Name: Flux
Price: 15 USD

Design Specification Development

Thought for 3 mins 23 sec

I updated the living spec to a v2 in Design Specification.

What changed:

Locked in USB-C, 500 mA USB budget, 1S 4400 mAh LiPo, and onboard battery protection
Recorded that external power should run the system first, with charging as secondary when source power is limited
Locked in USB priority over solar, with solar allowed to stay attached but not contribute when USB is present
Locked in always-on 3.3 V
Locked in mutually exclusive 5 V / 12 V, with manual switch preferred over jumper
Added clearer sections for power path, charging behavior, power budget, constraints, risks, and a decisions log
Narrowed the remaining uncertainties to a short list so we can finish requirements before schematic work

The remaining questions I still need from you are:

USB-C behavior
- Do you want:
  - simple USB-C sink-only behavior,
  - BC1.2-style charger detection, or
  - fuller USB-C current negotiation / PD-style behavior?
- Since your source budget is still 500 mA, my default recommendation would be USB-C sink-only unless you explicitly want more advanced negotiation.
Solar threshold
- What should count as “usable solar” before the board tries to charge or power from it?
- If you want, I can choose a conservative threshold rule and document it.
Live switching between 5 V and 12 V
- During switching while powered, is a brief interruption acceptable?
- Or must the downstream load see little to no interruption?
Rail quality / sequencing
- Any hard constraints on:
  - ripple,
  - startup time,
  - brownout tolerance,
  - rail sequencing?
- If not, I can define conservative defaults in the spec.
Battery connector
- Should I lock this now to a common JST family, or leave it as an implementation choice until part selection?

If you want, reply in a compact format like:

Choose defaults now

Use sink-only USB-C

Lock JST connector

Define rail limits

Talk to Flux to get started.