Switching Regulators vs LDOs: Which Regulator Fits Your Design

Every PCB with more than one voltage rail faces this decision: generate the lower rail with a switching regulator (buck converter) or an LDO (low-dropout linear regulator). The answer depends on three numbers: the input-to-output voltage difference, the load current, and the noise tolerance of the downstream circuit.

Efficiency: the voltage gap decides

An LDO burns the voltage difference as heat: Pdiss = (Vin - Vout) × Iload. Dropping 5 V to 3.3 V at 500 mA wastes 0.85 W. Dropping 12 V to 3.3 V at the same current wastes 4.35 W. At that point the LDO needs a heatsink and your enclosure gets warm. A buck converter at 85-90% efficiency wastes roughly 0.2-0.3 W for the same 12 V to 3.3 V, 500 mA conversion. No heatsink needed.

The crossover point where a switcher beats an LDO on total cost (IC + passives + board area + thermal management) is typically around 200-300 mA for a 5 V to 3.3 V conversion, and much lower (50-100 mA) for wider voltage differences. Below those currents, an LDO in SOT-23-5 with a single ceramic output capacitor is smaller, cheaper and simpler than a buck converter with its inductor, diode and multiple capacitors.

Noise: the LDO's killer advantage

A switching regulator produces ripple at its switching frequency (typically 100 kHz to 2 MHz) plus high-frequency ringing on the switch node. Output ripple for a well-designed buck converter is 10-30 mVpp. An LDO has no switching noise; its output noise is broadband thermal and flicker noise, typically 10-100 µVrms for a low-noise LDO.

For RF circuits, precision ADCs and PLL/VCO rails, LDO noise performance is essential. A TPS7A4700 (ultra-low-noise LDO, 4.17 µVrms from 10 Hz to 100 kHz) eliminates the switching artifacts that a buck converter would inject. A common architecture: buck converter for the first step (12 V to 5 V, high efficiency), followed by an LDO for the final regulation (5 V to 3.3 V analog, low noise). This cascaded approach optimizes both efficiency and noise.

PSRR: how well the LDO rejects input ripple

An LDO's power supply rejection ratio (PSRR) measures how much input ripple appears at the output. At low frequencies (100-120 Hz, mains ripple), most LDOs deliver 60-80 dB PSRR. At switching frequencies (100 kHz and above), PSRR drops significantly. An LM1117 specifies roughly 40 dB at 120 Hz but only 25 dB at 100 kHz. A modern LDO like the TPS7A20 (300 mA, ultra-low IQ) maintains 60 dB at 100 kHz and 40 dB at 1 MHz.

When cascading a buck + LDO, pick an LDO with high PSRR at your buck converter's switching frequency. If the buck runs at 500 kHz and produces 20 mVpp ripple, an LDO with 40 dB PSRR at 500 kHz (100× attenuation) reduces that to 200 µVpp at the output. An LDO with only 20 dB (10×) leaves 2 mVpp.

Quiescent current: battery life matters

In battery-powered designs, the regulator's own current consumption matters during light-load and standby conditions. An LM7805 burns roughly 5 mA of quiescent current even with zero load. Over 24 hours, that is 120 mAh from the battery. A modern LDO like the TPS7A02 (25 nA IQ) or the MCP1811 (250 nA) reduces that to microamp-hours.

Buck converters also have quiescent current, typically higher (tens to hundreds of µA) because the control circuitry runs continuously. For always-on battery rails at microamp loads, an LDO with nanoamp IQ is the clear winner. For higher loads where the efficiency gain of a switcher dominates, many buck converters now include a power-save or burst-mode that reduces switching frequency at light load, bringing quiescent current down to the 10-50 µA range.

Component count and PCB area

An LDO needs: the IC, an input capacitor, an output capacitor. Total: 3 components, roughly 20-30 mm² on a single-sided PCB.

A buck converter needs: the IC, input capacitor, output capacitor, inductor, bootstrap capacitor (for N-channel high-side FETs), feedback resistors, and optionally a soft-start capacitor and compensation network. Total: 7-12 components, roughly 80-150 mm².

For space-constrained designs with multiple rails, integrated power modules (Microchip MIC33M65x, TI TPSM series) package the inductor and IC into a single module, reducing area to roughly 50-80 mm² while keeping switching efficiency. The cost premium (2-4× vs discrete) buys simplicity and guaranteed EMI performance.

FAQ

Q: Can I parallel two LDOs for more current?

Not directly. LDOs do not share current actively. Slight differences in output voltage cause one LDO to source all the current while the other sits idle. Use a single higher-current LDO or add ballast resistors (50-100 mΩ) on each output to force sharing, accepting the voltage drop. Better option: use an LDO controller with an external pass transistor (e.g., TPS7A53 + N-channel MOSFET) for current above 3-5 A.

Q: What is the dropout voltage and why does it matter?

Dropout voltage (Vdo) is the minimum input-to-output differential required for regulation. An AMS1117-3.3 with 1.3 V dropout needs at least 4.6 V input to produce 3.3 V. If your battery drops to 3.8 V, the LDO loses regulation. A low-dropout regulator like the MCP1700-3302 (178 mV dropout at 250 mA) maintains regulation down to 3.48 V.

Q: When does it make sense to use an LDO after a switcher?

When the load demands noise below 1 mVpp and the switcher produces 10-30 mVpp, or when you need multiple clean rails from one efficient preregulator. The LDO filters the ripple and provides point-of-load regulation. The efficiency loss is small because the voltage drop across the LDO is minimal (e.g., 5 V from switcher to 3.3 V via LDO, wasting only 34% vs the switcher's 85% on the first stage).

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