What is the formula for LDO Efficiency (with Iq)?

The LDO Efficiency (with Iq) formula is η = (Vout × Iout) / (Vin × (Iout + Iq)), where η—Efficiency (fraction); Vout—Output voltage (V); Iout—Load current (A); Vin—Input voltage (V); Iq—Quiescent current (A).

LDO Efficiency (with Iq)

Q: How do you calculate LDO Efficiency (with Iq)?

Use the formula η = (Vout × Iout) / (Vin × (Iout + Iq)). The efficiency of a linear regulator is fundamentally limited by the output-to-input voltage ratio, because the dropout voltage is dissipated as heat. Enter the values you know in the calculator above and it solves for the remaining variable.

η = Vout·Iout / (Vin·(Iout + Iq))

Calculator

Efficiency

Output Voltage

Output Current

Input Voltage

Quiescent Current

Result

—

Formula

η = (Vout × Iout) / (Vin × (Iout + Iq))

Description

The efficiency of a linear regulator is fundamentally limited by the output-to-input voltage ratio, because the dropout voltage is dissipated as heat. Including the quiescent current in the input term shows the extra efficiency loss at light load. An LDO is efficient only when Vout is close to Vin; large step-downs waste most of the power.

Variables

η — Efficiency (fraction)
Vout — Output voltage (V)
Iout — Load current (A)
Vin — Input voltage (V)
Iq — Quiescent current (A)

Practical Notes

Ideal LDO efficiency ≈ Vout/Vin. Dropping 5 V to 1.8 V caps efficiency at 36% regardless of the part—use a switcher when the ratio is large.

Related Concepts

All Power & Efficiency formulas →

Linear Regulator Dissipation Efficiency Buck Converter Boost Converter Peukert Effect Energy After Losses Solar Charge Time LDO Minimum Output

Need more features?

Save calculations, import telemetry data, simulate battery discharge, and collaborate with your team.

Try the App