Low-dropout regulator

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A low-dropout or LDO regulator is a DC linear voltage regulator that can regulate the output voltage even when the supply voltage is very close to the output voltage. [1]

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.

Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential between two points. The difference in electric potential between two points in a static electric field is defined as the work needed per unit of charge to move a test charge between the two points. In the International System of Units, the derived unit for voltage is named volt. In SI units, work per unit charge is expressed as joules per coulomb, where 1 volt = 1 joule per 1 coulomb. The official SI definition for volt uses power and current, where 1 volt = 1 watt per 1 ampere. This definition is equivalent to the more commonly used 'joules per coulomb'. Voltage or electric potential difference is denoted symbolically by V, but more often simply as V, for instance in the context of Ohm's or Kirchhoff's circuit laws.

Contents

The advantages of a low dropout voltage regulator over other DC to DC regulators include the absence of switching noise (as no switching takes place), smaller device size (as neither large inductors nor transformers are needed), and greater design simplicity (usually consists of a reference, an amplifier, and a pass element). The disadvantage is that, unlike switching regulators, linear DC regulators must dissipate power, and thus heat, across the regulation device in order to regulate the output voltage. [2]

History

The adjustable low-dropout regulator debuted on April 12, 1977 in an Electronic Design article entitled "Break Loose from Fixed IC Regulators". The article was written by Robert Dobkin, an IC designer then working for National Semiconductor. Because of this, National Semiconductor claims the title of "LDO inventor". [3] Dobkin later left National Semiconductor in 1981 and founded Linear Technology where he was the chief technology officer. [4]

Electronic Design, founded in 1952, is the largest published print magazine for the electronic design industry published in the USA by Penton Media.

Robert C. Dobkin is an American electrical engineer, co-founder of Linear Technology Corporation, and veteran linear (analog) integrated circuit (IC) designer.

An integrated circuit or monolithic integrated circuit is a set of electronic circuits on one small flat piece of semiconductor material that is normally silicon. The integration of large numbers of tiny MOS transistors into a small chip results in circuits that are orders of magnitude smaller, faster, and less expensive than those constructed of discrete electronic components. The IC's mass production capability, reliability, and building-block approach to circuit design has ensured the rapid adoption of standardized ICs in place of designs using discrete transistors. ICs are now used in virtually all electronic equipment and have revolutionized the world of electronics. Computers, mobile phones, and other digital home appliances are now inextricable parts of the structure of modern societies, made possible by the small size and low cost of ICs.

Components

The main components are a power FET and a differential amplifier (error amplifier). One input of the differential amplifier monitors the fraction of the output determined by the resistor ratio of R1 and R2. The second input to the differential amplifier is from a stable voltage reference (bandgap reference). If the output voltage rises too high relative to the reference voltage, the drive to the power FET changes to maintain a constant output voltage.

The field-effect transistor (FET) is an electronic device which uses an electric field to control the flow of current. FETs are devices with three terminals: source, gate, and drain. FETs control the flow of current by the application of a voltage to the gate, which in turn alters the conductivity between the drain and source.

A differential amplifier is a type of electronic amplifier that amplifies the difference between two input voltages but suppresses any voltage common to the two inputs. It is an analog circuit with two inputs and and one output in which the output is ideally proportional to the difference between the two voltages

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.

Regulation

Low-dropout (LDO) regulators work in the same way as all linear voltage regulators. The main difference between LDO and non-LDO regulators is their schematic topology. Instead of an emitter follower topology, low-dropout regulators use open collector or open drain topology. In this topology, the transistor may be easily driven into saturation with the voltages available to the regulator. This allows the voltage drop from the unregulated voltage to the regulated voltage to be as low as the saturation voltage across the transistor. [2] :Appendix A

An open collector is a common type of output found on many integrated circuits (IC), which behaves like a switch that is either connected to ground or disconnected.

For the circuit given in the figure to the right, the output voltage is given as:

${\displaystyle V_{\text{OUT}}=\left(1+{\frac {R_{1}}{R_{2}}}\right)V_{\text{REF}}}$

If a bipolar transistor is used, as opposed to a field-effect transistor or JFET, significant additional power may be lost to control it, whereas non-LDO regulators take that power from voltage drop itself. For high voltages under very low In-Out difference there will be significant power loss in the control circuit. [5]

A bipolar junction transistor is a type of transistor that uses both electrons and holes as charge carriers.

The junction gate field-effect transistor is one of the simplest types of field-effect transistor. JFETs are three-terminal semiconductor devices that can be used as electronically-controlled switches, amplifiers, or voltage-controlled resistors.

Because the power control element functions as an inverter, another inverting amplifier is required to control it, which increases schematic complexity compared to simple linear regulator.[ citation needed ]

Power FETs may be preferable to reduce power consumption, but this poses problems when the regulator is used for low input voltage, as FETs usually require 5 to 10 V to close completely. Power FETs may also increase the cost.

Efficiency and heat dissipation

The power dissipated in the pass element and internal circuitry (${\displaystyle P_{\text{LOSS}}}$) of a typical LDO is calculated as follows:

${\displaystyle P_{\text{LOSS}}=\left(V_{\text{IN}}-V_{\text{OUT}}\right)I_{\text{OUT}}+V_{\text{IN}}I_{\text{Q}}}$

where ${\displaystyle I_{\text{Q}}}$ is the quiescent current required by the LDO for its internal circuitry.

Therefore, one can calculate the efficiency as follows:

${\displaystyle \eta ={\frac {P_{\text{IN}}-P_{\text{LOSS}}}{P_{\text{IN}}}}}$   where   ${\displaystyle P_{\text{IN}}=V_{\text{IN}}I_{\text{OUT}}}$

However, when the LDO is in full operation (i.e., supplying current to the load) generally: ${\displaystyle I_{\text{OUT}}\gg I_{\text{Q}}}$. This allows us to reduce ${\displaystyle P_{\text{LOSS}}}$ to the following:

${\displaystyle P_{\text{LOSS}}=\left(V_{\text{IN}}-V_{\text{OUT}}\right)I_{\text{OUT}}}$

which further reduces the efficiency equation to:

${\displaystyle \eta ={\frac {V_{\text{OUT}}}{V_{\text{IN}}}}}$

It is important to keep thermal considerations in mind when using a low drop-out linear regulator. Having high current and/or a wide differential between input and output voltage could lead to large power dissipation. Additionally, efficiency will suffer as the differential widens. Depending on the package, excessive power dissipation could damage the LDO or cause it to go into thermal shutdown.

Quiescent current

Among other important characteristics of a linear regulator is the quiescent current, also known as ground current or supply current, which accounts for the difference, although small, between the input and output currents of the LDO, that is:

${\displaystyle I_{\text{Q}}=I_{\text{IN}}-I_{\text{OUT}}}$

Quiescent current is current drawn by the LDO in order to control its internal circuitry for proper operation. The series pass element, topologies, and ambient temperature are the primary contributors to quiescent current. [6]

Many applications don't require an LDO to be in full operation all of the time (i.e. supplying current to the load). In this idle state the LDO still draws a small amount of quiescent current in order to keep the internal circuitry ready in case a load presented. When no current is being supplied to the load, ${\displaystyle P_{\text{LOSS}}}$ can be found as follows:

${\displaystyle P_{\text{LOSS}}=V_{\text{IN}}I_{Q}}$

Filtering

In addition to regulating voltage, LDOs can also be used as filters. This is especially useful when a system is using switchers, which introduce a ripple in the output voltage occurring at the switching frequency. Left alone, this ripple has the potential to adversely affect the performance of oscillators, [7] data converters, [8] and RF systems [9] being powered by the switcher. However, any power source, not just switchers, can contain AC elements that may be undesirable for design.

Two specifications that should be considered when using an LDO as a filter are power supply rejection ratio (PSRR) and output noise.

Specifications

An LDO is characterized by its drop-out voltage, quiescent current, load regulation, line regulation, maximum current (which is decided by the size of the pass transistor), speed (how fast it can respond as the load varies), voltage variations in the output because of sudden transients in the load current, output capacitor and its equivalent series resistance. [10] Speed is indicated by the rise time of the current at the output as it varies from 0 mA load current (no load) to the maximum load current. This is basically decided by the bandwidth of the error amplifier. It is also expected from an LDO to provide a quiet and stable output in all circumstances (example of possible perturbation could be: sudden change of the input voltage or output current). Stability analysis put in place some performance metrics to get such a behaviour and involve placing poles and zeros appropriately. Most of the time, there is a dominant pole that arise at low frequencies while other poles and zeros are pushed at high frequencies.

Power supply rejection ratio

PSRR refers to the LDO's ability to reject ripple it sees at its input. [11] As part of its regulation, the error amplifier and bandgap attenuate any spikes in the input voltage that deviate from the internal reference to which it is compared. [12] In an ideal LDO, the output voltage would be solely composed of the DC frequency. However, the error amplifier is limited in its ability to gain small spikes at high frequencies. PSRR is expressed as follows: [11]

${\displaystyle {\text{PSRR}}={\frac {\Delta V_{\text{IN}}^{2}}{\Delta V_{\text{OUT}}^{2}}}=10\log \left({\frac {\Delta V_{\text{IN}}^{2}}{\Delta V_{\text{OUT}}^{2}}}\right)\,{\text{dB}}}$

As an example, an LDO that has a PSRR of 55 dB at 1 MHz attenuates a 1 mV input ripple at this frequency to just 1.78 µV at the output. A 6 dB increase in PSRR roughly equates to an increase in attenuation by a factor of 2.

Most LDOs have relatively high PSRR at lower frequencies (10 Hz – 1 kHz). However, a Performance LDO is distinguished in having high PSRR over a broad frequency spectrum (10 Hz – 5 MHz). Having high PSRR over a wide band allows the LDO to reject high-frequency noise like that arising from a switcher. Similar to other specifications, PSRR fluctuates over frequency, temperature, current, output voltage, and the voltage differential.

Output noise

The noise from the LDO itself must also be considered in filter design. Like other electronic devices, LDOs are affected by thermal noise, bipolar shot noise, and flicker noise. [9] Each of these phenomena contribute noise to the output voltage, mostly concentrated over the lower end of the frequency spectrum. In order to properly filter AC frequencies, an LDO must both reject ripple at the input while introducing minimal noise at the output. Efforts to attenuate ripple from the input voltage could be in vain if a noisy LDO just adds that noise back again at the output.

Load regulation is a measure of the circuit’s ability to maintain the specified output voltage under varying load conditions. Load regulation is defined as:

${\displaystyle {\text{Load Regulation}}={\Delta V_{\text{OUT}} \over \Delta I_{\text{OUT}}}}$

The worst case of the output voltage variations occurs as the load current transitions from zero to its maximum rated value or vice versa. [6]

Line regulation

Line regulation is a measure of the circuit’s ability to maintain the specified output voltage with varying input voltage. Line regulation is defined as:

${\displaystyle {\text{Line regulation}}={\Delta V_{\text{OUT}} \over \Delta V_{\text{IN}}}}$

Like load regulation, line regulation is a steady state parameter—all frequency components are neglected. Increasing DC open-loop current gain improves the line regulation. [6]

Transient response

The transient response is the maximum allowable output voltage variation for a load current step change. The transient response is a function of the output capacitor value (${\textstyle C_{\text{OUT}}}$), the equivalent series resistance (ESR) of the output capacitor, the bypass capacitor (${\textstyle C_{\text{BYP}}}$) that is usually added to the output capacitor to improve the load transient response, and the maximum load-current (${\textstyle I_{\text{OUT,MAX}}}$). The maximum transient voltage variation is defined as follows:

${\displaystyle \Delta V_{\text{TR,MAX}}={\frac {I_{\text{OUT,MAX}}}{C_{\text{OUT}}+C_{\text{BYP}}}}\Delta t_{1}+{\Delta V_{\text{ESR}}}}$ [6]

Where ${\textstyle \Delta t_{1}}$ corresponds to the closed-loop bandwidth of an LDO regulator. ${\textstyle \Delta V_{\text{ESR}}}$ is the voltage variation resulting from the presence of the ESR (${\textstyle R_{\text{ESR}}}$) of the output capacitor. The application determines how low this value should be.

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References

1. Paul Horowitz and Winfield Hill (1989). The Art of Electronics. Cambridge University Press. pp. 343–349. ISBN   978-0-521-37095-0.
2. Jim Williams (March 1, 1989). "High Efficiency Linear Regulators". Linear Technology. Retrieved 2014-03-29.
3. Don Tuite (September 1, 2007). "Inventor Updates A Classic 30 Years Later".
4. Simpson, Chester. "Linear and Switching Voltage Regulator Fundamentals" (PDF). ti.com. Texas Instruments. Retrieved 18 June 2015.
5. Lee, Bang S. "Understanding the Terms and Definitions of LDO Voltage Regulators" (PDF). Texas Instruments. Retrieved 30 August 2013.
6. Mohammed, Habeeb. "Supply Noise Effect on Oscillator Phase Noise" (PDF).
7. Ramus, Xavier. "Measuring PSR in an ADC".
8. Pithadia, Sanjay. "LDO Noise Demystified" (PDF). Texas Instruments.
9. Pithadia, Sanjay. "LDO PSRR Measurement Simplified" (PDF). Texas Instruments. Archived from the original (PDF) on 2012-10-22.
10. Day, Michael. "Understanding Low Drop Out (LDO) Regulators" (PDF). Retrieved 16 September 2013.