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Design Specification
1. Document Purpose
This document is the living design specification for the power management board in this project. It captures requirements, architectural decisions, assumptions, open questions, constraints, and risks. It should be updated as requirements evolve and as implementation decisions are made.
2. Project Goal
Design a power management board for a low-power embedded system that can:
- Accept power from a solar panel, a 1S LiPo battery, and USB input
- Charge the battery when an external source is available
- Power the system from the battery when no external source is present
- Generate regulated output rails of 12 V, 5 V, and 3.3 V
- Prioritize USB input over solar input when both are connected
3. System Overview
The system is a multi-input, battery-backed power management board for a low-power embedded platform.
At a high level, the board must:
- Accept energy from a 6 W solar panel
- Accept power from a USB-C receptacle
- Interface with a 1S LiPo battery
- Manage battery charging from available external sources
- Provide regulated output rails for downstream electronics
- Enforce source prioritization so USB overrides solar when both are present
- Keep the 3.3 V rail available whenever any valid source exists
- Provide either 5 V or 12 V from a selectable boosted rail, one at a time
4. Requirements Snapshot
4.1 Inputs
| Input Source | Requirement | Notes |
|---|---|---|
| Solar panel | 6 W, Voc = 6 V | Exact operating point unknown; source is variable and must be qualified with a usable threshold |
| Battery | 1S LiPo, nominal ~3.7 V, max 4.2 V, 4400 mAh | Board shall provide battery protection |
| USB input | USB-C receptacle | USB 2.0 input, worst-case source budget 500 mA, current negotiation capability desired |
4.2 Outputs
| Rail | Requirement | Notes |
|---|---|---|
| 12 V | 100 mA continuous | Selectable boosted rail |
| 5 V | 500 mA continuous | Selectable boosted rail |
| 3.3 V | 500 mA continuous | Always-on rail when any valid source exists |
4.3 Power Behavior
- Battery shall charge when solar or USB is connected and sufficient source power is available.
- Battery shall power the system when no external source is available.
- When external power is present, external power should run the system and recharge the battery.
- If external power is insufficient for both load and charging, maintaining system operation takes priority and charging current shall be reduced or deprioritized.
- If both USB and solar are connected, USB shall be the active source and solar shall be prevented from supplying power.
4.4 Target Components
| Function | Preferred Component | Notes |
|---|---|---|
| Battery charging | CN3063 | Preferred starting point, subject to architecture validation |
| 3.3 V rail generation | RT8059 | Preferred starting point, subject to source/load validation |
| Selectable boosted rail | MT3608L | Preferred starting point for either 12 V or 5 V, one at a time |
5. Power Architecture
5.1 Block Diagram Description
Planned high-level architecture:
- External power sources: solar input and USB-C input
- Source qualification and prioritization stage
- Battery charging stage for the 1S LiPo battery
- System power-path / load-sharing stage
- 3.3 V rail generation stage
- Selectable boosted output stage producing either 5 V or 12 V
- Output distribution stage for downstream loads
5.2 Proposed Functional Partitioning
| Block | Function | Current Direction |
|---|---|---|
| Solar input front end | Accept solar input, protect and qualify source | Must include threshold behavior for usable charging/power contribution |
| USB input front end | Accept USB-C input, protect source, define USB current behavior | USB priority source |
| Source priority stage | Ensure USB has priority over solar | Solar may remain attached but must not contribute when USB is present |
| Battery charger | Charge 1S LiPo from external source | Prefer CN3063 if architecture fit is confirmed |
| Power path / load sharing | Run system from external source when available, fall back to battery otherwise | Required behavior |
| 3.3 V regulator | Generate 3.3 V rail | Prefer RT8059 if validated |
| Boost stage | Generate either 5 V or 12 V | Prefer MT3608L if validated |
| Output selection stage | Select one boosted output target at a time | Manual switch preferred over jumper |
6. Input Source Prioritization Strategy
The board must support automatic source prioritization with USB as the preferred source.
6.1 Intended Behavior
| Condition | Desired Behavior |
|---|---|
| USB only present | USB powers system and charges battery, subject to input budget |
| Solar only present | Solar powers system and charges battery when above threshold |
| USB and solar both present | USB powers system; solar remains attached but is prevented from contributing power |
| No external source present | Battery powers system |
| External source insufficient for load + charging | Load remains powered; charging current reduces or stops |
6.2 Candidate Implementation Direction
A source-selection stage is required ahead of the charger and/or system power path.
Potential implementation directions to evaluate later:
- Ideal-diode OR-ing with explicit priority control
- PFET-based power-path selection
- Load-switch or power-mux style selection
6.3 Requirements Locked So Far
- USB has priority over solar.
- Solar does not need to be physically disconnected.
- Solar must simply be prevented from contributing power while USB is present.
- External power should operate the system directly when available.
7. Battery Charging Strategy
The system shall charge a 1S LiPo battery whenever a valid external source is available and there is sufficient source margin beyond system load.
7.1 Intended Charging Sources
- Solar input
- USB-C input
7.2 Preliminary Strategy
- Use CN3063 as the initial preferred LiPo charging controller.
- Favor safe, conservative charging behavior rather than aggressive fast charging.
- Respect the USB input current budget.
- Allow solar charging whenever the solar source is available and above a defined usable threshold.
- Ensure charging current can be reduced or suspended when external power is insufficient for both charging and load operation.
7.3 Charging Design Considerations
| Item | Direction |
|---|---|
| Battery capacity target | 4400 mAh |
| Battery protection | Onboard |
| Battery connector | Common JST battery connector, exact family TBD |
| Charge aggressiveness | Conservative; fast charging is not a priority |
| Input qualification | Required, especially for solar |
| Reverse current blocking | Required |
| Thermal management | Must be considered in charger and power-path design |
8. Output Rail Generation Strategy
8.1 3.3 V Rail
Planned approach:
- Generate the 3.3 V rail using RT8059 as the preferred starting point.
- Support 500 mA continuous load, with some additional peak margin.
- Keep 3.3 V available whenever any valid source exists.
Open validation items:
- Confirm suitability over the full system input range.
- Confirm expected efficiency across the battery voltage range.
- Confirm ripple, transient, and thermal behavior against downstream needs.
8.2 5 V / 12 V Rail Generation
Planned approach:
- Use MT3608L as the preferred starting point.
- Generate either 5 V or 12 V, selected manually, one at a time.
- Prefer a manual switch rather than a jumper.
- Ideally support output selection while the board is powered.
Open validation items:
- Final selection topology
- Whether live switching requires output blanking, discharge, soft-transition, or interlock behavior
- Whether MT3608L is still the best fit after architecture review
8.3 Simultaneity Constraint
This requirement is now locked:
- 5 V and 12 V are not required simultaneously.
- 3.3 V should remain available regardless of whether 5 V or 12 V is selected.
9. Power Budget and Efficiency Considerations
9.1 Output Power Targets
| Rail | Voltage | Current | Output Power |
|---|---|---|---|
| 12 V | 12 V | 100 mA | 1.2 W |
| 5 V | 5 V | 500 mA | 2.5 W |
| 3.3 V | 3.3 V | 500 mA | 1.65 W |
9.2 Immediate Observations
- The listed outputs should be treated as continuous worst-case loads.
- Additional margin should be allowed for peak current events.
- The 5 V and 12 V rails are selectable, one at a time, which reduces the worst-case boosted-load burden versus a simultaneous-output design.
- USB input budget of 500 mA is likely to be a major system constraint during charge-plus-load operation.
- Solar performance cannot be budgeted accurately from Voc alone.
- Converter efficiency will strongly affect battery runtime and thermal performance.
9.3 Budget Items To Quantify Later
- Worst-case battery input current in each operating mode
- Worst-case USB current draw during load plus charging
- Realistic solar contribution under non-ideal conditions
- Conversion losses for the 3.3 V and boosted rail
- Runtime with 4400 mAh battery for representative load cases
- Thermal dissipation in charger, boost, buck, and priority path elements
10. Key Components and Rationale
| Component | Intended Role | Current Rationale | Status |
|---|---|---|---|
| CN3063 | 1S LiPo charger | Preferred by user; candidate for conservative charging implementation | Needs validation in full architecture |
| RT8059 | 3.3 V converter | Preferred by user for always-on 3.3 V rail | Needs validation against actual power-path conditions |
| MT3608L | Boost converter | Preferred by user for selectable 5 V / 12 V rail | Needs validation for both outputs, live switching behavior, and continuous-load targets |
11. Assumptions
- The LiPo battery is a single-cell chemistry with nominal voltage near 3.7 V and full-charge voltage of 4.2 V.
- The board is intended for indoor use.
- USB is the highest-priority external source whenever connected.
- 5 V and 12 V are mutually exclusive outputs.
- 3.3 V must remain available whenever any valid source exists.
- The project is currently in architecture/specification phase only; no schematic decisions are yet locked.
12. Open Questions
| Topic | Question | Priority |
|---|---|---|
| USB-C behavior | What exact USB-C feature level is required: simple sink-only CC configuration, BC1.2-style charging detection, or fuller USB-C current negotiation handling? | High |
| Solar input | What usable solar threshold should qualify the source for charging/system contribution? | High |
| Solar input | What is the panel operating voltage/current at maximum power, not just Voc? | High |
| Selectable output behavior | During live switching between 5 V and 12 V, what interruption or transient is acceptable to the downstream load? | High |
| Downstream requirements | Are there explicit ripple, startup-time, sequencing, or brownout constraints for the loads on 3.3 V, 5 V, or 12 V? | Medium |
| Battery connector | Should the exact JST family be specified now, or left as an implementation choice for later? | Low |
13. Constraints
- Solar input is variable and environment-dependent.
- USB input budget is limited to 500 mA.
- Battery voltage varies across charge and discharge states.
- One boosted rail must serve two output-voltage targets, implying mode selection and non-simultaneous operation.
- The design should remain appropriate for a low-power indoor embedded application.
14. Risks
| Risk | Description | Impact |
|---|---|---|
| Source-power insufficiency | Available input power may be insufficient for worst-case load plus charging | High |
| USB current limit conflict | USB source budget may conflict with charge-plus-load demand | High |
| Solar variability | Real solar power may be far below nominal depending on conditions | High |
| Architecture mismatch | Preferred ICs may not cleanly satisfy all required operating modes together | High |
| Rail selection transient | Switching between 5 V and 12 V may disturb powered loads | Medium |
| Thermal stress | Charger or converters may dissipate significant heat under worst-case conditions | Medium |
| USB-C implementation scope | Desired current negotiation capability may require more than a minimal power-only USB-C input stage | Medium |
15. Decisions Log
| Date | Decision | Status | Notes |
|---|---|---|---|
| 2026-04-15 | Create living design specification in Files tab before schematic work | Decided | Initial architecture capture |
| 2026-04-15 | Treat this document as the source-of-truth living spec for ongoing updates | Decided | Update in place as requirements evolve |
| 2026-04-15 | Use CN3063, RT8059, and MT3608L as preferred starting components | Initial direction | Subject to validation during architecture review |
| 2026-04-15 | Use USB-C receptacle for wired external power input | Decided | USB current negotiation capability desired |
| 2026-04-15 | Assume USB source budget of 500 mA worst case | Decided | Load support takes priority over charging |
| 2026-04-15 | Use 1S LiPo 4400 mAh battery target with onboard protection | Decided | Common JST battery connector to be chosen |
| 2026-04-15 | Keep 3.3 V available whenever any valid source exists | Decided | 500 mA continuous target with some peak margin |
| 2026-04-15 | Make 5 V and 12 V mutually exclusive outputs | Decided | Manual selection preferred |
| 2026-04-15 | Prefer live switching of 5 V / 12 V while powered if feasible | Goal | Final transient behavior still needs definition |
| 2026-04-15 | External power should run the system and charge the battery when possible | Decided | Charging is secondary to sustaining the load |
16. Next Update Targets
The next revision of this document should ideally resolve or refine:
- Exact USB-C implementation scope
- Solar qualification threshold and real operating point assumptions
- Acceptable transient behavior during live 5 V / 12 V switching
- Downstream rail quality and startup expectations
- Preliminary operating-mode power budget
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