Technical Background of LDO Dropout Voltage and PSRR Characteristics

Low Dropout regulators, commonly known as LDOs, are important power management ICs used to provide stable, low-noise DC output voltage from a higher input voltage. They are widely used in consumer electronics, automotive systems, industrial control, 5G communication equipment, medical devices, and precision analog circuits.

Among all LDO parameters, dropout voltage and Power Supply Rejection Ratio, or PSRR, are two of the most important performance indicators. Dropout voltage determines the minimum input-output voltage difference required for stable regulation, while PSRR measures the regulator's ability to suppress input ripple and noise.

For battery-powered systems, a lower dropout voltage helps extend usable operating time under low-voltage conditions. In RF transceivers, ADCs, medical electronics, and precision analog circuits, high PSRR is critical because power supply noise can directly affect signal integrity and measurement accuracy.

Main LDO Types Covered in the Test

LDO Type	Main Feature	Typical Application
General-purpose CMOS LDO	Low cost, mature process, balanced performance	Consumer electronics, basic industrial control
NPN BJT LDO	Stable regulation but higher dropout voltage	Legacy power supply designs
Ultra-low dropout regulator	Very low dropout voltage under full load	Wearables, IoT sensors, portable devices
Low-noise high-PSRR BiCMOS LDO	High ripple rejection and low output noise	RF, ADC/DAC, medical and audio equipment

Test Method for Dropout Voltage and PSRR

The test followed IEC 62007-1 and JEDEC JC-70 power management IC test specifications. Four groups of LDO samples were selected with the same SOT-223 package, rated 3.3V output voltage, and 1A maximum continuous output current. Each group included 20 samples to reduce the influence of individual process variation.

Dropout voltage was measured by setting the LDO output to 3.3V under 1A load and gradually reducing the input voltage from 5V. The input voltage point where the output dropped by 1% was recorded, and dropout voltage was calculated as the difference between input and output voltage.

PSRR was tested by injecting a 10mV sinusoidal ripple signal into the LDO input using a network analyzer. The ripple amplitude at the output was measured across frequencies from 10Hz to 10MHz, and PSRR was calculated in decibels.

Dropout Voltage Performance

LDO Type	Dropout Voltage at 25℃ / 1A	Key Observation
NPN BJT LDO	600mV	Highest dropout due to base-emitter voltage requirement
General-purpose CMOS LDO	300mV	Balanced dropout and cost
Ultra-low dropout regulator	100mV	Best low-voltage operating capability
High-PSRR BiCMOS LDO	200mV	Strong compromise between dropout, noise, and PSRR

The results show that dropout voltage increases with load current. At light load, the ULDO's dropout voltage dropped to around 10mV, while the BJT LDO still required about 150mV. Temperature also affected dropout performance, with most devices showing higher dropout voltage at 125℃.

PSRR Performance

LDO Type	PSRR at 100Hz	PSRR at 1MHz	Application Suitability
High-PSRR BiCMOS LDO	90dB	40dB	RF, ADC, precision analog systems
General-purpose CMOS LDO	70dB	20dB	General power regulation
ULDO	80dB	25dB	Battery-powered low-voltage systems
BJT LDO	Not specified	15dB	Low-frequency legacy applications

PSRR generally decreases as frequency increases. Even high-performance LDOs show reduced ripple rejection at high frequencies because of limited error amplifier bandwidth and package parasitic effects. The high-PSRR BiCMOS LDO maintained the strongest suppression capability and was the only tested type that remained suitable for high-frequency noise-sensitive applications.

Line Regulation, Load Regulation, and Output Noise

The high-PSRR BiCMOS LDO showed the strongest regulation performance, with line regulation of 0.01%/V and load regulation of 0.05%/A at 25℃ and 1A load. Its integrated output noise was only 10μVRMS across the 10Hz to 100kHz bandwidth.

LDO Type	Line Regulation	Load Regulation	Output Noise
High-PSRR BiCMOS LDO	0.01%/V	0.05%/A	10μVRMS
ULDO	0.02%/V	0.1%/A	30μVRMS
General-purpose CMOS LDO	0.05%/V	0.2%/A	50μVRMS
BJT LDO	Not specified	Not specified	80μVRMS

Long-Term Reliability

After 1000 hours of HTOL aging at 125℃, 12V input, and 1A full load, all tested samples showed no catastrophic failure. The high-PSRR BiCMOS LDO showed only a 3% increase in dropout voltage and a 3dB decrease in PSRR at 1kHz. The general-purpose CMOS LDO showed larger drift, with dropout voltage increasing by 8% and PSRR decreasing by 10dB at 1kHz.

After 1000 thermal cycles from -40℃ to 125℃, output voltage drift remained within 1% for all LDO types, meeting common industrial and automotive qualification expectations.

Process Factors Affecting LDO Performance

LDO performance is strongly influenced by semiconductor process technology, circuit architecture, pass device design, and package structure. BiCMOS processes provide higher transconductance and better error amplifier gain, which directly improves PSRR and load regulation. CMOS process scaling can reduce PMOS on-resistance and improve dropout voltage performance.

The bandgap reference circuit determines output voltage accuracy and temperature stability. The error amplifier affects PSRR, transient response, and overall regulation performance. Meanwhile, the PMOS pass device is the dominant factor affecting dropout voltage, especially in high-current designs.

Packaging also plays a significant role. SOT-223 packages offer good thermal performance, while DFN packages can reduce thermal resistance and package parasitics. Lower parasitic inductance helps improve high-frequency PSRR, especially in compact and high-speed power systems.

Commercial Application Status

Market Segment	Approximate Share	Typical Price Range	Main Applications
General-purpose CMOS LDO	About 55%	Around $0.15	Consumer electronics, home appliances, basic industrial systems
Low-noise high-PSRR LDO	About 20%	$0.5 to $1.5	RF front ends, ADC/DAC systems, medical devices, professional audio
ULDO	About 15%	$0.3 to $0.8	Wearables, TWS earbuds, IoT sensors, portable medical devices
High-voltage ruggedized LDO	About 10%	$0.8 to $3	Industrial automation, automotive powertrain, renewable energy systems
Automotive-grade LDO	Fast-growing segment	$0.4 to $2	BCM, BMS, ADAS, infotainment systems

Existing Technical Challenges

LDO regulators still face several important technical limitations. The first is the tradeoff between dropout voltage and quiescent current. A larger PMOS pass device can reduce dropout voltage, but it also increases gate capacitance and leakage current, which can raise quiescent current.

High-frequency PSRR degradation is another major issue. Most LDOs lose rejection capability above 1MHz, making it difficult to suppress switching noise from modern high-frequency DC-DC converters without additional external filtering.

Thermal management also remains a practical limitation. Since LDO power dissipation equals (VIN - VOUT) × IOUT, large input-output voltage differences can generate significant heat at high load current. This restricts LDOs mainly to low-power or low-voltage-differential applications.

Other challenges include transient response stability, wide-temperature performance drift, high-voltage versus low-dropout tradeoffs, and mass production consistency. These factors continue to shape the design direction of next-generation LDO regulators for automotive, industrial, RF, and portable electronics applications.