Charger Compatibility

Why resistor-strapped USB-PD trigger ICs (CH224D-class) were rejected in favor of a certified auto-negotiating controller, the tested-charger evidence behind that call, and the "verify 15V is in the PDO list" purchase rule.

Board P uses the STUSB4500 — a USB-IF certified, auto-run PD sink controller — instead of a simpler resistor-strapped trigger IC such as the CH224D family. This choice is inherited directly from the zudo-pd project, which tried the resistor-strapped approach first and measured real compatibility failures against it.

Why resistor-strapped triggers were rejected

A resistor-strapped PD trigger (CH224D and similar parts) requests a fixed voltage by presenting a configuration resistor value to the controller — there is no software PD stack behind it. That has two consequences that matter for a device meant to work with whatever USB-C charger someone already owns:

No retry logic. If the first negotiation attempt fails or a source's timing does not match what the simple hardware state machine expects, the part does not retry — it falls back to plain 5 V and stays there.
Passive against dynamic power management. Modern multi-port GaN chargers redistribute power as ports connect/disconnect and may re-advertise PDOs dynamically. A resistor-strapped sink has no mechanism to react to any of that; a certified sink controller with retry/re-negotiation logic (like the STUSB4500) does.

There is also a load-switch interaction that matters independently of which PD controller is used: if downstream DC-DC converters start drawing current the instant USB-C first connects at 5 V, that draw can collapse the 5 V rail before the negotiation IC ever completes a 15 V request. Board P's Q1 load switch exists specifically to gate downstream power off until the STUSB4500 confirms a live 15 V contract — see Board P's front-end page for the gate-drive network.

Tested-charger evidence

zudo-pd's own documentation records a real bench comparison of the resistor-strapped CH224D design against a range of USB-PD chargers:

Charger	Model	Power	Ports	Result with CH224D
Anker Nano II 65W	A2663	65 W	1× USB-C	✅ Works
Anker Prime 200W	A2683	200 W	4× USB-C + 2× USB-A	❌ All ports fail
Elecom EC-AC67150BK	—	150 W	3× USB-C + 1× USB-A	❌ All ports fail
Digiforce Square 65W	—	65 W	—	❌ Fails
CIO NovaPort SLIM 45W	—	45 W	2× USB-C	❌ Fails

That is 1 of 5 tested chargers working (20%) against the resistor-strapped controller — only the single-port Anker Nano II 65W succeeded.

Note

zudo-pd's own prose repeats a "~33% compatibility, 2 of 6 chargers worked" figure in several places (its CH224D component page, its STUSB4500 page, its BOM page). This audit could not find a documented 6-charger test table anywhere in zudo-pd's sources — including its own linked GitHub issue #33 — to back that number; every enumerated table found (reproduced above) lists exactly 5 chargers and exactly 1 success. This is carried forward here as an honest flag on the source material rather than silently repeated: the directionally-correct, well-evidenced claim is "most multi-port GaN chargers failed, the single-port unit worked"; the precise "33%" / "2 of 6" figure is unreconciled in zudo-pd's own docs and should not be quoted as a hard number without finding its source test session.

The pattern across every failure is the same, regardless of the exact fraction: single-port chargers worked; every multi-port GaN charger tested, failed. Symptoms on a failed charger: only a brief -12V LED flicker, then all output rails dead — the downstream DC-DC stage's inverting converter momentarily reacting to whatever residual voltage exists on a 5 V-only supply, then everything collapsing once the 5 V itself sags.

Recommendation for this lamp

Prefer a dedicated single-port USB-C PD charger. The one charger that worked cleanly in zudo-pd's testing (Anker Nano II 65W) is single-port. This is not proof every single-port charger will work — it is the strongest available signal that single-port, simpler power-management chargers are lower-risk than high-wattage multi-port GaN units for a hardware-only sink negotiation.
Avoid multi-port GaN chargers above roughly 100 W unless a specific model has been confirmed working with an STUSB4500-based design — none has been, as of this audit. The STUSB4500's certified retry/re-negotiation logic should do meaningfully better than the CH224D's zero-retry behavior against these chargers, but that expectation has not been bench-confirmed for this project (see the honesty note in Lessons Carried From zudo-pd).
Test before committing to a charger for the finished lamp, especially if reusing a charger already on hand rather than buying one specifically for this purpose.

The "verify 15 V is in the PDO list" rule

Before buying or reusing any charger for this lamp, check its listed PDO (Power Data Object) profiles for an explicit 15 V entry. Some PD chargers skip 15 V entirely — e.g. a charger advertising only 5 V / 9 V / 20 V. Board P's NVM is programmed to request 15 V/3 A specifically (see Lessons Carried From zudo-pd for why — the alternative, requesting 20 V, is out of scope for the D5/Q1 rating margins established in the ratings matrix). A charger whose PDO list has no 15 V entry will fail to negotiate the target contract regardless of how good its PD stack is — this is a charger-selection problem, not a Board P design defect.

Look for the voltage list printed on the charger itself or its packaging/listing (e.g. "5V⎓3A / 9V⎓3A / 15V⎓3A / 20V⎓3.25A"); a listing that jumps 9 V → 20 V without 15 V will not work with this design as programmed.

References

zudo-pd's CH224D component page, STUSB4500 component page, USB-PD adapter overview page, and BOM page — source of the charger test data and the unreconciled "~33%"/"2 of 6" figure flagged above.
zudo-pd's public GitHub issue #33 ("Consider replacing CH224D with STUSB4500 for better charger compatibility") — the same 3-row subset of the test table, no additional chargers.
Board P — USB-PD Front End for the load-switch circuit that gates power during negotiation.