Side-by-side comparison
| Parameter | LDO | Buck Converter |
|---|---|---|
| Regulation Topology | Linear — series PMOS/NPN pass element in active region | Switching — inductor/capacitor energy storage, MOSFET switch |
| Minimum Input-Output Voltage Difference | Dropout: 50 mV to 300 mV (TPS7A05: 70 mV at 200 mA) | Vin must exceed Vout (duty cycle D = Vout/Vin); minimum ~0.5 V headroom |
| Efficiency at Vin=3.7V, Vout=1.8V | ~49% (Vout/Vin = 1.8/3.7) | ~90% (TPS62840 datasheet) |
| Output Noise | Very low — 4 µVrms (TPS7A05); no switching artifacts | 10–50 mV ripple at switching frequency (4 MHz for TPS62840) |
| Quiescent Current | 1–100 µA depending on device | 75 nA (TPS62840) to 50 µA for standard bucks |
| Transient Response | Fast — 10–50 µs | Slower — 100–300 µs; limited by LC filter bandwidth |
| External Components | Input cap + output cap — minimal | Inductor (2.2–10 µH), input cap, output cap, bootstrap cap |
| EMI Generated | None | Significant — requires EMI filter, controlled di/dt |
| Suitable Output Voltage Step-Down Ratio | Best for Vin/Vout < 1.5 (small dropout) | Efficient for any ratio; optimal at Vin/Vout = 2–10 |
| Common Devices | AMS1117, TPS7A05, LP5907 (ultra-low noise) | LM2596, TPS62840, MP2315, LM3481 |
Key differences
An LDO dissipates (Vin − Vout)×Iload as heat — on a 4.2 V fully-charged cell dropping to 1.8 V at 100 mA, that is 240 mW of wasted heat, cutting battery life roughly in half. A synchronous buck like TPS62840 achieves 90% efficiency by switching the inductor current between Vin and GND, storing and releasing energy magnetically. The LDO's advantage: output noise of 4 µVrms (LP5907) is 1000× lower than a buck's 50 mV ripple — critical for ADC reference supplies, PLL VCO power, and RF amplifier bias where switching noise desensitizes the receiver. For moderate dropout ratios (Vin/Vout < 1.3), LDO efficiency becomes acceptable and noise advantage wins.
When to use LDO
Use an LDO (AMS1117, LP5907, TPS7A05) when the input voltage is close to the output voltage (< 1 V dropout), output noise must be below 10 µVrms, or the application is noise-sensitive — PLL power supplies, ADC reference rails, and RF front-end biasing.
When to use Buck Converter
Use a buck converter (TPS62840, MP2315, LM2596) when Vin/Vout ratio exceeds 1.5 and load current is above 50 mA — 5 V to 3.3 V for an STM32, 12 V to 5 V for a Raspberry Pi, or any battery-powered device where runtime matters.
Recommendation
Choose buck converter whenever Vin/Vout > 1.5 and power exceeds 200 mW — the efficiency gain is simply too large to ignore. Choose LDO when Vin−Vout < 1 V, load is light, or noise is critical. For RF and precision analog, follow a buck with an LDO post-regulator to combine the efficiency of switching with the clean noise floor of linear regulation.
Exam tip: Examiners ask you to calculate LDO efficiency — it is simply Vout/Vin (e.g., 3.3/5 = 66%) and the wasted power is (Vin−Vout)×Iload — contrast this with the ~90% efficiency of a well-designed synchronous buck.
Interview tip: Interviewers at IoT and mixed-signal IC companies ask why you would add an LDO after a buck converter — the answer is that the buck provides efficiency while the LDO's 5–10 µVrms noise floor protects sensitive analog and RF circuits from switching ripple.