⚡ Linear Voltage Regulator (LDO) Lab

About this simulation

A low-dropout (LDO) regulator holds a steady output voltage by continuously adjusting the resistance of a pass transistor connected between the raw input rail and the regulated output rail. An internal error amplifier compares a scaled-down copy of V_OUT against a fixed reference voltage and drives the pass device's gate to correct any difference — this is the "iris valve" you see pulsing between the input and output bus bars.

How it works

• Regulation region — while V_IN − V_SET stays above the dropout spec, the regulator holds V_OUT = V_SET exactly, absorbing the excess voltage across the pass transistor as heat.
• Dropout region — once V_IN gets too close to V_SET, the pass transistor is already fully "open" (minimum resistance) and can't drop any more voltage; V_OUT then simply follows V_IN − V_DO, tracking the input downward.
• Power dissipation — P = (V_IN − V_OUT) × I_LOAD. This heat is why linear regulators need headroom and why efficiency falls as V_IN rises far above V_OUT.
• Efficiency — approximately V_OUT / V_IN, unlike a switching regulator which can exceed 90% regardless of the voltage gap.

Controls

• V_IN — the raw, possibly noisy or sagging supply voltage feeding the regulator.
• V_SET — the regulated output voltage the internal reference targets.
• I_LOAD — current drawn by the downstream circuit; more current means more heat for the same voltage drop.
• V_DO — the device's dropout-voltage spec; lower numbers mean the regulator can keep working closer to V_SET.

Did you know?

"Low-dropout" simply means the minimum V_IN − V_OUT headroom needed for regulation is small — often under 0.3V — versus 2V or more for older linear regulators like the 7805. That extra headroom is entirely wasted as heat, so battery-powered designs favour LDOs with the smallest dropout spec that still fits the budget.