Linear regulators: LDO design and PSRR
Linear regulators are electronic circuits that provide a stable output voltage from a higher, often fluctuating input voltage. LDO (Low Dropout) regulators are a type of linear regulator capable of operating with a small difference between input and output voltages, making them efficient for low-voltage applications. PSRR (Power Supply Rejection Ratio) measures the regulator's ability to suppress variations or noise from the input supply, ensuring a clean and stable output for sensitive electronic devices.
Linear regulators: LDO design and PSRR
Linear regulators are electronic circuits that provide a stable output voltage from a higher, often fluctuating input voltage. LDO (Low Dropout) regulators are a type of linear regulator capable of operating with a small difference between input and output voltages, making them efficient for low-voltage applications. PSRR (Power Supply Rejection Ratio) measures the regulator's ability to suppress variations or noise from the input supply, ensuring a clean and stable output for sensitive electronic devices.
💡 Key Takeaways
- Understand how a low-dropout (LDO) regulator operates and why dropout voltage matters in design.
- Grasp what PSRR (Power Supply Rejection Ratio) is and how it affects noise rejection in LDO regulators across frequencies.
- Identify the key design elements that influence PSRR in LDOs, including the pass element, error amplifier, and input/output capacitors with proper ESR.
- Recognize common design trade-offs in LDOs (dropout, quiescent current, stability) and how they impact PSRR and overall performance.
❓ Frequently Asked Questions
What is an LDO and how does it work?
An LDO is a low-dropout linear regulator that uses a pass transistor and an error amplifier to keep the output voltage stable. It compares a reference to the output and adjusts the pass device to regulate. Dropout is the minimum input–output voltage difference needed for regulation.
What does PSRR mean and why is it important?
PSRR (Power Supply Rejection Ratio) measures how well the regulator rejects input noise from appearing on the output. Higher PSRR (in dB) means less ripple is transferred to the load.
How can PSRR be improved in LDOs?
Improve loop gain/bandwidth, choose an output capacitor with an appropriate ESR to create a helpful zero, optimize the error amplifier and pass transistor design, and ensure proper input bypassing. These steps reduce how much supply noise reaches the output.
How does frequency affect PSRR in LDOs?
PSRR is typically better at low frequencies due to higher loop gain. At higher frequencies, regulator dynamics limit attenuation, but the output capacitor ESR zeros can provide targeted noise suppression at certain bands.