KEMI Docs · Circuit Practice
LDO vs Buck Converter
Last updated: 2026-08-30
The first fork in any power design: linear (LDO) or switching (buck)? Only two variables actually decide it — the heat produced by voltage difference times current, and how noise-sensitive the load is. Here is how to decide with numbers.
1
One line each
An LDO is an automatically adjusted resistor that burns the excess voltage as heat — simple and quiet. A buck switches at hundreds of kHz to MHz, ferrying energy through an inductor instead of burning it — 85-95% efficient, at the price of switching noise and part count.
2
The thermal math that eliminates LDOs
LDO loss is one formula: P = (Vin − Vout) × Iout. From 12V to 3.3V at 500mA: P = 8.7 × 0.5 = 4.35W. A SOT-223's junction-to-ambient thermal resistance of roughly 60°C/W implies a 260°C rise — physically impossible without serious heatsinking. From 5V instead, P = 0.85W: manageable with copper pour as the heatsink. Large voltage difference × large current = LDO eliminated.
| Case | LDO loss | Verdict |
|---|---|---|
| 5V → 3.3V, 100mA | 0.17W | LDO fine — even SOT-23 class |
| 5V → 3.3V, 500mA | 0.85W | LDO possible with SOT-223 + copper heatsinking |
| 12V → 3.3V, 500mA | 4.35W | LDO impossible — buck required |
| 12V → 5V, 2A | 14W | not a discussion — buck |
| 3.6V (Li-ion) → 3.3V, 200mA | 0.06W | LDO ideal — quiet and cheap |
3
Noise — where bucks can't go
Buck outputs carry switching ripple and harmonics. Digital logic doesn't care; ADC references, RF supplies, precision analog front-ends and audio paths do. The standard answer is two stages: buck down to target +0.3-0.5V for efficiency, then a low-noise LDO for the final clean-up (12V → buck → 3.6V → LDO → 3.3V). With so little voltage across it, the LDO's heat is negligible.
4
Cost, area, design difficulty
- LDO: the IC plus two capacitors. High layout freedom, pennies in parts.
- Buck: IC + inductor + capacitors + feedback divider. Inductors are big and cost real money, and layout rules follow (short fat switch node, feedback routed away from noise).
- Module bucks with built-in inductors cost more but remove most layout risk — sensible for low-volume products.
- Copying the datasheet's reference layout is the cheapest insurance a buck design can buy.
KEMI's default decision order
1) Compute the loss — above about 1W, buck. 2) Analog-sensitive load — buck + LDO two-stage. 3) Otherwise the cheap quiet LDO. When unsure, calculate first: watts decide, not intuition.
FAQ
What is LDO dropout?
The minimum input-output difference that keeps regulation. A 0.3V-dropout LDO making 3.3V needs at least 3.6V in — critical when running to the end of a battery discharge curve.
Is buck efficiency always 90%?
No — it is a curve versus load. Many parts fall off hard at light load (a few mA); battery devices should check the light-load (PFM) behaviour on the datasheet curve.
When do boost and buck-boost apply?
Input below output: boost (3.7V→5V). Input straddling output (Li-ion 3.0-4.2V→3.3V): buck-boost. Same efficiency/noise trade-off framework.
See also
This article summarises the working rules KEMI uses in real design and fabrication work. For production, part datasheets and fab specifications take precedence.