Low Dropout Voltage Regulators, LDOs

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Linear regulators often need a good voltage across the series pass element, transistor, FET, etc to ensure that it can maintain a good level of regulation.

There are many instances where it might not be possible to drop the normal voltage across the series regulator element to reduce power dissipation, or because the power sources within the unit do not allow a large series voltage drop.

It is interesting to note that many low power portable electronic gadgets require a variety of different voltage rails, and local regulation of the supply is essential. USB can take 3.3V, analogue systems - 2.75V, core processing - 1.5V, memory 1.8V, modems - 2.8V and Bluetooth 2.8V. To accommodate all of these different voltage rails, it is necessary to have a variety of different regulators, most of which need to provide very clean voltage rails to ensure the correct performance of each of the circuits they power.

It is for circumstances like this where a low dropout regulator might be needed. To accommodate this need, there are many low drop out linear regulators available.

The LDOs, low drop out voltage regulators are able to provide good regulation with a minimum voltage across the series regulator element. They can be used as Point-of-Load, PoL, regulators that are placed in close proximity tot he device that will be using the regulated supply.

Although there is no exact definition of a low dropout regulator and where the boundary falls between standard regulators and low voltage ones, the consensus seems to be that any regulator that can operate with a dropout voltage of les than 1 V is considered to be an LDO.

In many cases a switch mode DC-DC converter might be considered, but these devices have much higher noise levels and spikes on the output, and there are many instances where a very clean, locally regulated supply is needed, and low dropout linear regulators fit the bill.

Low dropout linear regulator basics

Voltage regulators can be classified as standard linear voltage regulators or low drop out, LDO voltage regulators.

The difference between the two types is on the way the series pass element and the drive circuitry operates and designing an LDO circuit can present some interesting challenges.

Any voltage regulator will need to drop a certain amount of voltage across the series pass element so that it can regulate the output and ensure that it is free from ripple, spikes and noise as well as providing the required voltage. To achieve this, the series pass element must drop some voltage across it, so that it can perform the regulation.

Standard voltage regulators may drop three, four, five or more volts across the series pass element to provide the required performance.

Low drop out, LDO voltage regulators drop much less, sometimes much less than a volt - even a few hundred millivolts.

The reason why this is important is that less voltage is required at the input - if a voltage regulator provides an output of 5 volts and it must have at least three volts drop across it to provide the required regulation, then the input must be at least 5 + 3 = 8 volts.

Also the voltage drop across the regulator is dissipated as heat. This not only wastes energy and makes the regulator very inefficient, but it also increases the cost as larger transistors are required that can dissipate more heat, and also heatsinks and other aspects to the design arising from the heat dissipation mean that costs increase. The increases can be surprisingly large.

There are many instances where low voltage drops in a regulator are essential. Take for example a battery operated item such as a mobile phone, tablet or one of a host of battery operated items and gadgets we use these days.

Usually they are powered by lithium ion batteries typically providing between 3.3 and 3.6 volts. As many of the circuits will need a locally regulated supply of a particular voltage, some of which may have a rail volts only, say 100 or 200mV less than the battery voltage. This could not be provided by a traditional linear regulated power supply.

Instead, for applications such as these an LDO, low dropout regulator is a necessity. Not only is the power loss much less, but the rail voltage can also be preserved without any unnecessary reduction.

Also many of the circuits may often be placed into a sleep state when not in use to preserve battery life. LDO, low dropout regulators can normally be placed into a standby state where they consume a very small amount of quiescent current, something that not many standard regulators can do.

Low dropout regulator circuit technology

In order to develop a low dropout, LDO voltage regulator, it must be able to have a very low voltage across the series pass element, and still function to its requirements.

Traditional linear power supplies use an emitter follower format, often using a Darlington configuration to obtain the required current gain.

The very fact that the Darlington configuration has two base emitter voltage drops means that a higher voltage is needed to drive the base - for a silicon darlington this will be 1.2 - 1.3 volts. If it is to be driven hard to get a low collector emitter voltage then the drive circuitry must have a sufficiently high voltage to achieve this.

Also the saturation voltage between the collector and emitter is higher than for a single transistor or a FET, and this translates to a higher voltage between the input and output.

As the drive circuitry uses the incoming voltage as its power source, then it means that the incoming voltage must be at least 1.2 volts, and preferably more above the output voltage.

To overcome this issue, one solution is to use a FET series pass element - using a FET it is possible to achieve much lower input to output voltage levels.

The LDO low drop out regulator configuration often uses an N-channel FET, which can can have a very low series voltage drop - this is a function of the R_DS(on) figure for the FET.

Also the drive circuitry can be configured so that the driver circuitry can run from a rail taken from the incoming voltage without it being too far above the output voltage.

Although an N-channel FET is very often used, it is not the only solution. P-channel FET are also popular and even bipolar transistor configurations are found in some circuit designs

As would be expected, the overall low drop-out voltage regulator would consist of a series pass element - typically a low R_DS(on) FET - the gate drive circuitry, sensing of the output voltage, a voltage reference against which the sensed voltage can be compared, and then a differential amplifier which drives the gate drive circuits.

Another consideration is that the drive circuitry should take as little power as possible. In this way the quiescent current is reduced and the overall level of wasted power reduced and this is an important factor in many circuit designs where an LDO is needed.

An LDO, low drop-out regulator circuit such as this one could be used in a variety of ways - one example might be providing a 3V regulated supply from a 3.3 - 3.6 V lithium ion battery.

Factors affecting LDO drop-out voltage

Obviously one of the major performance factors for low drop out regulators is the actual dropout voltage. Particularly in low voltage circuit designs, this figure is critical.

Although several factors affect the minimum drop-out, one of the key factors os the level of R_DS(on) for the series FET, whether it is a discrete electronic component or contained within an LDO regulator chip.

The value of R_DS(on) required will obviously depend upon the anticipated current that will be drawn through the device, higher current applications needing lower values of R_DS(on).

In general, the value of R_DS(on) is dependent upon the size of the die used for the device. It is found that larger discrete FETs offer better performance than smaller ones contained within an IC. There is also a rough correlation between the IC package size and the values obtainable for R_DS(on).

However it must be remembered that there is not a full correlation because any IC LDO will also need some of the die area of the electronics circuits and not just the series FET output device itself.

Another factor affecting the LDO minimum drop-out resistance is input voltage. This results from the fact that in order to get a low value of R_DS(on), the series pass FET needs to be driven reasonably hard.

It is also worth noting that different manufacturers test drop out specifications differently, and this makes it more difficult to compare one device with another.

LDO efficiency

The efficiency of an LDO is normally of great importance. The very reason for using a low dropout regulator means that voltage drops need to be minimised to reduce power wastage.

Two main factors contribute to the overall efficiency: the voltage drop across the series pass device and the power consumption of the control and drive circuitry.

The quiescent current of the drive circuitry can be an important factor in determining the efficiency, especially for low power circuit designs.

The overall efficiency can be expressed:

Where:
    V_out = output voltage
    I_out = output current
    V_in = input voltage
    I_Q = quiescent current

Key LDO specifications

There are many specifications that are of importance to voltage regulators in general, and some that are particularly important for LDOs.

As many LDOs are available as integrated circuits, there are many different specifications associated with them. The specifications used by different manufacturers will differ, and this must be remembered when comparing ICs from different manufacturers.

• Input voltage, V_IN :   This is the input voltage range over which the regulator is guaranteed to operate correctly. In an under-voltage situation, some LDOs will lock-out and prevent the circuitry that is being powered from malfunctioning. The voltage at the input should not be allowed to go higher than the specified maximum otherwise excessive heat could be generated and the device and surrounding circuitry may be damaged or destroyed.

• Bias voltage (V_BIAS, V(_BAT)):   The bias or battery voltage is used as a power supply for the internal circuitry of the LDO sot hat it can provide sufficient drive for the series pass device, etc. A range of voltages will be given for the requirements of this supply.

• Output accuracy, V_OUT:   It is important that the output voltage is held within certain limits. The output specification for the LDO should be tighter than the requirements for the device being powered under all circuit conditions, load, current, temperature, etc.

• Quiescent current, I_QUIESC:   The quiescent current is the current of the internal circuitry minus the current associated with external features of the integrated circuit. There may be a variety of different options for quiescent current, so careful reading of these may be needed.

• Line regulation:   The line regulation is the change of the output voltage, V_OUT when there are changes to the input voltage.

• Load regulation:   The load regulation is the change in output for a given change in load or current. As LDOs are often used as Point-of-Load regulators it is possible that the current drawn may be relatively constant, but this is not always the case as many loads could change their the level of current drawn according to their state, etc.

• Dropout voltage V_DO:   The dropout voltage is the voltage between the input and output below which the regulator will cease to regulate effectively. It may be of the order of 100 mV or more, but be careful it could change with temperature.

• Max current, I_MAX:   There is a maximum current level for each device, beyond which the current limit circuitry on board the chip may start to operate. It is obviously essential to operate within the capacity of the chip.

There are many specifications associated with low dropout regulators. The actual specifications and the way they are stated will tend to vary between manufacturers, but despite this, it should be possible to gain a good comparison between the various devices on offer.

Popular LDO integrated circuits

The table below gives a summary of a few of the popular low dropout regulators - vast numbers of other devices are available and one of these is almost certain to be able to meet the requirements for any design.

Notes
  [1]   different variants of the LP2985 are available for different voltages in 100mV steps. It is not presettable.
  [2]   REG117 is available with fixed or variable voltages - adjustable voltage versions are set using two resistors. REG1117 with 800mA max output current has dropout voltage of 0.8A and REG1117A with dropout voltage of 1.3V has max output of 1A.

There are huge numbers of different low drop-out integrated circuits that are available to provide exactly the performance that is required.

Many of the devices are available as surface mount devices because they are widely used in low power equipment where surface mount technology is used. However some are also available as leaded devices because low dropout regulators are needed for higher power electronic circuit designs.

LDO, low dropout regulators are used in a large number of areas where they enable voltages to be reduced by relatively small amounts, but retaining the voltage regulation needed for these new voltage rails. With many items of electronic equipment being battery powered, comparatively small reductions are needed with a minimum of power loss to enable as little power as possible to be lost.

Although switch mode DC-DC converters could be used in some instances, the switching spikes present on the outputs of these modules means that linear voltage regulators with low drop out capabilities are needed to provide the clean supplies for various elements of the circuit.

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