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LDO Thermal Calculator

Calculate LDO regulator power dissipation, junction temperature, thermal margin, and minimum dropout voltage for thermal design validation.

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Formula

Pdiss=(Vin−Vout)⋅Iload,TJ=Tamb+θJA⋅PdissP_{diss} = (V_{in} - V_{out}) \cdot I_{load},\quad T_J = T_{amb} + \theta_{JA} \cdot P_{diss}

Reference: Texas Instruments Application Note SLVA061; IEC 60747-6

Pdiss— Power dissipation (W)
Vᵢₙ— Input voltage (V)
Vₒᵤₜ— Output voltage (V)
Iₗₒₐd— Load current (A)
TJ— Junction temperature (°C)
Tₐₘb— Ambient temperature (°C)
θJA— Thermal resistance junction-to-ambient (°C/W)

How It Works

The LDO thermal calculator determines junction temperature, power dissipation, and safe operating current for linear voltage regulators — essential for power management in noise-sensitive analog circuits, RF systems, and precision instrumentation. Analog design engineers, hardware architects, and reliability engineers use this tool to prevent thermal shutdown and ensure long-term reliability. According to TI application note SLVA118, LDO power dissipation Pdiss = (Vin - Vout) × Iload generates heat that increases junction temperature per Tj = Ta + (Pdiss × θJA). The thermal resistance θJA varies dramatically by package: SOT-23 exhibits 150-200°C/W, SOIC-8 provides 100-125°C/W, and DPAK (TO-252) achieves 40-60°C/W with proper PCB thermal design. Per JEDEC JEP122G, silicon junction temperature must remain below 125°C for 10-year MTBF — every 10°C increase above 85°C halves semiconductor lifetime according to the Arrhenius equation. Maximum safe current Imax = (Tj_max - Ta)/(θJA × ΔV), where ΔV = Vin - Vout represents the dropout headroom dissipated as heat.

Worked Example

Design an LDO power stage for a 5 V to 3.3 V converter at 800 mA with 55°C ambient in an industrial enclosure. Requirements: Tj < 110°C for reliability margin, no external heatsink. Step 1: Calculate power dissipation — Pdiss = (5 - 3.3) × 0.8 = 1.36 W. Step 2: Determine required thermal resistance — θJA_max = (110 - 55)/1.36 = 40.4°C/W. Step 3: Select package — SOT-223-4 with 62°C/W (datasheet typical) insufficient. Use DPAK (TO-252) with 35°C/W including 1 in² copper pour per TI SLMA002. Step 4: Verify junction temperature — Tj = 55 + (1.36 × 35) = 102.6°C (within spec). Step 5: Calculate safety margin — At 1 A maximum load: Pdiss = 1.7 W, Tj = 114.5°C (still acceptable). Step 6: Consider LDO selection — TI TPS73633 (DPAK, 150 mV dropout, 125°C max) provides integrated thermal shutdown at 160°C as backup protection.

Practical Tips

  • ✓Per TI TPS7A8300 datasheet, use thermal vias (0.3 mm diameter, 4-8 vias under exposed pad) to reduce θJA by 30-40% by conducting heat to inner ground planes
  • ✓Add 1 in² minimum copper pour connected to the GND pin for SOT-223/DPAK packages — this reduces θJA from 90°C/W to 50°C/W per Analog Devices thermal design guide
  • ✓Implement thermal shutdown monitoring via flag pin (where available) to trigger system-level power reduction before reaching Tj_max — prevents thermal cycling damage

Common Mistakes

  • ✗Using datasheet θJA without considering PCB copper area — SOT-23 θJA ranges from 205°C/W (minimum pad) to 120°C/W (1 in² copper) per TI SLMA002; real-world results may be 40% worse than datasheet values
  • ✗Ignoring dropout voltage increase at high current — LDO dropout rises from 150 mV at 100 mA to 300 mV at 1 A due to pass transistor Rds(on); Pdiss calculation must use actual dropout at operating current
  • ✗Operating continuously at Tj = Tj_max — per MIL-HDBK-217F, operating at 125°C versus 85°C reduces MTBF by 4×; design for Tj < 100°C in reliability-critical applications

Frequently Asked Questions

Thermal derating reduces maximum allowable current as ambient temperature increases. Per TI SLVA604, derating curve: Imax = (Tj_max - Ta)/(θJA × ΔV). Example: 1 A LDO at 25°C ambient can only deliver 500 mA at 75°C ambient with same thermal design. Critical: datasheet maximum current ratings assume specific Ta (typically 25°C) and θJA conditions.
Per Analog Devices AN-772, thermal resistance by package: SOT-23 (180-220°C/W), SOT-223 (60-90°C/W), SOIC-8 exposed pad (35-50°C/W), DPAK (25-40°C/W), D2PAK (15-25°C/W). Rule of thumb: each step up in package size provides 2× thermal capability. WLCSP packages offer lowest θJC (2-5°C/W) but require careful PCB design for heat spreading.
Yes — external heatsinks reduce θJA by 40-70%. For TO-220 package: θJA drops from 62°C/W (free air) to 23°C/W with small clip-on heatsink (Aavid 577002B00000G). For surface-mount packages, copper PCB area functions as heatsink — 2 in² copper reduces DPAK θJA from 40°C/W to 25°C/W per TI thermal modeling.
Above Tj_max (typically 125-150°C): output voltage accuracy degrades (±2% to ±5%), quiescent current increases 2-3×, and internal thermal shutdown activates at 150-160°C. Repeated thermal cycling above 125°C causes metal interconnect fatigue, leading to open circuits. Per JEDEC JEP122G, exceeding 150°C for >100 hours causes irreversible parametric shift.
Imax = (Tj_max - Ta)/(θJA × (Vin - Vout)). Example: Tj_max = 125°C, Ta = 40°C, θJA = 60°C/W (SOT-223), Vin = 5 V, Vout = 3.3 V. Imax = (125-40)/(60 × 1.7) = 833 mA. Always verify with actual dropout voltage at target current and add 20% margin for component variation and transient loads.

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