The oldest and most commonly used device for voltage regulation, an LDO (Low Dropout Linear Regulator) is likely to be used in almost any circuit design. While an LDO regulator is usually one of the lowest cost components in any given system, it is often one of the most valuable components from a cost/benefit perspective.

For most applications, the basic parameter specifications in the product datasheet are usually clear. Unfortunately, the datasheet does not list the parameters for every possible circuit condition, and many key performance parameters are not fully understood or at least not used to the maximum extent. Therefore, in order to fully utilize the advantages of LDO, engineers must have a deep understanding of the key performance parameters and their impact on specific loads.

This article will explore the main performance parameters of LDOs and their role in providing clean output voltages for different devices in electronic systems, from LDO concepts to systems. In addition, it will discuss the important factors that design engineers need to consider during system optimization.

Electronic Engineering World (ID: EEworldbbs) | Produced

Low dropout regulator (LDO) is a widely used voltage regulation solution that can convert a higher input voltage into a stable low voltage output under the condition of a small input-output voltage difference. As a linear regulator, LDO optimizes the voltage drop by adjusting the channel transistor, similar to an electronically controlled variable resistor that automatically adjusts the resistance value to ensure a stable output voltage at a specific output current.

LDO provides a simple and efficient voltage regulation method to generate a stable output voltage under constant load current. Its design structure has low dependence on external components, which helps reduce system complexity and material cost.

The typical LDO structure requires only a few external components, usually including capacitors for input decoupling and capacitors for output stabilization, and in some topologies, protection diodes can also be added to deal with reverse voltage conditions. This simple and efficient design makes LDO an ideal choice for stable voltage supply in a variety of applications. In summary, LDO contains four basic functional elements:

• Reference Voltage Source: provides a stable reference voltage for comparison with the output voltage;

• Feedback Network: Samples the output voltage and feeds it back to the error amplifier

• Error Amplifier: Compares the output voltage with the reference voltage and adjusts the control element based on the difference.

• Control Element: Usually a power transistor (such as MOSFET or BJT) that adjusts the input voltage to maintain a stable output voltage. The current mainstream types are PMOS and NMOS.

LDO regulator designs usually include four different pass elements: NPN transistor-based regulators, PNP transistor-based regulators, N-channel MOSFET-based regulators, and P-channel MOSFET-based regulators. Although the NPN and PNP transistor-based regulators have higher voltage dropouts than MOSFET-based regulators, the MOSFET quiescent current (Iq) is significantly lower than that of transistor-based regulators.

So, in summary, PMOS and NMOS LDO are the mainstream nowadays.

LDOs are generally easy to design and use, but in modern applications, systems often contain multiple analog and digital modules, so choosing a suitable LDO requires comprehensive consideration based on system characteristics and operating conditions.

In fact, the essence of LDO can be understood with a metaphor: imagine a duck crossing a pond. The water seems calm, but its feet are paddling vigorously under the water. The harder the duck paddles, the faster it runs out of energy and has to stop. Similarly, when the voltage regulator works harder to maintain a stable voltage, the energy consumption increases. Therefore, the key is to choose a voltage regulator that can use power efficiently and maintain stability to ensure consistent performance.

There are many parameters to judge the performance of LDO. If you want to use LDO well, you must understand the meaning of each performance parameter.

Pressure difference

The dropout voltage is the difference between the input and output voltages when the input voltage drops further and the LDO can no longer regulate. Under this dropout condition, the pass element operates in the linear region, similar to a resistor whose resistance is equal to the drain-to-source on-resistance RDS(ON). Today's LDO regulators typically use PMOS or NMOS FETs as pass elements, which can achieve a dropout voltage of 30mV to 500mV depending on load conditions. The dropout voltage formula is:

Among them, RDS (ON) includes the resistance of the path element, the on-chip interconnect resistance, the pin resistance and the wire bond resistance, and can be estimated by the voltage drop of the LDO. In the voltage drop mode, the variable resistance is close to zero, the LDO cannot adjust the output voltage, and the parameters such as input voltage and load regulation, accuracy, PSRR and noise are no longer meaningful.

Early LDO designs provided a dropout voltage of approximately 1.3 V, meaning that for an input voltage of 5 V, the maximum output that the device could regulate was only about 3.7 V. However, with today's more sophisticated design techniques and wafer fabrication processes, 'low' is roughly defined as <100 mV to around 300 mV.

Headroom voltage

The headroom voltage is the difference between the input and output voltages required for the LDO to meet its specifications, typically around 400 mV to 500 mV, but some LDOs may require a headroom voltage as high as 1.5 V. Data sheets often specify the headroom voltage as a condition for specifying other parameters. It is important to note that the headroom voltage should not be confused with the dropout voltage, which are equal only when the LDO is in dropout mode.

Quiescent Current

Quiescent current (Iq) is a common concept in analog. As the name implies, it refers to the current consumed when the system is in standby mode and under light load or no load conditions. Quiescent current applies to most integrated circuit (IC) designs, where amplifiers, buck-boost converters, and low dropout regulators (LDO) all affect the amount of quiescent current consumed. When the LDO is fully operational, the following formula is used for calculation:

The quiescent current may not seem to consume much power, but for products such as smart watches or mobile phones that are in standby mode for a long time, the smaller the LDO quiescent current, the longer the battery life. At the same time, to optimize the efficiency of the LDO, the quiescent current and the difference between the input and output voltages must be minimized. Since the difference between the input and output voltages directly affects the efficiency and power consumption, the lowest voltage difference is usually selected.

Ground current

Ground current (IGND) is the difference between input current and output current, including quiescent current. Low ground current can maximize the efficiency of LDO.

For high-performance CMOS LDOs, the ground current is typically less than 1% of the load current. As the load current increases, the ground current also increases because the gate drive of the PMOS pass element needs to increase to compensate for the voltage drop caused by RON. In the dropout region, when the driver stage enters saturation, the ground current also increases. CMOS LDOs are ideal for applications that require low power or small bias currents.

Shutdown current

The shutdown current is the input current consumed by the LDO when the output is disabled. Both the reference circuit and the error amplifier are not powered in shutdown mode. Higher leakage current causes the shutdown current to increase with temperature.

efficiency

The efficiency of an LDO is determined by the ground current and the input/output voltage. To achieve high efficiency, the headroom voltage and ground current must be minimized. In addition, the voltage difference between the input and output must be minimized. The input-to-output voltage difference is an intrinsic factor in determining efficiency, regardless of load conditions. For example, the efficiency of a 3.3 V LDO will never exceed 66% when powered by a 5 V supply, but when the input voltage drops to 3.6 V, its efficiency increases to a maximum of 91.7%.

noise

Since LDOs are electronic devices, they will generate noise. There are two ways to view this noise: viewing it across frequency and viewing it as an integrated value. To generate a clean power rail that will not affect system performance, it is important to select a low-noise LDO and take steps to reduce the noise. Since the closed-loop transfer function has limited effect on suppressing reference voltage noise, most low-noise LDOs require additional filters to block the noise from entering the closed loop.

In addition to selecting an LDO with low noise characteristics, some techniques can be applied to ensure that the LDO has the lowest noise characteristics. These techniques involve the use of noise reduction capacitors and feed-forward capacitors, or optimizing output noise and PSRR performance through additional pins and small capacitors to filter out noise on the bandgap.

Power Supply Rejection Ratio (PSRR)

PSRR is a common technical parameter that specifies the attenuation of an AC component of a specific frequency from the LDO input to the output.

For LDO, power supply rejection in the range of 100 kHz to 1 MHz is very important.

The power supply rejection ratio (PSRR) is often mistakenly considered to be a single static value, but this is not the case. Frequency, load current, LDO headroom (input to output voltage difference), output capacitance, input voltage, temperature, LDO architecture design, feedback network, and compensation all affect PSRR. For practical applications, the PSRR for a specific application can be improved by adjusting VIN - VOUT and output capacitance alone, but the factors that affect PSRR are not limited to these two items, but also include the following parameters:

When comparing the PSRR of LDOs, ensure that the test conditions are the same, especially considering the effects of headroom voltage and load current. In addition, the PSRR should cover different frequencies and provide typical operating performance curves. The output capacitor has an impact on the high-frequency PSRR. The impedance of a smaller capacitor is higher than that of a larger capacitor. Therefore, when comparing PSRR, the type and value of the capacitor must be consistent to ensure a valid comparison.

Line Regulation

Line regulation is the change in output voltage for a given change in input voltage. Since line regulation also depends on the performance of the pass element and the closed-loop DC gain, dropout operation is often not included when considering line regulation. Therefore, the minimum input voltage for line regulation must be above dropout.

Load Regulation

Load regulation is the change in output voltage for a given load change, which is usually from no load to full load. Load regulation reflects the performance of the pass element and the closed-loop DC gain of the regulator. The higher the closed-loop DC gain, the better the load regulation.

Transient response

LDO is widely used in fields with high requirements for load regulation, such as digital IC, DSP, FPGA and CPU. These applications require LDO to respond quickly to maintain voltage stability. Transient response is a key performance parameter of LDO, including load transient response and line transient response.

Load transient response refers to the change in output voltage when the load current changes, which is affected by the output capacitance, ESR, LDO control loop bandwidth and the rate of change of the load current. When the rate of change is slow, the LDO can track the change, but when it is too fast, it may cause abnormal behavior, such as excessive ringing.

Line transient response refers to the change in output voltage when the input voltage changes, which is affected by the LDO control loop bandwidth and the input voltage change rate. When the input voltage changes slowly, it may hide ringing or abnormal behavior.

The transient response depends mainly on the bandwidth of the closed-loop transfer function. To optimize the response, the closed-loop bandwidth should be as high as possible while maintaining sufficient phase margin to ensure system stability.

The above parameters are very important in practical applications. However, if you want to evaluate a high-performance LDO chip, there are three main standards: one is the power supply's noise suppression ratio, the second is the LDO's own output noise and its transient response capability to load changes, and the third is whether the temperature drift is small enough in high-precision sensor applications.

After knowing the above parameters, we can further digest these parameters from the manufacturer's actual products. After all, the parameters promoted by the manufacturer are definitely the most critical ones.

Although LDO is not a high-performance IC, the LDO chip market is extremely competitive. Major manufacturers include TI, ADI, ST, ONsemi, Infineon, Microchip, Diodes, Rhom, etc.

TI's LDO products are very rich and can help meet almost all regulator design challenges, from powering sensitive analog systems to extending battery life. Solutions include the industry's first intelligent AC/DC linear regulator, as well as multiple features such as low noise, wide input voltage range (VIN), small package size and low quiescent current (Iq). The overall characteristics are low noise, low Iq and high power density. TI's classification method is voltage range. In addition to low noise, low Iq, miniaturization, etc., TI also classifies automobiles into one category.

ST's LDO is mainly divided into three series: high PSRR LDO regulator, low Iq LDO regulator, and ultra-low dropout LDO regulator. Its product features are low dropout, low quiescent current (low IQ), fast transient response, low noise, and good ripple suppression.

Infineon Technologies offers a wide range of linear regulators (LDOs), including high-precision voltage followers, variable linear regulators, ultra-low quiescent current regulators (LDOs) and high-performance regulators. Unlike switching power supplies, low quiescent current regulators cannot be used with buck or boost converters. Buck and boost converters are able to "step down" or "step up" a voltage, respectively. They cannot be used with linear regulators because the input voltage must always be greater than the output voltage.

Renesas is a leading supplier of low dropout regulators (LDOs), providing low-power handheld applications equipped with world-leading single and dual LDOs, featuring ultra-high PSRR up to 3MHz, low input voltage range, compact packaging, ultra-low noise and quiescent current. Renesas' high-current LDOs support up to 3A current, with industry-leading fast load transient response, the tightest voltage output accuracy at full load and full junction temperature, and the lowest dropout voltage in its class. In addition, Renesas' low-cost linear regulators can also generate low-voltage bias supplies from intermediate distribution voltages commonly used in telecommunications and data communications. These devices can be used as startup or continuous low-power regulators.

ADI manufactures a wide range of high performance low dropout (LDO) linear regulators. These LDO linear regulators offer very low dropout voltage, fast transient response, and excellent line and load regulation, with features that add performance value in end applications in wired/wireless and audio systems, FPGA/DSP/µC power supplies, and RF and instrumentation. Our wide range of LDO linear regulators has a wide range of outstanding features to meet the needs of any design, whether it requires low noise, high PSRR, or compact packaging.

In recent years, more and more high-quality domestic LDOs have been born, such as Nanochip, SiRuiPu, Silergy, Lixun Microelectronics, Blue Arrow Electronics, Richtek Technology, Treis, U-Tai Semiconductor, and SiWang Electronics.

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