Power management

Description

LDO (Low Dropout Regulator) is an electronic component used to provide a stable output voltage and can work efficiently even when the input voltage is close to the output voltage. The following is a detailed description of the definition, classification, and application selection of Ldo:

1. Definition of LDO

An LDO is a linear regulator that maintains a steady output when the Dropout Voltage between the input voltage and the output voltage is very small. The core feature of LDO is:
• Low dropout: Conventional linear regulators (like the 7805) require an input voltage more than 2V higher than the output voltage, while LDOS can have a dropout as low as tens of mV (like 0.1V).
• Low noise: The output of an LDO is purer than that of a switching power supply and is suitable for noise-sensitive circuits.
• Easy to use: Usually only input/output capacitors are needed to work, without complex peripheral components such as inductors.

2. Classification of Ldos

Ldo can be classified according to different characteristics:
(1) Dropout Voltage
• Regular LDO: Dropout about 0.3V to 1V.
• Very low differential LDO (VLDO) : The differential can be as low as 50mV or less.
(2) By output current capacity
• Small current LDO: <100mA for low-power devices.
• Medium current LDO: 100mA to 1A, commonly found in general-purpose circuits.
• High current LDO: >1A, pay attention to heat dissipation design.
(3) According to functional characteristics
• Fixed output: The output voltage is fixed (such as 3.3V, 5V), and no external voltage resistor is required.
• Adjustable output: Adjust the output voltage through an external resistor (such as 1117-ADJ).
• Low noise type: Optimized for RF/analog circuits.
• High PSRR type: High power supply rejection ratio (PSRR), suitable for high frequency noise suppression.
• Low quiescent current (IQ) type: Suitable for battery-powered devices.
(4) By process technology
• PMOS LDO: Lower voltage drop, but higher cost.
• NMOS LDO: Faster dynamic response requires a higher gate drive voltage.
• Bipolar LDO: Low cost but high quiescent current.

3. Key points for LDO application selection

When choosing an LDO, the following parameters should be considered comprehensively:
(1) Key parameters
• Input voltage range: It should cover the actual input voltage (e.g. 2.5V to 5.5V).
• Output voltage: Fixed or adjustable, precision (e.g. ±1 percent).
• Dropout Voltage: The input voltage must be at least a certain value higher than the output voltage.
• Output current: Select according to the load requirements and leave margin.
• Static current (IQ) : In battery-powered scenarios, A model with a lower IQ (such as <10μA) should be selected.
• PSRR (Power Supply Rejection Ratio) : High frequency noise-sensitive applications (such as RF, ADC) require a high PSRR (e.g. 60dB@1kHz).
• Noise level: Low-noise LDOS typically have noise <10μVrms.
(2) Application scenarios
• Battery-powered devices: Choose a low IQ, low dropout LDO.
• Noise-sensitive circuits (such as sensors, ADC) : Choose high PSRR, low noise models.
• High-precision power supply: Choose a model with high output voltage accuracy (within ±1%) and low temperature drift.
• High current application: Pay attention to heat dissipation and choose an LDO with an enable pin (EN) to turn off and save power.
(3) Peripheral components
• Input/output capacitors: Must meet LDO stability requirements (e.g. Ceramic capacitors 1μF to 10μF).
• Heat dissipation design: Calculate power consumption for high current or high voltage difference (P=(VIN-VOUT)*IOUT), and add heat sinks if necessary.
(4) Other functions
• Enable pin (EN) : Supports power timing control.
• Power Good (PG) signal: For system monitoring.
• Reverse current protection: Prevents the output voltage from being higher than the input.

4. Typical LDO selection examples

Application scenarios    Features
Low-power IoT devices    IQ=0.8μA, differential voltage 150mV@150mA
Rf circuit power supply    Noise 4.7μVrms, PSRR 70dB@1kHz
5V to 3.3V general-purpose circuit    Differential voltage 1V@1A, low cost
High-precision sensor power supply    Noise 16μVrms, PSRR 79dB@1kHz

5. Notes

• Stability: Some Ldo require specific ESR capacitors (such as tantalum capacitors), consult the manual.
• Thermal management: Power consumption may cause overheat protection when the voltage difference is large or the current is high.
• Cost: High-performance Ldo(such as ultra-low noise) are more expensive and need to weigh the requirements.
With proper selection, Ldo can provide efficient and stable power solutions for electronic systems.