LDO Calculator — Dropout, Power Dissipation & Thermal
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
What is an LDO voltage regulator?
An LDO (Low Dropout Regulator) is a linear voltage regulator that can operate with a very small input-to-output voltage differential (dropout voltage), typically 100-500mV. Unlike switching regulators, LDOs produce clean output with minimal noise, making them ideal for powering analog circuits, ADCs, and RF stages.
How do I calculate LDO power dissipation?
P = (Vin - Vout) × Iload. For example, 5V to 3.3V at 500mA: P = (5 - 3.3) × 0.5 = 0.85W. This power is entirely converted to heat. Always verify the junction temperature: Tj = Tambient + P × θJA. If Tj exceeds 125°C (most LDOs' max), you need a heatsink, copper pour, or a switching regulator instead.
When should I use an LDO vs a switching regulator?
Use LDO when: (1) Low noise required (analog/RF/audio), (2) Vin - Vout < 2V, (3) Current < 500mA, (4) PCB space is tight, (5) Cost is critical. Use switching when: (1) Efficiency matters (battery), (2) Large Vin-Vout differential, (3) High current, (4) Step-up needed. LDO efficiency = Vout/Vin, so 3.3V from 5V = 66% vs ~90% for a buck converter.
What is dropout voltage?
The minimum Vin - Vout for the LDO to maintain regulation. True LDOs: 100-300mV at rated current. Quasi-LDOs: 500-700mV. Standard regulators (like 7805): 1.5-2V. Dropout increases with load current. Always check the datasheet graph 'Dropout vs. Load Current' at your operating temperature.
How do I choose input and output capacitors for an LDO?
Output cap: 1µF-10µF ceramic (X5R/X7R) minimum, close to output pin. Provides stability and transient response. Input cap: 1µF-10µF ceramic, close to input pin. Reduces source impedance. Some older LDOs require ESR in a specific range for stability — check the datasheet. Modern LDOs are stable with low-ESR MLCC.
What are popular LDO ICs?
3.3V: AMS1117-3.3 (1A, cheap), MCP1700 (250mA, low Iq), AP2112K (600mA, low noise). 5V: LM7805 (classic, 1.5A, not true LDO). Adjustable: LM317 (classic), TLV757P (1A, ultra-low noise), AP7361C (1A, 300mV dropout). For battery: ME6211 (500mA, 100mV dropout, 40µA Iq).
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