LDO Efficiency (with Iq)

η = Vout·Iout / (Vin·(Iout + Iq))

Quick answer

This calculator solves η = (Vout × Iout) / (Vin × (Iout + Iq)) for Efficiency. With Output Voltage 12 V, Output Current 20 mA, Input Voltage 26.4 V, Quiescent Current 44 mA, the result is 14.2 %. It solves in 4 directions, so you can enter the values you have and get the one you need.

Calculator

Efficiency (%)
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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.

Symbols and units

Symbol Meaning Default unit Accepted units
eta Efficiency % —, %
Vout Output Voltage V pV, nV, µV, mV, V, kV
Iout Output Current mA pA, nA, µA, mA, A, kA
Vin Input Voltage V pV, nV, µV, mV, V, kV
Iq Quiescent Current mA pA, nA, µA, mA, A, kA

Step by step

  1. 1. What you know: Output Voltage 12 V, Output Current 20 mA, Input Voltage 26.4 V, Quiescent Current 44 mA
  2. 2. The relation: η = (Vout × Iout) / (Vin × (Iout + Iq))
  3. 3. Substitute: Vout = 12 V, Iout = 20 mA, Vin = 26.4 V, Iq = 44 mA
  4. 4. Result: Efficiency = 14.2 %

Related quantities from the same inputs

  • P = V × I = 0.24 W

Worked examples

Minimum range

Given: Output Voltage 0.33 V, Output Current 100 µA, Input Voltage 0.726 V, Quiescent Current 0.22 mA

Result: Efficiency = 14.2 %

Low range

Given: Output Voltage 3.3 V, Output Current 1 mA, Input Voltage 7.26 V, Quiescent Current 2.2 mA

Result: Efficiency = 14.2 %

Typical

Given: Output Voltage 12 V, Output Current 20 mA, Input Voltage 26.4 V, Quiescent Current 44 mA

Result: Efficiency = 14.2 %

High range

Given: Output Voltage 0.23 kV, Output Current 0.5 A, Input Voltage 0.506 kV, Quiescent Current 1.1 A

Result: Efficiency = 14.2 %

Maximum range

Given: Output Voltage 2.3 kV, Output Current 5 A, Input Voltage 5.06 kV, Quiescent Current 11 A

Result: Efficiency = 14.2 %

Solving for each variable

Solve for Efficiency

Output Voltage 0.33 V, Output Current 100 µA, Input Voltage 0.726 V, Quiescent Current 0.22 mA → 14.2 %

Solve for Output Voltage

Efficiency 0.5 %, Output Current 100 µA, Input Voltage 0.726 V, Quiescent Current 0.22 mA → 11.62 mV

Solve for Input Voltage

Efficiency 0.5 %, Output Voltage 0.33 V, Output Current 100 µA, Quiescent Current 0.22 mA → 20.63 V

Solve for Quiescent Current

Efficiency 0.5 %, Output Voltage 0.33 V, Output Current 100 µA, Input Voltage 0.726 V → 8.991 mA

How Efficiency changes with Output Voltage

Output Voltage (input) Efficiency (result)
1.2 V 1.42 %
3 V 3.551 %
6 V 7.102 %
9 V 10.65 %
12 V 14.2 %
18 V 21.31 %
24 V 28.41 %
48 V 0.5682 %
84 V 0.9943 %
120 V 1.42 %

The same result in other units

Unit Value
% 0.142
% 14.2

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Frequently asked questions

How do you calculate LDO Efficiency (with Iq)?

Use η = (Vout × Iout) / (Vin × (Iout + Iq)). With Output Voltage 12 V, Output Current 20 mA, Input Voltage 26.4 V, Quiescent Current 44 mA, Efficiency works out to 14.2 %.

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

The formula is 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.

How do you find Output Voltage when you know the other values?

Rearrange for Output Voltage: with Efficiency 0.5 %, Output Current 100 µA, Input Voltage 0.726 V, Quiescent Current 0.22 mA, Output Voltage is 11.62 mV.

How do you find Input Voltage when you know the other values?

Rearrange for Input Voltage: with Efficiency 0.5 %, Output Voltage 0.33 V, Output Current 100 µA, Quiescent Current 0.22 mA, Input Voltage is 20.63 V.

What units does LDO Efficiency (with Iq) use?

Efficiency is shown in %, %. Values are held in SI base units internally, so you can mix prefixes without converting by hand.

Related Concepts

All Power & Efficiency formulas →
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