Your MCU needs 3.3 V from a 5 V or 12 V supply. An LDO works but burns power as heat. A buck converter fixes that — if you get the design right. Here’s what goes wrong.
Inductor selection
The inductor value determines the current ripple. Too small: high ripple current, core saturation risk, EMI. Too large: slower transient response, bigger and more expensive.
The standard formula: L = (Vin - Vout) × Vout / (Vin × ΔIL × f)
Where ΔIL is your target peak-to-peak ripple (typically 20–40% of max output current) and f is switching frequency.
For a 5 V → 3.3 V converter at 500 kHz, 1 A max, 30% ripple target:
L = (5 - 3.3) × 3.3 / (5 × 0.3 × 500000)
L = 5.61 / 750000
L = 7.5 µHUse the buck converter calculator to verify. The next standard value up is 10 µH, which gives lower ripple — fine.
The critical spec is saturation current. Your inductor must handle peak current without saturating: Ipeak = Iout_max + ΔIL/2. At 1 A output with 0.3 A ripple, Ipeak = 1.15 A. Choose an inductor rated for ≥1.5 A saturation current with margin.
Temperature affects saturation current. An inductor rated at 2 A saturation at 25 °C may saturate at 1.6 A at 85 °C. Check the datasheet curves if your design runs hot.
Output capacitor
The output cap determines voltage ripple and transient response. The ESR (equivalent series resistance) of the capacitor dominates ripple at higher frequencies:
ΔVout ≈ ΔIL × ESR
For 300 mA ripple and a ceramic cap with 5 mΩ ESR: ΔVout = 0.3 × 0.005 = 1.5 mV. Negligible.
Use X5R or X7R ceramic capacitors for output filtering. Avoid Y5V — capacitance drops 50–80% with DC bias and temperature. A 10 µF Y5V capacitor on a 3.3 V rail may present 3 µF of actual capacitance. You’ll see oscillation and poor transient response while the datasheet value looks fine.
Minimum capacitance for stability depends on the converter IC’s compensation. Check the datasheet — most modern converters specify a minimum Cout range with their internal compensation.
Stability and the feedback resistor divider
The output voltage is set by a resistor divider feeding the feedback pin. Vout = Vref × (1 + R1/R2).
Use 1% resistors at minimum. Solder paste voids under the feedback resistors cause field failures that are nearly impossible to debug. If the output voltage is slightly off, check feedback resistor values before touching anything else.
Keep the feedback trace short and away from the switching node. The inductor and diode/FET create a large dV/dt node — if the feedback trace picks up noise, the converter sees a false error signal and hunts.
Layout is not optional
Buck converter layout problems cause EMI failures and instability. The hot loop — Vin bypass cap, high-side switch, inductor, output cap, and back to Vin ground — must be as small as possible. Every extra millimeter of loop area is an antenna.
Practical rules:
- Place the input capacitor immediately adjacent to the converter’s Vin pin, with a short path to GND
- Place the output capacitor close to the inductor output and load
- Keep the switching node (between FET and inductor) small — don’t let it snake around the board
- Separate the power ground and analog ground if you have sensitive analog circuitry; connect them at one point near the input bulk cap
When to use an LDO instead
A buck converter makes sense when (Vin - Vout) × Iout > ~200 mW. Below that, the efficiency improvement over a good LDO is modest and the added complexity isn’t worth it.
Check with the LDO calculator: for 3.3 V at 50 mA from 5 V, an LDO dissipates 85 mW and needs no external components. A buck converter for 85 mW at typical 90% efficiency recovers about 8 mW. Not worth it.
For anything over 300 mA from a supply more than 1–2 V above the output, a buck converter pays for itself quickly in thermal and battery savings.