Guide to Choosing the Best LDO for Your Application

A revised version of this article is available here

By Moises Robinson, Ph.D. and Jerry Rudiak, M.S.E.E. (Vidatronic)

ABSTRACT

LDOs are so common inside portable devices, state of the art power management integrated circuits (PMICs) for smartphones and other portable devices include over a dozen LDOs. To know which LDO you need, you must first define the application of your LDO and then examine which parameters are most important when dealing with that application. With so many different LDO applications and the multiple parameters that characterize a particular LDO, it is not easy to determine which LDO is best suited.

To help you figure this out, we have put together this reference. This guide presents a comprehensive list of all of the different LDO parameters with definitions, the most common applications of LDOs, and which parameters are critical for each.

INTRODUCTION

Power management is an important design consideration for most products, especially for portable devices that rely on a battery for their power.

A voltage regulator is an electronic component that automatically provides a regulated and controlled output voltage to the various components inside an electronic device. The two main categories of voltage regulators are low dropout linear regulators (LDOs) and switching regulators. There is much confusion in the marketplace with respect to LDOs. The majority of development and marketing budgets are centered around switch mode power supplies for the benefits they provide in power density and overall efficiency; however, LDOs are used in a wide variety of applications today. For example, LDOs are used to power multiple components inside handheld and portable devices due to their clean output voltage, low noise, small size, low shutdown current and low cost when compared to switching regulators. The clean voltage they provide makes them an ideal choice for high frequency, noise sensitive applications.

With all of the possible LDO applications and many parameters that can be optimized for each LDO, it’s not easy to determine which LDO is best suited for a particular application. There is usually a trade-off between achieving very low quiescent current (Iq) while also meeting other key parameters like excellent dynamic performance, low output noise, and high power supply rejection. This whitepaper is intended as a tool to help system designers choose the best LDO for their application.

LDO PARAMETERS

Different applications impose different requirements on LDOs. By clearly understanding the different LDO parameters and the requirements from different applications, system designers can choose the proper LDO for their application. Listed below are the important parameters and what they mean.

Capless operation. Most LDOs require an output capacitor for proper operation. LDOs that can provide their stated performance without an output capacitor are called capless. Capless LDOs are ideal for applications with challenging space requirements, such as smartphones and wearable devices.

DC line regulation. Change in output voltage for a change in input voltage. This measurement is made under conditions of low power dissipation.

DC load regulation. Change in output voltage for a static change in output current/load current. Dropout voltage. Input/Output differential voltage for which the regulator output no longer maintains regulation against further reductions in input voltage. Typically, the LDO dropout voltage is specified as the input voltage level that causes the output voltage to drop 100mV below its programmed, regulated value. As an LDO must provide the required constant voltage as the battery discharges, a small dropout voltage is very important as the smaller the dropout voltage, the greater the useful input voltage from the battery and therefore the longer the run time.

Efficiency. Power efficiency is defined as the percentage of the input power that is delivered to the output and it is expressed as,

LDO Efficiency (%) = 100 × (Iout × Vout)/((Iout + Iq) × Vin)

In order to maximize the LDO efficiency, the difference output voltage to the input voltage (Vout/ Vin), and the quiescent current (Iq) must be minimized.

Input voltage (Vin) range. The input voltage range determines the maximum and minimum allowable input supply for the LDO. Input supplies higher than the maximum allowable input can damage the LDO. The lowest input supply must be higher than the LDO output voltage plus the dropout voltage.

Maximum output current (Iout). Maximum output current that the LDO can provide while meeting the datasheet parameters.

No load current operation. Several applications need the LDO to hold the output and provide good performance under a no current load condition (e.g. CMOS RAM keep-alive applications). Some LDOs might suffer from degraded performance under no load conditions.

Output voltage noise. RMS output noise voltage generated only by the LDO over a given frequency range (typically 10Hz to 100KHz) under a constant output current and a clean input voltage. Most of the noise comes from flicker noise in the internal LDO bandgap reference.

Output voltage (Vout) accuracy. The output voltage accuracy describes the typical and worst case deviation of the output voltage with respect to the nominal LDO output voltage. The overall output voltage accuracy also includes the effects of line regulation and load regulation.

Output voltage range. The output voltage can be fixed or adjustable. If the output voltage is adjustable, it is important to know the maximum and minimum allowable output voltage as well as the programming steps.

Over-current protection. This feature limits the maximum amount of current that the LDO can source. This limit is established in order to protect the LDO and the system under a short-circuit or high current condition.

Over-temperature protection. This feature shuts down the LDO when the die temperature exceeds the specified high temperature level. The LDO is re-enabled when the temperature drops to a safe value.

Power supply rejection ratio (PSRR). A measure of how well the LDO rejects electrical noise at the input voltage when measured at the output voltage.

Quiescent current (Iq). Also called ground current, is the current used to operate the LDO, and is not delivered to the load. Measured when the LDO is enabled and the output/load current is zero (0). A small quiescent current is needed to maximize LDO output efficiency, reduce heat, and extend battery life in battery-operated applications.

Soft-start operation. Guarantees that the output voltage will ramp-up slowly from zero to the required output voltage. Soft-start is useful to avoid inrush current during start-up operation. Soft-stop operation. Guarantees that the output voltage will ramp-down slowly in a controlled fashion when the LDO is disabled.

Shutdown current (Isd). Leakage current through the LDO, when the LDO is disabled or powered down.

Start-up or turn-on time. Start-up time is the time between the rising edge of the enable signal and the output voltage reaching 90% of its nominal value.

Transient line regulation. Ability of the LDO to maintain a constant output voltage with a transient step at the input.

Transient load regulation. Change in output voltage for a dynamic (step) change in output current. Load transient regulation includes overshoot (difference between the maximum Vout and the initial Vout during a transient load regulation test when the output current changes to a lower value), as well as undershoot (difference between the minimum Vout and the initial Vout during a transient load regulation test when the output current changes to a larger value).

LDO SELECTION PROCEDURE

With so many LDO parameters, it is usually difficult for system designers to select an appropriate or best LDO for a certain application. To make the LDO selection process easier, we recommend dividing the LDO parameters into three groups:

• Basic Parameters. The LDO must meet these to provide the required functionality for a particular application. A typical basic parameter is the input voltage range.
• Performance Parameters. The LDO must meet these to provide the performance required for the particular application. A typical performance parameter is PSRR.
• Optional Parameters. These may or may not be required for a particular application. For example, if the LDO is part of a large SOC which already includes a temperature sensor, a temperature sensor inside the LDO may not be useful.

Table 1 shows the parameters separated into each of these three groups. Efficiency is not included because it is a function of the input/output voltage ratio (Vout/Vin) and the quiescent current (Iq).

Table 1. LDO parameters divided into three groups

To select the best LDO for your application, we recommend the following four step process:

1. Select a group of LDOs that meet the basic parameters for your application. For example, do you need a capless LDO for an embedded SOC application? What is the required dropout voltage? What input voltage range does the LDO need to support, etc.?
2. From the group of LDOs chosen in Step #1, choose the LDOs that meet the parameters from the performance column of Table 1 that are important for your application.
3. In this step, narrow your selection to the one performance parameter that is most important for your application.
4. Now that you have made your LDO selection, find out which optional parameters the LDO IP Core offers for your application.

LDO APPLICATIONS

Listed below are a few LDO application and the LDO performance parameters that are important for each. Please contact Vidatronic at sales@vidatronic.com if your particular application is not discussed in this white paper. We will gladly help you with your particular application.

Cameras (CMOS image sensor)

The analog supply voltage for most CMOS sensors used in cameras ranges from 2.4V to 3.1V. This voltage must be very clean for the sensor to support the camera specifications. Critical LDO parameters for this applications are a high PSRR, typically at 100 KHz, and a low output noise.

AMOLED or LCD Displays

The embedded analog circuitry inside AMOLED or LCD Displays requires a supply between 2.2V and 3.6V provided by an LDO. The key LDO performance parameters required to enhance image quality are high PSRR, fast transient load regulation, and tight output voltage accuracy.

Digital IC Loads

LDOs are used to power multiple types of logic circuits including microprocessors, microcontrollers, embedded memories, and digital signal processor blocks. The input voltage ranges for these digital blocks continue to be pushed lower (e.g. sub 1 Volt) and with tighter tolerances. To guarantee that the LDO output voltage always stays within the minimum and maximum levels, the LDO must have good output voltage accuracy, good line and load DC regulation, and fast transient line and load regulation with small overshoot/undershoot. The requirement for good DC line regulation directly translates into a requirement for good low frequency PSRR, since both parameters are a measure of how much the output voltage changes as a function of a change in the input voltage.

If the logic circuits are used inside handheld devices, low quiescent current (Iq) and low shutdown current (Isd) are important in order to minimize system power consumption in sleep mode. A fast start-up or turn-on time is also required when the digital blocks come out of sleep mode, and a small dropout voltage is highly desirable to power digital IC loads in order to optimize power efficiency.

Phase Locked Loops (PLLs) and Clock and Data Recovery Circuits

LDOs provide the clean power supply for most voltage controlled oscillators (VCOs) used inside Phase Locked Loops (PLLs) and Clock and Data Recovery (CDR) circuits. Such PLLs and CDRs are used to meet tight phase noise and clock jitter specifications in high-speed transceiver and serializer/ deserializers (SERDES) blocks. Critical LDO performance parameters for these applications include tight output accuracy, high PSRR, and low output noise.

High-Speed and High-Accuracy Data Converters

To get the optimum performance from high-speed or high-accuracy analog to digital (ADC) and digital-to-analog (DAC) converters, the data converter must be supplied with very clean DC power supplies and reference/bias voltages [1]. Because noise on the supply can affect the converter noise level as well as the spurious performance of the converter, the power supply rejection ratio (PSRR) of the voltage regulator that generates the supply or reference voltages for the data converter is the most important parameter for data converter applications. PSRR in excess of -40dB for frequencies greater than or equal to10MHz may be required to avoid any high frequency noise degrading the data converter performance.

RF Sections in Portable Devices

The RF section in portable devices runs at higher voltages than the baseband functions and processor, reducing the difference between the input and output voltage. The current consumed by all RF blocks is typically small (< 100mA) except for the transmitter power amplifier. The low power and small difference between the input and output voltage make the RF sections of portable devices an ideal application for LDOs. Some critical blocks inside the RF section include input low noise amplifiers (LNAs), mixers, and PLLs. A clean power supply is required to meet tight RF receiver sensitivity and to avoid issues with transmit spectral purity which are subject to government compliance standards. Critical LDO parameters for such applications include very low output noise, high PSRR at high frequencies, and fast start-up.

Audio CODECs

Portable devices must process several types of audio signals including voice and MP3 files. The audio CODECS in portable devices require clean power supplies in order to meet challenging resolution, Harmonic Distortion (THD), and dynamic range requirements of the Analog-to-Digital (ADC) and Digital-to-Analog (DAC) converters. Important LDO parameters for audio applications are tight output accuracy, high PSRR, and low output noise.

There are many other noise sensitive applications which require LDOs with low noise as well as high PSRR. These applications include test and measurements, medical equipment, base stations and communications equipment, among others [2].

VIDATRONIC HIGH PERFORMANCE LDOs

Vidatronic capless LDO Intellectual Property (IP) Cores offer unparalleled performance which make them ideal for a host of power applications. Using our Noise Quencher technology, our LDO IP Cores have several features that add significant value to the overall system solution:

• Smallest overall solution due to capless operation (no need for an output capacitor)
• Output voltage accuracy better than +/- 1%, without trimming
• Output current (Iout) higher than 1A
• Power supply rejection ratio (PSRR) better than -40dB up to 10 MHz
• Overshoot/undershoot less than +/- 50mV without an output capacitor
• Unconditional stability regardless of output capacitance used including capless
• Controlled soft-start and soft-stop operation
• Over-current and over-temperature protection
• Dropout voltages of less than 30mV for 100mA output current
• Small silicon area and reduced pin count through capless operation

SUMMARY

Today’s portable devices are complex systems that continue to add more and more functions that require multiple supply voltage levels. Low dropout (LDO) linear voltage regulators are a key component used in multiple applications, including portable devices. With this simple procedure and parameter/application reference, system designers can select the best LDO for any application.

Contact Vidatronicto learn how we can provide the best LDO IP Core for your application.

REFERENCES

[1] Michael Cobb, “Guidelines for Supplying Power to High Speed ADCs,” EETimes, Europe: Power Management, December 13, 2010.

[2] Sureena Gupta, “How to pick a linear regulator for noise-sensitive applications,” Texas Instruments Analog Applications Journal, 1Q 2013, pages 25-27.

For more information, please visit vidatronic.com