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LDO Linear Regulator Dropout Calculator

Calculate LDO regulator power dissipation, junction temperature rise, minimum input voltage, efficiency, and headroom for linear regulator design

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Formula

PD=(Vin−Vout)×Iload,η=Vout/Vin×100P_D = (V_in − V_out) × I_load, η = V_out / V_in × 100%
V_in— Input voltage (V)
V_out— Output voltage (V)
I_load— Load current (A)
V_DO— Dropout voltage (V)
θ_JA— Thermal resistance (°C/W)

How It Works

The linear regulator dropout calculator determines minimum input voltage, power dissipation, and efficiency for voltage regulation circuits — essential for post-switching-regulator filtering, precision voltage references, and noise-sensitive instrumentation. Power supply designers, analog engineers, and embedded developers use this tool to ensure adequate headroom while minimizing thermal losses. According to Horowitz & Hill's 'The Art of Electronics' (3rd ed.), linear regulators dissipate excess voltage as heat: Pdiss = (Vin - Vout) × Iload, achieving maximum efficiency of η = Vout/Vin. For the classic LM7805 series (fixed 5 V output), dropout voltage is 2-2.5 V per ON Semiconductor datasheet, requiring Vin ≥ 7 V for guaranteed regulation. Modern LDO (Low Dropout) regulators reduce this to 50-300 mV using PMOS pass transistors with Rds(on) < 0.5 Ω. Per TI application note SLVA079, dropout voltage increases approximately linearly with load current: Vdropout ≈ Rds(on) × Iload. The internal reference accuracy (typically ±1-2%) and line/load regulation (5-50 mV variation) determine output voltage stability under varying conditions.

Worked Example

Design a post-regulator stage using linear regulation after a buck converter. Requirements: convert 5 V buck output to 3.3 V for ADC reference, 50 mA load, <50 µV output noise. Step 1: Select regulator type — LM317 (1.5 V dropout) insufficient margin with 5 V input transients. Use LDO: TI TPS7A4901 (250 mV dropout, 15 µV RMS noise). Step 2: Verify headroom — At 50 mA: Vdropout = 90 mV typical, 250 mV max. Vin_min = 3.3 + 0.25 = 3.55 V (5 V input provides 1.45 V margin). Step 3: Calculate power — Pdiss = (5 - 3.3) × 0.05 = 85 mW. SOT-23-5 package (θJA = 180°C/W): ΔTj = 15°C — no heatsink required. Step 4: Verify noise — TPS7A4901: 15 µV RMS (10 Hz - 100 kHz) with proper decoupling. Use 10 µF + 0.1 µF ceramics on input/output. Step 5: Verify PSRR — 70 dB at 1 kHz attenuates buck converter 30 mV ripple to 9 µV at output.

Practical Tips

  • ✓Per Analog Devices AN-1072, cascade two LDOs for ultra-low-noise performance: first stage provides 20 dB PSRR, second stage adds 60 dB — achieving >80 dB total rejection of switching noise
  • ✓Use adjustable LDOs (TPS7A49, LT3045) with 0.1% precision resistors for output voltage accuracy better than ±1% — fixed-voltage LDOs typically specify ±2-3% tolerance
  • ✓Add soft-start capacitor on adjust pin to limit inrush current — 100 nF provides 1-10 ms startup ramp, protecting against input voltage droop with limited source impedance

Common Mistakes

  • ✗Confusing standard regulators with LDOs — LM7805 requires 2 V headroom minimum (Vin ≥ 7 V), while modern LDOs (ADP3338) operate with only 190 mV dropout at 1 A
  • ✗Ignoring internal resistance effect — dropout = Rds(on) × Iload; a 0.3 Ω LDO exhibits 150 mV dropout at 500 mA but 450 mV at 1.5 A, potentially exceeding spec
  • ✗Using insufficient input decoupling — per TI SLVA115, LDOs require 1-10 µF input capacitor within 10 mm of pins to prevent oscillation; ESR <1 Ω for ceramic, or 0.5-5 Ω for tantalum

Frequently Asked Questions

Dropout voltage is the minimum Vin - Vout differential for maintaining regulation within specified output tolerance. Per ON Semiconductor application note, it represents the voltage across the pass transistor at saturation (PMOS: Rds(on) × Iload, NPN: Vce(sat)). Below dropout, output voltage drops proportionally with input, and PSRR degrades to 0 dB.
Low dropout enables: (1) longer battery runtime — 200 mV lower dropout extracts additional 5-10% battery capacity from Li-ion cells, (2) reduced power dissipation — at 1 A, 200 mV less dropout saves 200 mW of heat, (3) cascaded regulation — enables post-regulator stages without excessive voltage budget. Per TI, modern ultra-LDOs achieve 50 mV dropout at 1 A.
Vin_min = Vout + Vdropout + Vmargin (typically 100-200 mV). Use maximum dropout specification at operating current and temperature. Example: 3.3 V output, 200 mV max dropout, 150 mV margin → Vin_min = 3.65 V. For battery applications, calculate battery voltage at end-of-discharge and verify Vin_min is achievable.
Pdiss = (Vin - Vout) × Iload + Vout × Iq, where Iq is quiescent current (typically 1 µA - 5 mA). At high load current, Iload term dominates. Example: 12 V to 5 V at 500 mA: Pdiss = 7 V × 0.5 A = 3.5 W — requires significant heatsinking (θJA < 15°C/W for safe operation at 85°C ambient).
Per TI power topology selection guide: Use linear regulator when (1) efficiency >80% acceptable (Vout/Vin > 0.8), (2) low noise critical (<100 µV), (3) simplicity/cost priority, (4) PCB space limited (<0.5 cm²). Use switching regulator when (1) efficiency critical (battery life), (2) Vin >> Vout (>2×), (3) power >2 W. Hybrid approach: switching regulator + LDO post-regulator achieves both efficiency and low noise.

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