regulators
turn one voltage into another: smooth, stable, and safe for your parts.
caution
rule of thumb: big drop + medium/high current → use a buck. small drop or tiny current → an ldo is fine.
two main types
- ldo (linear) — quiet, simple, but the extra voltage becomes heat.
- buck (switching) — efficient, handles big drops, needs inductor/diodes and good layout.
quick chooser
| input → output | current | efficiency need | pick |
|---|---|---|---|
| 5 v → 3.3 v | ≤ 150–200 mA | low/med | ldo (low dropout) |
| 5 v → 3.3 v | > 200 mA | med/high | buck |
| 1-cell li-ion (4.2→3.3 v) | ≤ 200 mA | low | ldo (ultra-low Iq if battery-powered) |
| 12 v → 5 v | any | med/high | buck (always) |
| 2–3s li-ion → 5 v/3.3 v | any | high | buck |
| always-on micro @ µA–mA | tiny | sleep life | ldo with low Iq |
ldo basics
dropout: the minimum headroom the ldo needs to stay in regulation.
Vin ≥ Vout + Vdropout
fact
linear ≠ always bad: for small drops and light loads, an ldo can waste less total power than a buck.
- heat: P ≈ (Vin − Vout) × Iout.
- caps matter: follow datasheet for input/output caps (values, ESR).
- noise: usually quieter than buck, good for analog/rf.
- dropout spec: pick low-dropout if Vin is close to Vout.
buck basics
what it does: high-frequency switch + inductor converts extra voltage into current instead of heat.
caution
keep the buck’s switch node tiny and away from sensors/antennas → this trace is noisy.
- efficiency: often 85–95% with good parts/layout.
- inductor + caps: pick values per datasheet tables.
- frequency: 500 kHz–2 MHz typical; higher = smaller parts, but more emi.
- layout: place input cap at VIN/GND pins; short loops only.
when to choose what
- ldo if: small drop, small current, low noise, or dead-simple.
- buck if: big drop (12→5), current >200 mA, efficiency matters.
- combo: buck to just above, then ldo for clean final rail.
mini cookbook
usb 5 v → 3.3 v (≤200 mA)
bench notes
ap2112 (ldo) is a hobbyist favorite; dropout ≈ 250 mV, 600 mA max.
- pick 300–500 mA ldo with low dropout.
- 1–10 µF in/out caps, ceramic ok.
1s li-ion → 3.3 v wearable
try this
for battery loggers, aim for Iq ≤ 2 µA; extends sleep life a lot.
- ultra-low Iq ldo (TPS7A02, MCP1810, etc).
- watch dropout near 3.3–3.5 v as cell empties.
12 v wall → 5 v logic (0.3–2 a)
- synchronous buck 2–3 a, ~500 kHz–1.5 MHz.
- use datasheet “typical app” values for L + caps.
- add input tvs/pi filter if cables are long/noisy.
quiet analog rail (3.3 v from noisy 5 v)
fact
classic trick: buck to ~3.8 v, then ldo to 3.3 v → cleans ripple.
- route analog ground separately after ldo if sensitive.
numbers to check
- Iout max + surges
- Vin range (min/max, hot-plug)
- dropout (ldo) / duty limits (buck)
- Iq if battery-powered
- thermal: P × θJA → temp rise
- caps: type, ESR, values
common mistakes
- ignoring dropout margin.
- random cap values that break stability.
- letting the buck switch node run across the board.
- no thermal margin on ldos.
quick math
Pldo ≈ (Vin − Vout) × Iout
ηbuck ≈ Pout / (Pout + losses) ≈ 85–95%
parts to look into
ldo (3.3 v): MCP1700, TLV700, AP2112, XC6206
ultra-low Iq: TPS7A02/03, MCP1810
buck (5/3.3 v): MP1584 modules, TPS621xx, MP1495