metronome/hardware/LAYOUT_REFERENCES.md

# PM_K-1 — Per-Stage Layout References

`LAYOUT.md` has *our* rules (general principles for this board). This file points at the
**manufacturers' own reference layouts, EVMs, and layout app-notes** for each stage — so
whoever routes the board can follow *proven* examples instead of working from first
principles. Two of these are essentially our exact circuit.

⭐ = a reference design / EVM that closely matches what we built (highest value).

## Power — switching supply (TPS65131) — the most layout-critical stage
- ⭐ **TPS65131EVM-839** (the eval board) + its **User's Guide SLVUAW7**.
  https://www.ti.com/tool/TPS65131EVM-839 · https://www.ti.com/lit/ug/slvuaw7/slvuaw7.pdf
  *Takeaway:* 4-layer, all parts top side; **switching nodes isolated from the feedback
  network**, careful high-frequency current routing, **separate analog & power grounds**,
  feedback components small/closely-spaced. TI explicitly says "follow the EVM layout."
- Datasheet layout section: TPS65131 (SLVS493E) — already in `datasheets/`.

## Power — clean ±15 V LDOs (TPS7A4901 / TPS7A3001)
- ⭐ **TPS7A30-49EVM-567** — a dual EVM with **−15 V and +15 V outputs** = *our exact rails*.
  https://www.ti.com/tool/TPS7A30-49EVM-567 (search TI for the user's guide)
  *Takeaway:* the reference layout for the exact LDO pair we use.
- Datasheet rule (TPS7A49/TPS7A30): **separate IN and OUT ground planes joined only at the
  GND pin**; keep the NR/SS cap and feedback divider tight; ~2 dB PSRR from good grounding.
  https://www.ti.com/lit/ds/symlink/tps7a49.pdf · https://www.ti.com/lit/ds/symlink/tps7a30.pdf
  E2E thread on layout + resistor selection: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/238266/

## MCU (RP2350)
- ⭐ **Official RP2350 "Minimal" KiCad design files** (schematic + symbols + footprints +
  the minimal layout): https://datasheets.raspberrypi.com/rp2350/Minimal-KiCAD.zip
  *Takeaway:* fork this — it's the authoritative layout for the core SMPS inductor loop,
  decoupling, crystal, QSPI, and USB. (Also a clean source for the RP2350 + QFN footprints.)
- **Hardware design with RP2350** (RP-008280) — in `datasheets/` (decoupling-per-pin,
  crystal, QSPI-short, USB 27 Ω series, VREG_AVDD filter).
- Third-party open board: **Olimex RP2350-PICO2-BB48** (full open KiCad).
  https://www.olimex.com/Products/RaspberryPi/PICO/RP2350-PICO2-BB48/open-source-hardware

## DAC (PCM5102A)
- Datasheet layout section: PCM5102A (SLAS859C) — in `datasheets/`.
- Widely-cloned reference: the **GY-PCM5102 module** + community layouts.
  diyAudio: https://www.diyaudio.com/community/threads/dac-design-based-on-pcm5102a.410231/ ·
  KiCad forum: https://forum.kicad.info/t/how-do-i-design-a-pcm5102a-dac/56441
  *Takeaway:* **one solid ground plane** (well-segmented), decoupling (10 nF + 100 nF) within
  ~5 mm of VDD, full copper pour under the IC, ground-via grid, short I²S traces.

## Balanced audio (THAT1240 receiver / THAT1646 driver)
- THAT product pages (have app guidance + reference circuits):
  https://thatcorp.com/that-1606-1646-balanced-line-driver-ics/ · audio design resources:
  https://thatcorp.com/Audio_Design_Resources.php
- Community reference layouts: theslowdiyer THAT1646 board
  https://theslowdiyer.wordpress.com/tag/that1646/ · Elektor "Balanced Audio Line Driver"
  https://www.elektormagazine.com/labs/balanced-audio-line-driver
  *Takeaway:* 22 Ω + decoupling caps right at the ±15 V pins; DC-block "link" caps; RFI
  filter (series R/ferrite + small cap) at the connector; keep input and output apart;
  route HOT/COLD as a tight pair.

## Op-amps (OPA1641 / OPA1612 — filter, summer, DI buffer)
- **TI SBOA092B** — *Handbook of Operational Amplifier Applications*:
  https://www.ti.com/lit/an/sboa092b/sboa092b.pdf
- **TI SLOA046 / SLOA102** — amplifier layout app-notes:
  https://www.ti.com/lit/an/sloa046/sloa046.pdf
- TI Precision Hub "**The basics: how to layout a PCB for an op amp**":
  https://e2e.ti.com/blogs_/archives/b/precisionhub/posts/the-basics-how-to-layout-a-pcb-for-an-op-amp
  *Takeaway:* decoupling < 0.1 in from the supply pins; **feedback resistor right at the
  inverting input** (minimal loop area / parasitic C); short input traces; ground plane under;
  keep input and output apart for channel isolation.

## RTC (RV-8803-C7)
- **RV-8803-C7 Application Manual** (rev 1.6) — §"Dimensions and Solder Pad Layout" +
  "Recommended Thermal Relief" (≈ p65–66):
  https://www.microcrystal.com/fileadmin/Media/Products/RTC/App.Manual/RV-8803-C7_App-Manual.pdf
  (EM Microelectronic mirror: https://www.emmicroelectronic.com/sites/default/files/products/datasheets/rv-8803-c7-mn01.pdf)
- Ready footprint (verify against ours): SnapEDA RV-8803-C7-TA-QC.
  *Takeaway:* confirms the SON-8 pad land pattern we built + thermal relief.

## USB (USB-C receptacle + USBLC6-2SC6 ESD)
- USB-C routing guides: PCBWay https://www.pcbway.com/blog/PCB_Design_Layout/USB_Type_C_PCB_Design_Guidelines_Layout_and_Routing_Best_Practices_55bc0c39.html ·
  Cadence https://resources.pcb.cadence.com/blog/2024-impedance-matching-for-usb-interfaces-in-pcbs ·
  Altium (2-layer USB) https://resources.altium.com/p/routing-requirements-usb-20-2-layer-pcb
  *Takeaway:* **D± = 90 Ω ±10 % differential pair**, skew < 15 ps (~100 mil), ground on both
  sides (gap ≤ 3× trace width). Place the **USBLC6-2 within ~100–200 mil of the connector,
  before any other component, no vias** between connector and diode (vias slow the clamp).

## Class-D speaker amp (PAM8302A, DNP)
- TI Class-D layout app-note (applies to filterless BTL output): **SLAA896 / SLAA902**
  https://www.ti.com/lit/an/slaa896/slaa896.pdf
  *Takeaway:* keep the BTL output traces short and symmetric; add the EMI ferrite/cap near the
  output if cabling is long; the switching output is noisy — keep it away from the analog input.

## Relay (TQ2SA), ULN2003, LM393
- Straightforward digital/contact layout: relay contact traces wide enough for the switched
  current; ULN2003 flyback (COM to the coil supply) close; LM393 standard comparator layout.
  Panasonic TQ-SMD datasheet (in `datasheets/`) has the recommended mounting pad.

---

### The three to prioritize (the hard parts)
1. **TPS65131EVM-839 / SLVUAW7** — copy the switcher layout almost verbatim.
2. **TPS7A30-49EVM-567** — it's literally the ±15 V dual-LDO layout we need.
3. **RP2350 Minimal-KiCAD.zip** — fork the MCU/USB/crystal/QSPI layout.

Hand these (plus `LAYOUT.md`) to whoever routes the board; they cover every stage where
layout — not schematic — determines whether it works.

## Saved locally (in `hardware/datasheets/`, git-ignored)
The three priority references are downloaded for offline use:
- **`TPS65131EVM_SLVUAW7.pdf`** — switcher EVM, includes the full **4-layer PCB plots**
  (Top / Inner 1 / Inner 2 / Bottom) + BOM (which matches our TPS65131RGER / 4.7 µH / MBRM120).
  *Copy this layout for the switcher block.*
- **`TPS7A30-49EVM_SLVU405.pdf`** — the ±15 V dual-LDO EVM (schematic + layout + thermal + BOM).
- **`RP2350-Minimal-KiCAD.zip`** — official RP2350 reference. **Nested:** unzip it, then extract
  `RP-006440-...RP2350A Minimal Board Kicad archive.zip` (RP2350A / QFN-60 = our part) for the
  schematic + PCB + footprints; the `RP-006442` archive is the QFN-80 (RP2350B) variant.