The source sees the whole tree
Power-Tree Rail Budget Calculator
A child converter is load on its parent. Budget direct loads, conversion loss, and evidence quality together across steady, peak, and sleep states—without counting internal rail transfers twice.
Updated 2026-07-15
Short answer
Start at the leaves. Convert each child rail's output demand into input demand, add it to the parent's direct load, and continue upstream. Use the minimum operating source voltage for a worst-case current budget. The result is useful only when every voltage, load state, efficiency, ground-current, dropout, and current-limit value has an honest evidence label.
Deterministic worksheet
Budget the tree, one assumption at a time
The loaded buck efficiency entered here already includes converter IQ losses. Do not add IQ again. A truly zero-load buck state needs no-load input current, which is a different datasheet or measured value.
Starter voltages and loads are synthetic worksheet assumptions. Conversion fields are intentionally blank: enter values for the exact part, VIN, VOUT, load, mode, and temperature.
The interactive worksheet supports a maximum of six rails.
Calculated boundary
Tree result
unresolved
Steady boundary
Weakest evidence: Unknown / unresolved
- Source current
- Unresolved
- Source power
- Unresolved
- Delivered load
- Unresolved
- Modeled loss
- Unresolved
- Tree efficiency
- Unresolved
Named issues
- — 3.3 V linear child: steady dropout voltage is unresolved.
- — 3.3 V linear child: steady uses unknown evidence.
- — 5 V buck: steady load or child demand is unresolved.
- — 5 V buck: steady uses unknown evidence.
- — steady result uses unknown evidence.
| Stage | Output load | Input current | Loss | Headroom | Boundary |
|---|---|---|---|---|---|
| 5 V buck | Unresolved | Unresolved | Unresolved | — | unresolved |
| 3.3 V linear child | 50 mA | Unresolved | Unresolved | 1.7 V / Unresolved required | unresolved |
What the arithmetic means
- Direct delivered load
- VOUT × the load attached directly to that rail. Child-converter demand is an internal transfer and is not counted again as delivered load.
- Linear stage
- IIN = total output current + entered ground current. Loss is input power minus output power. The entered dropout maximum must fit inside the available input-to-output headroom for each state.
- Loaded buck stage
- PIN = POUT ÷ entered total operating-point efficiency. Enter efficiency at the actual stage VIN, VOUT, load, mode, and temperature—not a headline value or another rail's curve. The entered efficiency already includes converter IQ losses; adding IQ again would double-count them.
- Zero-load buck state
- Use explicit no-load input current. IQ is measured under a different condition and is not a valid automatic substitute.
Evidence travels upstream too
The weakest entered evidence used by a child state propagates to its parent and the source result. A typical curve remains typical. A measurement applies to the measured board and condition. Unknown evidence can show arithmetic, but it cannot produce a pass boundary.
Three synchronized whole-tree states
Every rail is evaluated in the same named state at once; this v1 does not enumerate mixed per-rail combinations.
- Steady: sustained operating demand.
- Peak: bounded operating load, not startup inrush.
- Sleep: entered light/no-load behavior, not IQ cosplay.
What this deliberately excludes
The v1 worksheet does not model startup or inrush current, soft-start timing, sequencing, pre-bias behavior, boost or isolated stages, transient response, thermal impedance, or fault energy. Peak operating load is not permission to call startup solved.
It also does not estimate battery runtime. Runtime needs capacity versus load and temperature, converter behavior across the discharge curve, state durations, self-discharge, and aging. A one-line watt-hour division would be spreadsheet theater.
Continue the decision journey
Power-tree architecture
Place protection, conversion, monitoring, and sequencing jobs before filling the worksheet.
LDO vs buck for 3.3 V
Choose the operating model before assigning its loss inputs.
Linear thermal calculator
Turn a linear-stage loss into a bounded thermal and dropout estimate.
Bring-up checklist
Measure the actual source and rails under controlled first power.
INA219
Review an exact in-corpus current/power monitor without treating it as a universal measurement answer.
Official sources and claim boundaries
- Texas Instruments — Calculating Efficiency (SLVA390A)
Supports operating-point buck efficiency and loss relationships; it does not provide a generic efficiency default for another device or condition.
- Texas Instruments — IQ: What it is, what it isn't, and how to use it (SLYT412)
Distinguishes IQ from no-load input current and states that total efficiency curves already include IQ losses.
- Texas Instruments — Understanding LDO Terms and Definitions (SLVA079)
Supports the entered linear-stage relationship IIN = IOUT + IGND and the corresponding efficiency boundary.
- Texas Instruments — Managing Inrush Current (SLVA670A)
Explains why startup capacitor charging and controlled rise time are transient checks outside this operating-state worksheet.
- Analog Devices — Measuring Buck Converter Efficiency
Supports PIN = VIN × IIN, POUT = VOUT × IOUT, and efficiency as POUT / PIN while warning that measurement location changes the observed result.
Found an error? Submit a correction — we verify every correction against the manufacturer’s datasheet.