A linear regulator, or LDO, makes a lower voltage by dropping the difference across itself. That difference, times the current, becomes heat. Laptop boards use small LDOs for low-current rails — audio, clock generators, always-on logic — where their simplicity and low noise matter more than efficiency. This calculator shows how much heat an LDO makes and whether its package can survive it.

How to use it

  1. Enter the input voltage, output voltage and load current.
  2. Optionally add the quiescent current and the package's thermal resistance θJA from the datasheet.
  3. Enter the ambient temperature around the part — inside a laptop, 40–50 °C is realistic.
  4. Read the dissipation, efficiency and estimated junction temperature.

The formulas

  • Dissipation: P = (Vin − Vout) × Iout + Vin × Iq
  • Efficiency: η = Vout × Iout ÷ (Vin × (Iout + Iq)) — roughly Vout ÷ Vin
  • Junction temperature: Tj = Ta + P × θJA

Most regulators shut down somewhere around 150 °C junction temperature; long-term operation is usually rated to 125 °C.

Worked example: an LDO that keeps shutting down

A 1.8 V LDO is fed from 3.3 V and supplies 300 mA. It drops 1.5 V, dissipating 0.45 W. In a small package with θJA of 100 °C/W at 45 °C ambient, its junction would reach about 90 °C — warm but within limits. Feed the same LDO from 5 V by mistake (a wrong jumper or a design variant) and the dissipation rises to about 0.96 W, the junction to about 141 °C, close to thermal shutdown: the rail drops out every few minutes. The calculation shows the cause before you replace a good part. The component is explained under ldo and in LDO Regulators on Laptop Boards.

Where it matters on a laptop board

  • Diagnosing LDOs that get very hot or shut down intermittently.
  • Checking that a load on an LDO rail is not drawing far more than it should — a partial short heats the LDO first.
  • Understanding why high-current rails use buck converters instead (see Buck Converter Calculator (Duty Cycle and Inductor Ripple)).

FAQ

Is a hot LDO always faulty?

No. An LDO dropping a large voltage at moderate current runs hot by design. Calculate the expected dissipation before suspecting it.

Why is LDO efficiency so low?

Everything not delivered to the load is burned in the regulator. Efficiency is roughly Vout ÷ Vin.

What is dropout voltage?

The minimum difference between input and output for the LDO to regulate. Below it, the output follows the input down.