20261024-2200-air-hub-mini-diode-or-input Air Hub Mini: Schottky diode-OR instead of the LM74502 M8 input switch

• status: completed
• priority: P2
• owner: worker/session-ybo-46
• createdAt: 2026-10-24 22:00

Description

On 2026-10-24 the owner reviewed off-the-shelf power modules for the Air Hub Mini and chose to keep the discrete LMR36520 buck ("1，保持现状"). No module covers 4.3 V (5 V USB through a Schottky) to 36 V with a 53 V TVS clamp at 0.56 A peak: the Mornsun K7803 parts need at least 6 V, the K7803JT-500R3 gives only 0.5 A, and the TI TPSM365R6 gives only 0.6 A.

The owner also approved the side item from that review ("3，也顺便做"). Now that the Mini USB-C input is 5 V only, the M8 reverse-polarity switch and the USB-priority cut-off are replaced by a plain diode-OR:

• Removed: the LM74502, the DMN6040SSD FET pair, the 100 nF / 100 V VS capacitor, the 220 nF / 100 V VCAP capacitor and the 100 kΩ / 39.2 kΩ OV divider on VBUS.
• Added: one 60 V / 3 A Schottky (B360A, the same part as on the USB path) from the fused, TVS-protected V+_F to the filter input V+_P. It blocks a reversed harness input.
• Behaviour change: the higher input feeds VIN. With the M8 harness connected (at least 9 V), M8 supplies the board and the USB Schottky is reverse biased. USB-C 5 V supplies the board only when M8 is absent. This replaces "USB-C priority" for the Mini only; the Linux carrier keeps USB priority.

Acceptance criteria:

1. Schematic: ERC 0 and net verification 0 mismatches, generated with tools/mini/run_mini.sh.
2. PCB: tools/mini/pcb_mini.sh gives DRC 0 unconnected, 0 clearance errors and 0 parity. The EMC rerun has no errors.
3. Cost: the cost sheet is updated.
4. Docs:
   • The spec describes the Mini supply priority (§2.4 supply row, GW-05).
   • The design appendix and the reference-board page describe the diode-OR (§3, §4, §5, §6.1, §7, §8).
   • The workspace README follows.
5. Quality gate: bun run docs:lint && bun run docs:build && git diff --check passes.

ActiveForm

Replacing the Air Hub Mini M8 input switch with a Schottky diode-OR

Dependencies

• blocked by: (none)
• blocks: (none)

Notes

Results (2026-10-24):

• Schematic: 47 placed parts in 38 BOM lines (was 52 in 41). ERC 0; net check 0 mismatches on 79 nets. The block-diagram text on the root sheet and the power-sheet notes describe the diode-OR.
• PCB: tools/mini/pcb_mini.sh gives DRC 0 unconnected, 0 errors, 0 parity, with the three known lib_footprint_mismatch warnings. The new Schottky sits where the FET pair was.
  • EMC: 18 findings, 0 errors. The 10 warnings are the USB pair and MG24_SWCLK on the outer layer, a 2.9 mm (16 ps) skew on the connector-side USB pair, reference-plane gaps on CANH / CANL at the harness entry (93–94 % coverage), two ground domains by design, and buck harmonics.
• Cost: 18.87 → 17.96 USD per board at 100 boards. Electronics are $15.3 and the unit is $21.0–26.3.
• Docs: spec R1.17, design appendix R1.12 and reference board R1.9 are updated; the README, the Mini schematic and PCB diagrams, and the product-directory copies follow.
• Open: the switch-over transient when M8 is plugged or unplugged with USB connected (VIN steps between about 8.5 V and 4.5 V) needs a bench check. B360A reverse margin against the TVS clamp also needs checking, if the TVS moves to a 36 V stand-off part (clamp about 58 V vs 60 V).