LDO headroom + dissipation calculator (linear regulator)

A linear LDO holds its output only while the input stays above the output by at least its dropout voltage. The voltage it drops becomes heat: power = (V_in − V_out) × the load current. A 5 V rail regulated to 3.3 V at 550 mA burns 1.7 V × 0.55 A = 0.94 W, which is what actually limits the part. Set your own below.

Inputs

Input voltageVThe rail feeding the LDO (e.g. 5 V from USB, or a battery).Output voltageVThe regulated rail you want (e.g. 3.3 V).Dropout voltageVFrom the LDO datasheet, at your load current. Low-dropout parts are ~0.1–0.5 V.Load currentmAThe worst-case current the rail has to deliver.

Result

0.94 W

dissipated as heat

Regulates: 1.70 V headroom is at or above the 0.3 V dropout.

Regulation

✓ Holds the output

Headroom: will it even regulate?

The gap between input and output, V_in − V_out, is the headroom. The LDO needs that gap to stay at or above its dropout voltage, or the output starts following the input down and the rail sags. Dropout grows with load current, so read it from the datasheet at the current you actually draw, not the headline figure. A battery is the case to watch: as it discharges, V_in falls toward V_outand a regulator that was fine at full charge can drop out near empty.

Dissipation is the real limit

A linear LDO passes its full output current straight through, so the input current roughly equals the output current. Every volt of headroom times that current turns into heat in the package. A big input-to-output gap at high current cooks the part: the same 0.94 W on a tiny SOT-23 has nowhere to go. The calculator shows the watts so you can check it against the package’s thermal rating before you trust it.

From a real board

The OTD L1.01 board regulates USB’s 5 V down to 3.3 V with an RT9080 linear LDO. At the board’s Wi-Fi-transmit peak near 550 mA that is roughly 1 W transient through the regulator (One Thousand Drones, L1.01 / L1.03 design 2026), which is exactly why it is sized as a 600 mA part with a thermal pad and not a tiny one.

When a switcher beats an LDO

An LDO is small, quiet, and cheap, and it is the right call for a small drop or a low current. When the drop is large and the current is high, the wasted heat is the problem, and a switching (buck) converter that steps the voltage down efficiently makes more sense. The trade is size, cost, and switching noise. See the battery and power module course.

References

Your LDO’s datasheet for the dropout voltage (at your load current) and the package thermal resistance.
One Thousand Drones. ESP32-S3 USB-C breakout (L1.01), RT9080 3.3 V rail. Build the board.

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