LDO Dropout Calculator

How to Use This Calculator

This calculator helps you determine if your LDO (Low Dropout) voltage regulator has sufficient input voltage headroom for stable operation. It also calculates power dissipation and efficiency.

1. Output Voltage — The regulated output voltage (Vout) from the LDO
2. Dropout Voltage — The minimum voltage drop across the LDO (from datasheet)
3. Load Current — The current drawn by your load
4. Input Voltage — Your actual input voltage supply

The calculator will show whether your design has adequate headroom and warn you if the input voltage is too low for stable regulation.

Understanding LDO Dropout Voltage

Dropout voltage is the minimum difference between input and output voltage required for an LDO regulator to maintain proper regulation. When the input voltage drops below Vout + Vdropout, the LDO can no longer regulate and the output follows the input.

Factors Affecting Dropout

• Load Current — Dropout voltage typically increases with load current
• Temperature — Dropout may increase at higher temperatures
• Pass Element — PNP/NPN transistors have higher dropout than PMOS
• Package — Thermal performance affects maximum current and dropout

Types of LDO Regulators

• Standard LDO — 1-2V dropout (e.g., LM7805, LM317)
• Low Dropout — 0.3-1V dropout (e.g., LM1117, AMS1117)
• Ultra-Low Dropout — Under 0.3V dropout (e.g., MCP1700, AP2112)

Why Headroom Matters

Headroom is the extra voltage above the minimum required input. Having adequate headroom ensures stable operation under varying conditions.

Recommended Headroom

• Minimum: 100mV — Absolute minimum for basic operation
• Typical: 300-500mV — Good margin for most applications
• Conservative: 500mV+ — Accounts for temperature, aging, and ripple

Low Headroom Problems

• Output voltage sags below specification
• Increased output ripple and noise
• Poor transient response
• Oscillation or instability
• Reduced PSRR (Power Supply Rejection Ratio)

Power Dissipation and Thermal Considerations

LDOs dissipate the voltage difference between input and output as heat. This is the main drawback of linear regulators compared to switching regulators.

Thermal Management

• Check maximum power dissipation for your package
• Calculate junction temperature: Tj = Ta + (Pd × θja)
• Consider thermal pads, copper pour, or heatsinks for high power
• Derate current at high ambient temperatures

Efficiency Comparison

LDO efficiency is simply Vout/Vin × 100%. A 3.3V output from 5V input is only 66% efficient, with 34% of power wasted as heat. For high current or large voltage drops, consider a switching regulator instead.

LDO Selection Guide

When to Use an LDO

• Small voltage difference (Vin - Vout under 1-2V)
• Low noise requirements (audio, RF, sensors)
• Low current applications (under 500mA)
• Space-constrained designs (no inductor needed)
• Cost-sensitive applications

When to Consider Alternatives

• Buck converter — Large voltage drops, high current, efficiency critical
• Buck + LDO — High efficiency with low noise output
• Charge pump — Low current, small form factor

Key Specifications to Check

• Dropout voltage at your load current
• Output current capability
• Quiescent current (important for battery applications)
• Output capacitor requirements
• Thermal resistance (θja)
• PSRR and noise specifications