What is a Low Dropout Regulator & Its Working

At present, the size of modern electronic components & devices is decreasing. However, the efficiency of the battery is changing very much so this is a factor for pushing the limits over power management systems. In the manufacturing of semiconductors, technical development has led to System on Chip (SoC) architecture, wherever the subsystems like analog, digital & RF are incorporated into particular silicon die that means, the different system blocks use different power supplies. A power management system uses different power supply circuits such as DC-DC Converter, linear voltage regulator, switching regulator & a low dropout regulator, or LDO. This article discusses an overview of low dropout regulators.

What is a Low Dropout Regulator?

The term LDO stands for “Low dropout regulator” and it is a cost-effective & simple voltage regulator. The main function of this regulator is to obtain a regulated o/p voltage from a high input voltage.
The main feature of this regulator is its capability to include an extremely low-voltage drop across it whenever a regulated o/p voltage is provided. So this allows the regulator to use in the applications of power critical battery wherever the input voltage of the battery is near to the necessary regulated o/p voltage.

Low Dropout Voltage Regulator
Low Dropout Voltage Regulator

The low dropout voltage regulator uses a variable input to give a steady, constantly controlled, low-noise DC o/p voltage. This is a linear voltage regulator that includes a small voltage drop among the input as well as the output that works even when the o/p voltage is extremely close to the i/p voltage not like the linear voltage regulator that needs a huge voltage drop among the input & the output to function appropriately. As compared to other voltage regulators like linear type, this voltage regulator includes no switching noise & a smaller device size.

The main functions of a low dropout voltage regulator include; it controls the input voltage supply toward the required voltage throughout the load. The next function is to supply an extremely low-noise o/p voltage in the existence of noise in the i/p power supply. So, the most frequently used LDO regulators are AMS1117, RT9193 &MIC29302.

Brief Note on Linear Voltage Regulators

In-circuit design, there are different kinds of linear voltage regulators are used like 7805 otherwise 7812. This is one kind of circuit or a device through a changeable input voltage as well as a stable, constantly controlled DC o/p voltage with low-noise.

Linear Voltage Regulator Circuit
Linear Voltage Regulator Circuit

The regulator’s stable output voltage is an outcome of nonstop alteration of its inside resistance as compared to the changes within the load resistance.


The simple constant voltage regulator’s output voltage can be given through the below equation:



If any load is not present, then the o/p voltage is highest & is equivalent to the i/p at the regulator. When the load is present, the o/p voltage will be low as compared to the highest achievable value. So, the main dissimilarity between the utmost output voltage & output voltage through the load is known as output voltage error that is signified through EVO.

Usually, this output voltage error is signified like percentage disparity among the highest o/p voltage & output voltage through the load.


The percentage of error in terms of i/p, as well as load resistances, can be given as


This error must be reduced, so that feedback is essential. So, the feedback circuit is used to detect the changes happening within the load & regulates the changeable internal resistance so that the internal resistance ratio toward load resistance will stay constant.

Low Dropout Regulator Theory

LDO is a Low Dropout and it can work at a low potential variation among input as well as output. Sometimes, it is called a low-loss type otherwise saturation type linear regulator. Generally, the lowest voltage at which the voltage regulator can function steadily is low than 1V.

Dropout Voltage

In a linear regulator, a transistor is located among the VIN & VO & the least potential dissimilarity required to get constant transistor operation is known as the dropout voltage. Once the voltage disparity among the input & output drops under the dropout voltage then the transistor cannot keep constant operation & the o/p voltage reduces.

In this manner, for both the regulators like linear regulators as well as low dropout, the minimum required input voltage can be set to ensure the process. In this case, VO + Dropout Voltage is the minimum operating voltage. Once the i/p voltage is under the least operating voltage then the o/p voltage will not be constant.

Structure of LDO

The below image shows the basic Block Diagram for LDO. The main components of an LDO are the Reference voltage, Differential amplifier (error amplifier), and Pass element (field-effect transistor).

Low Dropout Voltage Regulator Block Diagram
Low Dropout Voltage Regulator Block Diagram

The block diagram of LDO is shown below and the essential components used in this are the Error amplifier (Differential amplifier), Reference voltage & FET (field-effect transistor).

The positive input of the differential amplifier checks the division of the output measured through the fraction of Resistors like R1 & R2 whereas the i/p at the negative pin of the amplifier can be from a steady voltage reference.

Low Dropout Regulator Working

The LDO regulator working is similar to a normal linear voltage regulator but it includes three essential components like pass element, reference voltage source & error amplifier. Usually, the pass element is a P-channel & N-channel FET, however, it is also called PNP or NPN. In the following diagram of LDO, the input voltage is given to a pass element like an N-channel FET.

The operating of this FET can be done in the linear area to reduce the input voltage below to the necessary output voltage.

Error amplifier or differential amplifier detects the output voltage by comparing it to the reference voltage. So this kind of amplifier modifies the gate terminal of the FET toward the suitable operating end to make sure that the o/p is at the accurate voltage. Once the input voltage alters, then the differential amplifier changes the field-effect transistor to keep a stable output voltage. In steady-state working environments, this kind of regulator works like a simple resistor.

Some kinds of LDO regulators are existing in fixed as well as changeable o/p voltage editions to regulate the o/p voltage based on the necessity. These regulators also include an Enable pin, used to operate the regulator so that it assists the designers to control the regulator so that battery usage can be prevented when it is not used.

Elements of LDO Regulator

The main elements of low dropout regulator mainly include the following.

Voltage Reference

In any voltage regulator, the voltage reference is the initial point because it situates the operating end of the differential amplifier. Generally, a band-gap kind voltage reference can be used because it per mitts to function at low voltage supply.

Differential Amplifier/Error Amplifier

The main requirement in the design of the error amplifier is that it should draw as minimum current as possible. The output resistance of the amplifier must be as low as possible as the gate capacitance of the pass transistor will be large.

The o/p voltage which is balanced down through the voltage divider network is the error amplifier’s one input whereas the other input can be the reference voltage. So, after contrast, this amplifier regulates the pass element’s resistance.


The divider feedback of resistive voltage is dependable for scaling down the o/p voltage & it allows it to be evaluated through the reference voltage using the differential amplifier.

Pass Element

In LDO, the pass element is accountable to transfer the current from input to load & is driven through the differential amplifier within the feedback loop. Generally, MOSFETs are used like pass elements.

Output Capacitor

In an LDO, this is an essential component because it ensures that the flow of current can be delivered instantly toward the load throughout load transients until the differential amplifier is ready.

The Equivalent Series Resistance (ESR) of the capacitor is extremely significant because it stops the flow of current from the capacitor toward the load. Therefore, for a capacitor (1µF ) with equivalent series resistance ranges from 10mΩ – 300mΩ, the feasible capacitor types are ceramic, polymer electrolytic & low-ESR tantalum.

LDO Parameters

The different parameters of LDO are discussed below.

Quiescent Current

The Quiescent can be defined as a condition otherwise phase of inactivity. So this current can be drawn throughout the system within standby mode once light otherwise no load is connected.

Both the currents like Quiescent & shutdown are different terms. Quiescent current is the current drawn throughout the system once the light otherwise no load is connected whereas the shutdown current is the current drawn once the device is deactivated, however, the battery is still allied toward the device.

PSRR (Power Supply Rejection Ratio)

The Power Supply Rejection Ratio (PSRR) can be defined as the ability of LDO to refuse AC elements such as ripple voltage. PSRR can be expressed through the following formula:

PSRR (dB) = 20 log (Vripple(in)/Vripple(out))

Load Regulation

This kind of regulation can be defined as the ability of the circuit to maintain a specific output voltage below changing load conditions. So, this load regulation can be expressed as

Load Regulation = ∆Vout/ ∆Iout

Line Regulation

Line regulation can be defined as the ability of the circuit to keep the specific output voltage by changing input voltage. This can be expressed like the following.

Load Regulation = ∆Vout / ∆Vin

Transient Response

The transient response can be defined as the highest permissible o/p voltage difference for a load current step change. It can also be called like line step response. This is the output capacitor value (Cout) function, the ESR (equivalent series resistance) of the o/p capacitor, the Cb (bypass capacitor) & the (Iout, max) highest load-current. The highest transient voltage difference can be expressed like the following.

∆Vtr, max = (Iout, max / Cout + Cb) ∆t1 + ∆VESR

Features of LDO

The main features of LDO mainly include the following.

  • Lockout of Under-voltage
  • Current Limit
  • TSD (Thermal Shut Down)
  • Output Discharge

Advantages & Disadvantages

An LDO or low-dropout regulator is a DC linear voltage regulator. This kind of regulator is used to regulate the o/p voltage even once the voltage supply is extremely near to the o/p voltage.

As compared to DC to DC regulators, this type of regulator has many benefits like the nonexistence of switching noise, device size is small, and the simplicity of design is greater. The drawback of this regulator is, not like switching regulators, linear DC regulators should dissolve power across the regulation device to control the output voltage.


The main applications of LDO mainly include the following.

  • Cellular Telephones
  • Linear Power Supplies with High-Efficiency
  • Palmtop Computers, Laptop & Notebook
  • DC/DC Modules & SMPS Post-Regulator
  • Equipment with Battery-Powered
  • Personal Electronics or Consumer Electronics
  • VPP Regulation or Switching & PCMCIA VCC
  • The LDOs like Low-Noise & High-PSRR is used for Wireless & Wired Communications.
  • Low-Power and Small-Size types are used for Portable Equipment.
  • High-Voltage withstand for Automotive & Industrial Applications.
  • High-Power LDOs are used for Digital Core Supply.

Thus, this is an overview of a low dropout voltage regulator or LDO. In a power management system, it is the most important component especially the devices which operate through a battery. These regulators can give several voltage levels through a stable output. This regulator’s output voltage is independent of the load impedance, temperature & the changes within the input voltages. This is a linear voltage regulator which operates at extremely low potential disparity among the input as well as the output. Here is a question for you, what are the different kinds of voltage regulators available in the market?