Linear Dropout Regulator (LDO) Calculator

An LDO (Low-Dropout Regulator) is a circuit component that converts a higher DC voltage down to a stable lower voltage — commonly used to power microcontrollers, sensors, and other electronics. Enter your Output Voltage, Input Voltage, Load Current, and Max Dropout Voltage into the LDO Calculator to find the Minimum Input Voltage your regulator needs to stay in regulation. Secondary outputs include Voltage Headroom, Power Dissipation, Efficiency, and — if you provide thermal resistance and ambient temperature — the estimated Junction Temperature.

V

Regulated output voltage from the LDO

V

Actual input supply voltage

A

Current drawn by the load

V

Maximum dropout voltage from LDO datasheet

A

Ground current consumed by LDO (optional)

°C/W

Junction-to-ambient thermal resistance (optional)

°C

Operating ambient temperature (optional)

Results

Minimum Input Voltage

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Voltage Headroom

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Power Dissipation

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Efficiency

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Junction Temperature

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Frequently Asked Questions

What happens if I exceed the dropout voltage?

If the input voltage falls below the minimum required voltage (Vout + Vdropout), the LDO will go out of regulation. The output voltage will drop and become unstable, potentially causing your circuit to malfunction.

Why does dropout voltage increase with load current?

Dropout voltage is primarily determined by the internal resistance of the LDO's pass element. As load current increases, the voltage drop across this internal resistance increases according to Ohm's law (V = I × R).

Can I use an LDO to drop 12V to 3.3V?

Yes, but consider the power dissipation. With a large voltage drop, the LDO will dissipate significant power as heat (P = (Vin - Vout) × Iload). You'll need adequate heat sinking and thermal management.

What is quiescent current and why does it matter?

Quiescent current is the current consumed by the LDO itself for its internal operation. It affects efficiency, especially at light loads, and is important for battery-powered applications where every microamp counts.

Why do some LDOs need specific output capacitors?

Output capacitors provide stability and improve transient response. Different LDO architectures have specific ESR (Equivalent Series Resistance) requirements for their output capacitors to maintain stability and prevent oscillation.

How do I know if my LDO needs a heatsink?

Calculate the junction temperature using: Tj = Ta + (Pd × θJA). If Tj exceeds the maximum junction temperature rating (typically 125-150°C), you need better thermal management like a heatsink or copper pour.

What's the difference between dropout and headroom?

Dropout voltage is the minimum voltage difference required across the LDO for regulation. Headroom is the actual voltage difference you have (Vin - Vout). Headroom should be greater than dropout for stable operation.

Why is LDO efficiency important?

LDO efficiency (Vout/Vin × 100%) determines how much input power is wasted as heat. Low efficiency means more power dissipation, higher temperatures, and shorter battery life in portable applications.