Power budget for a multi-rail circuit

How the power budget is calculated

A power budget analysis in electronics works from the loads back to the source. For each rail, start from its own load current, add the input current of every regulator it feeds, and that total is what its own regulator has to deliver. Then its input current follows from the regulator type:

• Switching regulator: PIN = POUT / η, loss = POUT × (1/η − 1), and IIN = PIN / VIN, where VIN is the voltage of whatever feeds it.
• LDO: IIN = IOUT + IQ, because a linear regulator passes its load current and adds its ground current (TI SLVA118A). Its loss is (VIN − VOUT) × IOUT + VIN × IQ.
• Source: the current is the sum of the input currents of the rails it feeds directly, and the overall efficiency is the power reaching the loads divided by the power drawn from the source.

Converters in series multiply their efficiencies, as EDN's Efficiency calculations for power converters sets out, so the chain column is the product of the stage efficiencies from the source down to each rail. In the worked example, the 3.3 V rail sits behind a 90 % stage and its own 88 % stage: only 79.2 % of the power drawn for it at the battery reaches its loads.

What “first order” leaves out

Each switching regulator here has one fixed efficiency. Real converter efficiency depends on the load current and the input voltage — it typically falls at light load, where fixed switching and bias losses dominate — and on temperature. A single figure can be optimistic or pessimistic depending on where on its curve each rail really runs, and tolerance on the input voltage and the loads moves every number in the table.

That is the part a spreadsheet, or this page, cannot do well. The editor solves the same tree with load-dependent efficiency, tolerance corners and thermal estimates, and keeps it as a design you can export and share.

LDO rails

An LDO's loss is set by the voltage it drops, not by an efficiency, so it is entered with its ground current instead. The check column links each LDO to the LDO power dissipation calculator with its input, output and current filled in, to confirm the junction temperature in its package.

Battery runtime

With the source marked as a battery, the runtime is the usable capacity divided by the average current. TI's Increase Your Battery Life With Nano Quiescent Current LDO (SBVA084) computes battery life as capacity over total current, with the average current weighted over active and standby time. Here that is two states: the source current from the rails while active, and the sleep current for the rest:

• Iavg = Iactive × d + Isleep × (1 − d)
• Runtime = C × k / Iavg

k is the usable fraction of the rated capacity: set it below 100 % for low temperature, ageing and the cut-off voltage your circuit stops at. SBVA084 also counts battery self-discharge and output-capacitor leakage; if they matter for your design, add them to the sleep current.

Instead of a power budget spreadsheet

The arithmetic is the same as a spreadsheet's. What this page adds is the tree: a rail's current is carried up to whatever feeds it, so moving a regulator to a different parent updates every upstream number without editing formulas.

References

1. Texas Instruments, Linear Regulator Design Guide for LDOs, SLVA118A.
2. Texas Instruments, Increase Your Battery Life With Nano Quiescent Current LDO, SBVA084.
3. EDN, “Efficiency calculations for power converters”.