How to Select the Right LDO for Your Application
Key Selection Parameters
Selecting the right LDO requires understanding several key parameters: **Dropout Voltage**: The minimum input-to-output voltage difference required for regulation. Lower dropout allows operation with lower input voltages. For battery applications, choose LDOs with dropout voltage under 300mV at your maximum current. **Quiescent Current (IQ)**: The current consumed by the LDO when enabled but delivering no load current. For battery-powered devices, ultra-low IQ (under 10μA) is essential for maximizing battery life during standby. **PSRR (Power Supply Rejection Ratio)**: Measures how well the LDO rejects input noise. Higher PSRR (70dB+) is needed for noise-sensitive analog circuits like ADCs and RF systems. **Output Noise**: Voltage noise at the LDO output. Low noise (under 50μVRMS) is critical for sensitive analog circuits.
Application Considerations
**Battery-Powered IoT Devices**: Prioritize ultra-low quiescent current (1-5μA) over other parameters. The LP3990 series with 1μA IQ is ideal for these applications. **Noise-Sensitive Analog Circuits**: Choose high PSRR (80dB+) and low output noise LDOs like the LP5907 series. These are essential for powering precision ADCs, DACs, and RF circuits. **General Digital Circuits**: Standard LDOs with moderate PSRR (60-70dB) are sufficient. Focus on dropout voltage and current capability for your specific load requirements. **Multi-Rail Systems**: Use a combination of LDO types - ultra-low IQ for always-on circuits, high PSRR for analog, and standard LDOs for digital loads.
💡 FAE Insights
⚠️ Common Pitfalls
- ✗ Ignoring quiescent current in battery applications
- ✗ Insufficient dropout margin for battery end-of-life
- ✗ Inadequate PSRR for sensitive analog circuits
- ✗ Thermal issues from high input-output differential
- ✗ Using one LDO type for all rails in complex systems
📋 Customer Cases
SensorTech IoT
IoT Devices
Challenge
Needed to select appropriate LDO for battery-powered temperature sensor requiring 2+ year battery life
Solution
Applied selection methodology from this guide, chose LP3990-33 with 1μA quiescent current
Results
Achieved 2.5 year battery life on single CR123A cell, exceeding customer requirements
Frequently Asked Questions
1. What is the most important parameter for battery-powered applications?
Quiescent current (IQ) is the most critical parameter for battery-powered applications. A 1μA IQ LDO consumes 100x less power in standby than a 100μA IQ LDO. For devices in standby most of the time, IQ often dominates battery life more than active current consumption.
2. How do I calculate dropout voltage requirements?
Calculate dropout as: VIN_MIN - VOUT, where VIN_MIN is your minimum input voltage (battery end-of-life voltage). For a Li-ion battery (3.0V min) to 3.3V output, you need negative dropout - impossible. You would need a boost converter instead. For 3.7V to 3.3V, dropout is 400mV - choose LDO with dropout under 400mV at your load current.
3. What PSRR do I need for my application?
PSRR requirements depend on your application: 60-70dB sufficient for digital circuits, 70-80dB recommended for mixed-signal, 80dB+ required for sensitive analog like precision ADCs and RF. Also consider PSRR at your specific noise frequencies - PSRR typically decreases at higher frequencies.
4. When should I use an LDO vs a DC-DC converter?
Use an LDO when: voltage differential is small (under 1V), noise must be minimized, simplicity is preferred, or cost is critical. Use a DC-DC converter when: voltage differential is large, efficiency is critical, or heat generation must be minimized. Many systems use both - DC-DC for main conversion, LDO for post-regulation of sensitive rails.
5. How do I calculate power dissipation in an LDO?
Power dissipation is calculated as: P = (VIN - VOUT) × ILOAD + VIN × IQ. For example, with 5V input, 3.3V output, 100mA load, and 10μA IQ: P = (5 - 3.3) × 0.1 + 5 × 0.00001 = 0.17W + 0.00005W ≈ 0.17W. The IQ term is usually negligible compared to load-dependent dissipation.