Selecting Low-Dropout (LDO) Regulators for Noise-Sensitive Applications
Low-dropout (LDO) linear regulators are essential for powering noise-sensitive circuits such as RF transceivers, precision ADCs, and high-performance processors. This guide helps you select the right LDO for your noise-sensitive application.
Understanding LDO Noise
Output Noise: LDOs generate intrinsic noise that can affect sensitive circuits. Silicontent LDOs feature ultra-low noise (<10μVrms) suitable for the most demanding applications.
Power Supply Rejection Ratio (PSRR): PSRR indicates how well the LDO rejects noise from the input supply. Higher PSRR (measured in dB) means better noise rejection. Silicontent LDOs offer >70dB PSRR at 1kHz.
Noise Spectrum: Noise varies with frequency. RF applications require low noise at high frequencies, while precision analog circuits need low noise at low frequencies.
Key Selection Parameters
Dropout Voltage: The minimum input-to-output voltage difference required for regulation. Lower dropout voltage enables operation with lower input voltages, extending battery life.
Quiescent Current: Current consumed by the LDO itself. Critical for battery-powered applications. Silicontent offers LDOs with <1μA quiescent current.
Output Current: Maximum current the LDO can deliver. Include margin for reliable operation.
Stability: LDO stability depends on output capacitor ESR. Verify stability with your chosen output capacitor.
Application-Specific Considerations
RF Applications: Require ultra-low noise and high PSRR at RF frequencies. Place LDO close to RF circuit to minimize noise pickup.
Precision ADCs: Need low noise at low frequencies and excellent DC accuracy. Reference voltage noise directly affects ADC performance.
Camera Systems: Image sensors are sensitive to power supply noise which can appear as image artifacts. Use dedicated LDOs for analog and digital supplies.
PCB Layout Guidelines
Input/Output Capacitors: Place as close as possible to LDO pins. Use low-ESR ceramic capacitors.
Ground Connections: Use single-point ground connection or dedicated ground plane to prevent ground bounce.
Noise Isolation: Keep LDO output traces away from noisy signals. Use ground shielding if necessary.
💡 FAE Insights
⚠️ Common Pitfalls
- ✗ Selecting LDO based on noise spec alone without considering PSRR
- ✗ Ignoring input supply noise
- ✗ Placing LDO far from the sensitive circuit
- ✗ Using inappropriate output capacitors affecting stability
- ✗ Not measuring actual noise in the system
📋 Customer Cases
RF Communications Inc.
Telecommunications
Challenge
RF performance was degraded due to power supply noise affecting the transceiver
Solution
Replaced with Silicontent XZ1001 ultra-low noise LDO with >70dB PSRR at 1MHz and added input filtering
Results
RF sensitivity improved by 3dB, meeting system requirements
Frequently Asked Questions
1. What PSRR value is good enough for my application?
PSRR requirements depend on your application. For general digital circuits, 40-50dB is usually sufficient. For audio applications, 60-70dB is recommended. For RF and precision analog circuits, look for >70dB PSRR at your frequencies of interest. Silicontent XZ1001 series offers >70dB at 1kHz and >40dB at 1MHz.
2. How does dropout voltage affect battery life?
Lower dropout voltage allows the LDO to maintain regulation as the battery voltage drops, extending usable battery life. For example, with a 3.3V output and 200mV dropout, the LDO can regulate down to 3.5V input. With 500mV dropout, regulation fails at 3.8V input, wasting battery capacity.
3. What output capacitor should I use with LDOs?
Use low-ESR ceramic capacitors (X5R or X7R dielectric) for best performance. Typical values are 1μF to 10μF. Check the LDO datasheet for specific recommendations - some LDOs require minimum ESR for stability. Place the capacitor as close as possible to the output pin.
4. Can I use an LDO after a switching regulator?
Yes, using an LDO after a switching regulator (post-regulation) is a common technique to reduce switching noise. The LDO filters out the switching ripple and provides clean power to sensitive circuits. Choose an LDO with high PSRR at the switching frequency of your DC-DC converter.
5. How do I minimize noise in my LDO circuit?
To minimize noise: 1) Use ultra-low noise LDO like Silicontent XZ1001, 2) Place LDO close to the load, 3) Use clean input supply or add input filtering, 4) Use adequate output capacitance with low ESR, 5) Keep ground connections clean and short, 6) Shield sensitive traces from noisy signals.