Micropower, 150mA Low-Noise Ultra Low-Dropout CMOS Voltage Regulator# Technical Documentation: LP3985IBL31 Ultra-Low Noise, 150mA CMOS Voltage Regulator
Manufacturer : National Semiconductor (NS)
## 1. Application Scenarios
### Typical Use Cases
The LP3985IBL31 is a 150mA, ultra-low dropout (ULDO) CMOS voltage regulator designed for noise-sensitive, battery-powered portable electronics. Its primary use cases include:
* Portable Audio/Video Devices: Provides clean, stable power to analog audio codecs, headphone amplifiers, and sensitive RF sections in MP3 players, digital cameras, and portable media players, minimizing audible noise and interference.
* Wireless Communication Modules: Powers the voltage-controlled oscillators (VCOs), phase-locked loops (PLLs), and low-noise amplifiers (LNAs) in Bluetooth® modules, Wi-Fi radios, and cellular handset transceivers, where power supply noise directly impacts signal integrity and receiver sensitivity.
* Medical Monitoring Devices: Used in battery-powered equipment like pulse oximeters and portable ECG monitors, where stable analog front-end power is critical for accurate signal acquisition.
* Precision Analog Circuits: Supplies reference voltages and power to high-resolution analog-to-digital converters (ADCs), digital-to-analog converters (DACs), and sensor interfaces, where supply ripple can introduce measurement errors.
### Industry Applications
* Consumer Electronics: Smartphones, tablets, wearables, and handheld gaming systems.
* Industrial IoT: Sensor nodes, wireless gateways, and portable data loggers.
* Automotive Infotainment: Auxiliary power for noise-sensitive audio subsystems.
### Practical Advantages and Limitations
Advantages:
* Ultra-Low Noise: Typical output noise of 30µVrms (10Hz to 100kHz) is ideal for sensitive analog/RF loads.
* High Power Supply Rejection Ratio (PSRR): >70dB at 1kHz, effectively filtering input ripple.
* Very Low Dropout Voltage: Typically 85mV at 150mA load (for the 3.1V `BL31` variant), maximizing battery life and efficiency.
* Low Quiescent Current: 75µA typical, crucial for long standby times in portable devices.
* Stable with Ceramic Capacitors: Requires only a small 1.0µF ceramic capacitor on the output, saving board space and cost.
* Fixed Output Voltage: The `BL31` suffix denotes a fixed 3.1V output (±1.5% accuracy).
Limitations:
* Fixed Output Current: Limited to 150mA continuous output. Not suitable for high-power loads like motors or high-brightness LEDs.
* Fixed Voltage Option: The `BL31` is a fixed-voltage variant. Applications requiring adjustable output must select a different part number from the LP3985 family.
* Thermal Considerations: In SOT-23-5 package, maximum power dissipation is limited (~300mW). Continuous operation at high input-output differentials and full load may require thermal analysis.
## 2. Design Considerations
### Common Design Pitfalls and Solutions
* Pitfall 1: Input Capacitor Omission. While stable with just an output capacitor, omitting the input capacitor can lead to instability during line transients or when the regulator is far from the primary power source.
* Solution: Always include a 1.0µF ceramic capacitor (X5R or X7R) placed as close as possible to the `IN` and `GND` pins.
* Pitfall 2: Exceeding Absolute Maximum Ratings. Applying an input voltage (`VIN`) greater than 6V (absolute max) will damage the device.
* Solution: Ensure
