Micropower, 150mA Low-Noise Ultra Low-Dropout CMOS Voltage Regulator# Technical Documentation: LP3985IBL28 Ultra-Low Noise LDO Regulator
*Manufacturer: National Semiconductor (NS)*
## 1. Application Scenarios
### Typical Use Cases
The LP3985IBL28 is a 150mA ultra-low noise, high-PSRR (Power Supply Rejection Ratio) low-dropout (LDO) linear regulator optimized for noise-sensitive analog and RF circuits. Its primary use cases include:
- Portable Battery-Powered Devices : Extends battery life through low quiescent current (typically 75µA) and low dropout voltage (typically 110mV at 150mA load).
- RF/Communication Systems : Provides clean power to VCOs (Voltage Controlled Oscillators), PLLs (Phase-Locked Loops), and mixers where supply noise directly impacts phase noise and signal integrity.
- Medical Instrumentation : Powers precision analog front-ends, sensors, and data acquisition systems requiring minimal supply-induced error.
- Audio Equipment : Supplies low-noise bias to high-fidelity audio codecs, amplifiers, and DACs/ADCs to preserve signal-to-noise ratio (SNR).
### Industry Applications
- Wireless Infrastructure : Base station power amplifiers, transceiver modules.
- Test & Measurement : Precision signal generators, spectrum analyzers.
- Consumer Electronics : Smartphones (for RF and audio sections), digital cameras, portable media players.
- Industrial Control : Sensor interfaces, data loggers, process control systems.
### Practical Advantages and Limitations
Advantages:
- Exceptional Noise Performance : Integrated bypass capacitor pin enables very low output noise (typically 30µVrms, 10Hz–100kHz).
- High PSRR : Typically 70dB at 1kHz, effectively attenuating upstream switching regulator noise.
- Stable with Ceramic Capacitors : Designed for stability with small, low-ESR ceramic output capacitors (≥2.2µF).
- Protection Features : Includes current limit, thermal shutdown, and reverse current protection.
- Small Package : 1.5mm×1.5mm micro SMD package (DSBGA-4) saves board space.
Limitations:
- Limited Output Current : Maximum 150mA, unsuitable for high-power loads.
- Fixed Output Voltage : The "28" variant provides a fixed 2.8V output; other voltages require a different suffix.
- Thermal Dissipation : In small packages, maximum power dissipation is limited by junction temperature; careful thermal design is needed for high current loads.
- Dropout Voltage : While low, it may still be higher than some newer nano-cap LDOs for very low input-output differentials.
## 2. Design Considerations
### Common Design Pitfalls and Solutions
- Insufficient Input/Output Capacitance : Causes instability or poor transient response.
*Solution*: Use ≥1µF on input and ≥2.2µF on output (ceramic, X5R or better), placed as close as possible to the IC pins.
- Ignoring Thermal Limits : Risk of thermal shutdown in high ambient temperatures or with high power dissipation.
*Solution*: Calculate power dissipation \(P_D = (V_{IN} - V_{OUT}) × I_{LOAD}\). Ensure junction temperature \(T_J = T_A + (P_D × θ_{JA})\) remains below 125°C. Use thermal vias or copper pours for heat sinking.
- Misusing Bypass (BP) Pin : Leaving BP pin unconnected or poorly decoupled increases output noise.
*Solution*: Connect a 0.01µF ceramic capacitor from BP to GND, placed within 1mm of the pin.
- Input Voltage Exceeding Absolute Maximum : The device is sensitive to input transients above 6V
