Low-Dropout (LDO) Linear Regulators | FAQ

March 24, 2009
Frequently asked questions regarding Low-Dropout (LDO) Linear Regulators

What are the typical applications for a low-dropout (LDO) linear regulator?
LDO linear regulators usually are employed in systems that require a low-noise power source instead of a switching regulator that might upset the system. LDOs also find use in applications where the regulator must maintain regulation with small differences between the input supply voltage and output load voltage, such as battery-powered systems. Their low dropout voltage and low quiescent current make them a good fit for portable and wireless applications.

How much output current can an LDO provide? LDOs with an on-chip power MOSFET or bipolar transistor typically provide outputs in the 50-to 1000-mA range.

What is a typical LDO regulator circuit?
An LDO voltage regulator operates in the linear region. Shown in the figure (below) is Analog Devices ADP121, whose circuit is basically similar for most LDOs. Its main components are a series pass transistor (bipolar transistor or MOSFET), differential error amplifier, and precise voltage reference.

What characteristics affect an LDO’s performance?
The key operational factors for an LDO are its dropout voltage, power-supply rejection ratio, output noise, and quiescent current.

What is dropout voltage?
Low dropout refers to the difference between the input and output voltages that allow the IC to regulate the output load voltage. That is, an LDO can regulate the output load voltage until its input and output approach each other at the dropout voltage. Ideally, the dropout voltage should be as low as possible to minimize power dissipation and maximize efficiency. Typically, dropout is considered to be reached when the output voltage has dropped to 100 mV below its nominal value. The load current and pass transistor temperature affect the dropout voltage.

What is power-supply ripple rejection?
Power-supply ripple rejection (PSRR) affects the LDO’s ability to prevent output voltage fluctuations caused by variations in input voltage. PSRR is usually specified at a specific frequency, for example, 60-dB rejection at 120 Hz. Low-ESR (equivalent series resistance) output capacitors and added reference voltage bypass capacitors improve the PSRR performance. Battery-based systems should employ LDOs that maintain high PSRR at low battery voltages.

What affects an LDO’s output noise?
An LDO’s internal voltage reference is a potential noise source, usually specified as microvolts rms over a specific bandwidth, such as 30 µV rms from 1 to 100 kHz. This low-level noise causes fewer problems than the switching transients and harmonics from a switch-mode converter. In the figure, the LDO has a (voltage-reference) bypass pin to filter reference voltage noise with a capacitor to ground. Adding the datasheet-specified input, output, and bypass capacitors usually results in a non-problematic noise level.

What is quiescent current?
Another important characteristic is the quiescent or ground current (the current flowing through the system when no load is present), which creates a difference between the input and output currents. The series pass element, topologies, and ambient temperature are the primary contributors to quiescent current. Quiescent current and input to output voltage limit LDO efficiency and should be minimized.

How does the output capacitor affect LDO performance?
Controlling the LDO’s frequency compensation loop to include the load capacitor reduces sensitivity to the capacitor’s ESR, which allows a stable LDO with good quality capacitors of any type. In addition, output capacitor placement should be as close as possible to the output.

What circuit features can enhance LDO performance?
An enable input permits external control of LDO turn-on and turn-off, which allow the sequencing of supplies in multi-rail systems. Soft-start limits inrush current and controls output voltage rise time during power-up. A sleep state minimizes power, particularly in battery-based systems. A bypass pin enables an external capacitor to reduce reference voltage noise. An error output indicates if the output is going out of regulation, also known as a voltage-good or power-good output. Thermal shutdown turns the LDO off.

What factors determine the optimum LDO for a specific application?
Considerations include the type and range of the applied input voltage, required output voltage, maximum load current, minimum dropout voltage, quiescent current, power dissipation, and shutdown current.

How does output-capacitor ESR affect LDO performance?
The output capacitor’s ESR can affect LDO control loop stability. For example, a minimum of 1-µF capacitance with an ESR of 500 mΩ or less is usually recommended to ensure the stability of a CMOS LDO. Transient response to changes in load current is also affected by output capacitance. Plus, using a larger value of output capacitance improves the transient response of the LDO to large changes in load current.

What type of output capacitors should be employed with an LDO?
Any ceramic capacitors that are of good quality can be used with most LDOs, as long as they meet the minimum capacitance and maximum ESR requirements. Ceramic capacitors are manufactured with a variety of dielectrics, each with different behavior over temperature and applied voltage. Capacitors must have a dielectric adequate to ensure the minimum capacitance over the necessary temperature range and dc bias conditions. X5R or X7R dielectrics with a voltage rating of 6.3 V or 10 V are recommended. Y5V and Z5U dielectrics are not recommended, due to their poor temperature and dc bias characteristics.

What is the effect of the input bypass capacitor?
Connecting a 1-µF capacitor from input to ground reduces the circuit sensitivity to printed circuit board (PCB) layout, especially when long input traces or high source impedances are encountered. If the output requires more than a 1 µF capacitor, the input capacitor should be increased to match it.

What are some typical commercially-available LDOs?
150 mA, Low Quiescent Current, CMOS Linear Regulator
Analog Devices’ ADP121 is a low quiescent current, low dropout, linear regulator that operates from 2.3 V to 5.5 V and provides up to a 150 mA output. The 135 mV dropout voltage at 150 mA load improves efficiency and allows operation over a wide input voltage range. The 30 μA of quiescent current at full load makes the ADP121 ideal for battery-operated portable equipment.

The ADP121 is available in 16 fixed output voltage options ranging from 1.2 V to 3.3 V. The part is optimized for stable operation with a small 1 μF ceramic output capacitor. The ADP121 delivers good transient performance with minimal board area. Short-circuit protection and thermal overload protection circuits prevent damage in adverse conditions. PSRR is 70 dB from 1 kHz to 10 kHz with a 150 mA load. The output voltage accuracy is ±1%. Noise output 40µV rms at VOUT = 1.2 V. Shutdown current is <1 µA. The ADP121 is available in a 5-lead TSOT and 4-ball 0.4 mm pitch WLCSP packages and utilizes the smallest footprint solution to meet a variety of portable applications. 250mA LDO Withstands 80V Input and Operates from -55°C to +125°C.

250mA LDO Withstands 80V Input and Operates from -55°C to +125°C

Linear Technology’s LT3013MP is high reliability (MP) grade, wide temperature range micropower LDO with input voltage capability up to 80V. The MP grade device is offered in the TSSOP package and operates from -55°C to +125°C junction temperature, featuring low dropout voltage of only 400mV while delivering up to 250mA of output current. The wide VIN capability of 3.3V to 80V and adjustable output from 1.24V up to 60V makes it ideal for automotive, avionics, 48V telecom backup supplies and industrial control applications. With its very low quiescent current of 65µA (operating) and 1µA (shutdown), the LT3013 improves battery life in battery-powered memory "keep alive" systems that require extended run times. The LT3013's Power Good flag feature is programmable and indicates output regulation.

For high voltage applications that require large input-to-output differentials, the LT3013 offers a very compact and thermally effective solution. The thermally enhanced TSSOP package provides thermal resistances comparable to much larger conventional packages. The device operates with very small, low cost ceramic output capacitors, and is stable with only a 3.3uF output capacitor, compared to prior regulators of comparable output current and high voltage requiring 10uF to more than 100uF. These tiny external capacitors can be used without any necessary series resistance as is common with many other regulators. Internal protection circuitry includes reverse-battery protection, current limiting, thermal limiting, and reverse-current protection.

Low-Noise 500mA LDO Regulators in a 2mm x 2mm TDFN Package
Maxim’s MAX8902A/MAX8902B are low-noise linear regulators that can deliver up to 500mA of output with only 16µVRMS of output noise in a 100kHz bandwidth. These regulators maintain their output voltage over a wide input range, requiring only 100mV of input-to-output headroom at full load.
The LDOs maintain a low 80µA typical supply current, independent of the load current and dropout voltage. The regulator control circuitry includes a programmable soft-start circuit and short circuit, reverse current, and thermal-overload protection. Other features include an enable input and a power-OK output (MAX8902B only).

The MAX8902A has a 1.7V 5.5 input voltage range. Output voltage can be set to 1.5V, 1.8V,2.0V, 2.5V, 3.0V, 3.1V, 3.3V, 4.6V, or 4.7V using its SELA and SELB inputs. The MAX8902B output voltage can be set between 0.6V and 5.3V with an external resistor voltage-divider. Output accuracy is ±1.5% over load, line, and Temperature.

PSRR is 92 dB at 5 kHz. Dropout is 100mV (max) with a 500mA Load. Shutdown supply current is < 1µA.


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