What is a common limitation of an LDO regulator when delivering high currents?

• A. It cannot regulate under any load.
• B. Efficiency degrades drastically at high loads but PSRR improves.
• C. Output voltage becomes unpredictable at high load.
• D. Dropout voltage increases with current, limiting performance under heavy load.

Answer: (D)

The key idea is the headroom that an LDO needs across its pass element to regulate. As you pull more current, the voltage drop across the pass device increases, so the input voltage must be higher above the desired output by a larger amount. This dropout voltage grows with current because the pass transistor’s on-resistance (for a MOSFET pass element) or the saturation voltage (for a BJT pass element) increases as current climbs. If Vin minus Vout isn’t large enough to cover this increasing dropout, the regulator can’t hold the output steady, and the output voltage will sag or lose regulation under heavy load. That limitation is what typically constrains high-current use of LDOs. Other effects like efficiency and PSRR can vary, but they don’t define the fundamental high-current constraint in the same way dropout does. In practice, ensure sufficient input headroom for the expected load, or choose a regulator topology better suited for high current.