LDO Rechner — Dropout, Verlustleistung & Thermik
Dropout, power dissipation, and thermal analysis
Required Parameters
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Quick Answer
LDO calculator: P = (Vin − Vout) × Iload. Example 5 V→3.3 V @ 500 mA → 0.85 W heat. Check Tj = Ta + P×θJA stays below 125 °C.
LDO Calculator — Linear Regulator Thermal Design
Use this LDO calculator to check dropout, power dissipation, and junction temperature before you commit a linear regulator to the PCB.
Core formulas
| Quantity | Formula |
|---|---|
| Power dissipation | P = (Vin − Vout) × Iload |
| Junction temperature | Tj = Ta + P × θJA |
| Efficiency | η = Vout / Vin |
Worked example — 5 V → 3.3 V @ 500 mA
- Dropout: 5 − 3.3 = 1.7 V
- P = 1.7 × 0.5 = 0.85 W (all converted to heat)
- With θJA = 50 °C/W and Ta = 25 °C: Tj = 25 + 0.85×50 = 67.5 °C ✓
If the same load ran at 1 A, P = 1.7 W → Tj = 110 °C — still OK on many packages, but verify the datasheet curve.
When to use LDO vs buck
- LDO: low noise (ADC, RF, audio), small Vin−Vout (< 2 V), I < 500 mA
- Buck: battery-powered, large step-down, high current — 85–95% efficiency vs ~66% for 5→3.3 V LDO
Capacitor selection
- Output: 1–10 µF X5R/X7R ceramic, placed at the LDO output pin
- Input: 1–10 µF ceramic at the input pin — reduces source impedance during load steps
Related tools
- LDO Regulator Calculator — interactive thermal check
- Buck/Boost Converter Calculator — when efficiency matters
- Component Derating Calculator — cap voltage margin after regulation
- Electrical Power Calculator — P = VI verification
Design Notes
LDOs are simple but waste power as heat. Efficiency = Vout/Vin. For a 5V→3.3V conversion at 500mA, you dissipate 0.85W. Always check the dropout voltage spec at your load current — it increases with current. Use input and output capacitors (typically 1µF-10µF ceramic) for stability.
Common Mistakes
- 1
Forgetting that dropout voltage increases with load current — check the datasheet curves.
- 2
Ignoring thermal limits — a SOT-23 package can only dissipate ~0.4W without a heatsink.
- 3
Using an LDO where a switching regulator would be far more efficient (large Vin-Vout differential).
Engineering Handbox
1. Dropout = 5 − 3.3 = 1.7 V 2. P = 1.7 × 0.5 = 0.85 W 3. Tj = 25 + 0.85 × 50 = 67.5 °C
Knowledge Base
Was ist ein LDO-Spannungsregler?
Linearregler mit sehr geringer Dropout-Spannung (100–500 mV). Saubere Ausgangsspannung, ideal für ADC, RF und Audio — im Gegensatz zu Schaltreglern.
Wie berechne ich die Verlustleistung?
P = (Vin − Vout) × Iload. Beispiel: 5 V → 3,3 V bei 500 mA: P = 0,85 W. Tj = Ta + P × θJA.
LDO oder Schaltregler?
LDO bei geringem Rauschbedarf, kleinem Vin−Vout (< 2 V), I < 500 mA. Schaltregler bei Batteriebetrieb und hoher Effizienz.
Welche Kondensatoren am LDO?
Ein- und Ausgang: je 1–10 µF X5R/X7R Keramik, nah am IC. Stabilisiert Transienten und reduziert Quellenimpedanz.
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