection.

Using a linear regulator

Linear regulators can be constructed using discrete components but are usually encountered in integrated circuit forms. The most common linear regulators are three-terminal integrated circuits in the TO-220 package.

Common solid-state series voltage regulators are the LM78xx (for positive voltages) and LM79xx (for negative voltages). Alternatives are low-dropout regulators like the AMS1117 and Holtek HT7xxx series, both also available for voltages below those supported by the LM78xx series. The Holtek regulators have a quiescent current of <5 µA (approximately 1000 times less than the LM78xx series) making them better suited for battery-powered devices.

Common fixed voltages are 1.8 V, 2.5 V, 3.3 V (both for low-voltage CMOS logic circuits), 5 V (for transistor-transistor logic circuits) and 12 V (for communications circuits and peripheral devices such as disk drives).

In fixed voltage regulators the reference pin is tied to ground, whereas in variable regulators the reference pin is connected to the centre point of a fixed or variable voltage divider fed by the regulator's output. A variable voltage divider such as a potentiometer allows the user to adjust the regulated voltage.

See also

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References

  1. When I[ who? ] designed my AM pocket radio powered by a 3.7 V lithium-ion battery, the 1.5–1.8 V power supply required by the TA7642 chip was provided using a Zener regulator using a red LED (with a forward voltage of 1.7 V) in forward in place of the Zener diode. This LED also doubled as the power indicator.
  2. "Archived copy" (PDF). Archived from the original (PDF) on 2015-06-13. Retrieved 2015-06-11.CS1 maint: archived copy as title (link), Datasheet of L78xx Showing a model that can output 2 A