Ldo ppt

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Originally written by: Edgar Sanchez-Sinencio, Ph. Revised: January by Stephen M. Nolan Vidatronic, Inc. Power management of battery-powered electronic devices is becoming increasingly more important for the microelectronics industry. This white paper details the difference between low dropout LDO voltage regulators that use external output capacitors and those that do not, and how your system designs can benefit from not using an output capacitor.

Well-designed capless LDO voltage regulators can have multiple benefits, and they are presented here. The demand for battery-powered electronic devices with better performance and higher efficiency continues to increase, driving engineers to design systems that provide best-in-class performance, while consuming minimal power.

Low Drop-Out Voltage Regulators - PowerPoint PPT Presentation

Subsequently, the power management of such devices is increasingly more important for the microelectronics industry. For systems to be successful in the market, it is vital to increase the operating time of the device and the battery while reducing the total system cost. Therefore, an increasing number of voltage regulators are required. The low drop-out LDO voltage regulator is often used to provide a stable and highly-accurate low-noise output voltage.

A typical LDO voltage regulator normally requires an external capacitor.

ldo ppt

This external capacitor is usually necessary to have acceptable transient response, power supply noise rejection PSRand stability of the LDO. The external capacitor, however, is usually bulky, occupies valuable board space, decreases long-term reliability, and consumes valuable pins in SoC devices. Additionally, the practical selection of a real capacitor that meets all the requirements of the LDO over a broad range of operating conditions can be very difficult.

An ideal solution would not require an external capacitor. Some LDO voltage regulators use architectures that do not require the external load capacitance.

The absence of the external capacitor brings many economic advantages; however, significant degradation can often be seen in areas of transient performance and power supply noise rejection. As a result, significant design challenges accompany the capacitor-free LDO voltage regulator that must be economically and efficiently overcome. A typical LDO voltage regulator consists of a voltage reference, an error amplifier, a pass transistor, and a resistor feedback.

The output voltage of the regulator is fed back as one of the inputs to the error amplifier. Therefore, LDO voltage regulators are a closed-loop architecture circuit. Because feedback exists, a poorly designed voltage regulator can become unstable in response to a transient condition at the input or output and can produce ringing or, worst-case, continuous oscillations at the output.

When designing a voltage regulator, loop-response analyses must be done to ensure stable operation. Gain and phase margin are two important loop stability analyses that are considered in any good regulator design. Traditional LDO voltage regulators use off-chip output capacitors in the range of a few microfarads. This topology has one dominant pole at the output, which yields good stability in the loop-response analyses. Offchip capacitors also contribute to good power-supply noise rejection, line regulation and load regulation.

There are, however, several practical concerns that must be considered when selecting external output capacitors in those applications where they are required.

External, discrete capacitors have non-idealities that can be critical. There exist several elaborate models of a real capacitor. In Figure 1, above, we include a relatively simple model of a real capacitor to illustrate its nature. It consists of four impedances, connected as shown.

The impedance on the left is the ESR, the middle is the ideal capacitor connected in parallel with a large leakage ieee 69 bus matlab code, and on the right is the ESL.A low dropout regulator LDO consists of a voltage reference, an error amplifier, a feedback voltage divider, and a series pass element, usually a bipolar or CMOS transistor see Figure 1.

Output current is controlled by the PMOS transistor, which in turn is controlled by the error amplifier. This amplifier compares the reference voltage with the feedback voltage from the output and amplifies the difference. If the feedback voltage is lower than the reference voltage, the gate of the PMOS device is pulled lower, allowing more current to pass and increasing the output voltage.

ldo ppt

If the feedback voltage is higher than the reference voltage, the gate of the PMOS device is pulled higher, allowing less current to pass and decreasing the output voltage.

Analog Devices LDOs are designed to be stable over the specified operating temperature and voltage ranges when the recommended capacitors are used. The LDOs response to rapid changes in load current, i. Using a larger value of output capacitor improves the transient response of the LDO; however, it can increase the start-up time. LDO regulators are used to derive lower output voltages from a main supply or battery.

The output voltage is ideally stable with line and load variations, immune to changes in ambient temperature, and stable over time. LDOs should have as low a difference between the input and output voltage as possible, called the dropout voltage. For example, in a battery-powered design using a lithium-ion cell connected to a 2.

In some systems LDOs are used for postregulation. The LDO connects to the output of a high-efficiency switching regulator and provides noise filtering, as well as a constant and stable output voltage.

What are some key selection criteria used for selecting an LDO? Depending on your particular design your selection criteria may differ.

However, as a general rule you should use the list below in the order that they appear. Input voltage range Output voltage, fixed or adjustable Output accuracy over line, load, and temperature Load current requirement Dropout voltage Power supply rejection ratio PSRR Output noise Quiescent current and shutdown current. Does the output bypass capacitor value affect LDO performance? Increasing the load capacitance above the recommended value can improve load transient response.

However, when a larger output capacitor is chosen, the input bypass capacitor should be increased to match it. Note: the input and output capacitors should be placed as close as possible to the LDO. Any good quality ceramic capacitors can be used, as long as they meet the minimum capacitance and maximum effective series resistance ESR specifications listed on the LDO data sheet. Ceramic capacitors using X5R or X7R dielectrics are highly recommended as these have good temperature stability and a low voltage coefficient.

Does LDO ground current vary with load current? What is power supply rejection ratio PSRR?Toggle navigation. Help Preferences Sign up Log in. Featured Presentations. Membrane Electrodes Just a note before we begin discussing how to use the LDO system - We will be using 2 different systems until all of the permits Application type should be one of the following We will be using 2 different systems until all of the permits Agenda - Agenda LDO Measuring DO Winkler Titration EPA Interferences, positive and negative Atomic positions.

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ldo ppt

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ldo ppt

Why would I want to change a system that benefits me? My designator has always promoted well against others.The shunt regulator operates by maintaining a constant voltage across its terminals and it takes up the surplus current to maintain the voltage across the load. One of the most common examples of the shunt regulator is the simple Zener diode circuit where the Zener diode acts as the shunt element. As such the shunt voltage regulator is an essential element within linear power supply technology.

The basic operation of a shunt voltage regulator can be seen from the diagram. Essentially the load is operated with a resistor in series with the voltage source and the shunt regulator then in parallel with the load. In order to keep the voltage across the load constant, a level of current must be drawn through the series resistor to maintain the required voltage across the load. The load will take some and the remaining current is drawn by the shunt voltage regulator.

The circuit is designed so that at maximum load current the shunt regulator draws virtually no current and at minimum load current, the shunt voltage regulator passes the full current. As a result, it can be seen that shunt regulators are inefficient because maximum current is drawn from the source regardless of the load current, i.

One of the most common and simple forms of shunt regulator is the simple Zener diode regulator circuit shown below. Its operation is very straightforward. Once over its small minimum current, the Zener diode maintains an almost constant voltage across its terminals. In this circuit, the series resistor drops the voltage from the source to the Zener diode and load. As the Zener diode maintains its voltage, any variations in load current do not affect the voltage across the Zener diode. It takes up the current variations required to ensure the correct drop across the series resistor.

Shunt Voltage Regulator: shunt regulator

In this way it shunts sufficient current to maintain the voltage across its terminals and hence the load. In this shunt voltage regulator circuit, the Zener diode must be capable to dissipating the power from the maximum amount of current it is likely to handle. This is most likely to be a little more than the maximum current supplied to the load as the Zener diode will need to pass all the current when load current is zero. Thus the total maximum current that will be passed by the diode is the load current plus an allowance for current to maintain the reference voltage when the load is taking its maximum current.

It should also be noted that for the shunt regulator circuit, the series resistance is comprised of the series resistor value, plus any source resistance. In most cases the value of the series resistor will dominate and the source resistance can be ignored, but this may not always be the case.

The basic shunt voltage regulator above does not have any feedback, i. As imagined, the performance of this form of shunt regulator is sufficient for many applications, but much higher levels of performance can be achieved by providing feedback based on the output voltage of the shunt voltage regulator and feeding this back into the system to ensure that the required output voltage is accurately maintained.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.

The advantages of a low dropout voltage regulator over other DC to DC regulators include the absence of switching noise as no switching takes placesmaller device size as neither large inductors nor transformers are neededand greater design simplicity usually consists of a reference, an amplifier, and a pass element.

The disadvantage is that, unlike switching regulatorslinear DC regulators must dissipate power, and thus heat, across the regulation device in order to regulate the output voltage. 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. Low-dropout LDO regulators work in the same way as all linear voltage regulators. 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. If a bipolar transistor is used, as opposed to a field-effect transistor or JFETsignificant additional power may be lost to control it, whereas non-LDO regulators take that power from voltage drop itself.

The Fundamentals of LDO Design and Applications

For high voltages under very low In-Out difference there will be significant power loss in the control circuit.

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.

Power FETs may also increase the cost. However, when the LDO is in full operation i. It is important to keep thermal considerations in mind when using a low drop-out linear regulator. Additionally, efficiency will suffer as the differential widens. Depending on the packageexcessive power dissipation could damage the LDO or cause it to go into thermal shutdown. Among other important characteristics of a linear regulator is the quiescent currentalso 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:.

Quiescent current is current drawn by the LDO in order to control its internal circuitry for proper operation.


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