Project Status and Plan

Current progress and future plans for the USB-PD powered modular synthesizer power supply.

🎯 Project Goal

Low-noise power module supplying ±12V/+5V for modular synths from USB-C PD 15V input

Protection circuit safe for modular synth beginners
All parts available from JLCPCB (stable supply, low cost)
Low-noise design with <1mVp-p ripple
Easy to use with USB-C PD

✅ Completed Items

1. Circuit Design (100% Complete)

4-Stage Architecture Completed:

USB-C 15V ──┬─→ +13.5V (DC-DC) ──→ +12V (LDO) ──→ +12V OUT
            │
            ├─→ +7.5V  (DC-DC) ──→ +5V  (LDO) ──→ +5V OUT
            │
            └─→ -15V (Inverter) ──→ -13.5V (DC-DC) ──→ -12V (LDO) ──→ -12V OUT

✅ Stage 1: USB-PD Power Supply (STUSB4500)
✅ Stage 2: DC-DC Converters (LM2596S × 3 + LM2586 inverted SEPIC)
✅ Stage 3: Linear Regulators (LM7812/7805/7912)
✅ Stage 4: Protection Circuit (PTC + Fuse + TVS)

2. Parts Selection (100% Complete) ✅

All parts confirmed: All JLCPCB part numbers finalized

✅ USB-PD Controller: STUSB4500 (C2678061)
✅ DC-DC Converter: LM2596S-ADJ × 3 (C347423)
✅ Voltage Inverter: LM2586SX-ADJ/NOPB (C181324)
✅ Linear Regulators: L7812CV-DG (C2914) / L7805ABD2T-TR (C86206) / CJ7912 (C94173)
✅ Inductors: 100µH 4.5A × 3 (C19268674)
✅ TVS Diodes: SMAJ15A, SD05
✅ Fuses: 1.5A × 2 confirmed (C95352)

~~Unconfirmed Parts~~ → All Confirmed! ✅

✅ PTC Resettable Fuses × 3
- PTC1: 1.1A 16V (1812) → C883148 (BSMD1812-110-16V)
- PTC2: 0.75A 16V (1206) → C883128 (BSMD1206-075-16V)
- PTC3: 0.9A→1.1A 16V (1812) → C883148 (BSMD1812-110-16V) ※
✅ 2A SMD Fuse × 1 (for +12V) → C5183824 (6125FA2A)

※ 0.9A part out of stock. 1.1A provides sufficient protection margin for -12V actual load of 800mA.

3. Documentation (100% Complete)

✅ Complete circuit diagrams (all 10 stages)
✅ Detailed BOM (with JLCPCB part numbers)
✅ Design philosophy and architecture explanation
✅ Protection circuit operation description

4. Cost Estimation (100% Complete) ✅

Total with all parts confirmed: $4.68/board

Stage 1: $0.43
Stage 2: $2.09
Stage 3: $0.37
Stage 4: $1.79 (all parts confirmed price)

5. PCB Layout (100% Complete) ✅

PCBA v2 layout complete — DRC clean as of 2026-04-18:

✅ R14 (470 Ω) placed — VBUS_IN to VBUS_VS_DISCH (pin 18) pull
✅ R15, R16 (4.7 k each) placed — I2C pull-ups for NVM programming interface
✅ J2 placed — pogo pad footprint for NVM programming access
✅ All components placed and routed
✅ DRC run with zero errors

🔄 Current Status

Where Are We Now?

PCBA v2 schematic and PCB layout complete — ready to generate manufacturing files

Full project history to date:

✅ Circuit design complete — Working 4-stage design finalized
✅ All parts selection complete — JLCPCB part numbers confirmed (100%), optimized to high-stock parts
✅ BOM fully confirmed — Cost $4.68/board
✅ PCBA v1 ordered and tested — Board powered up but STUSB4500 failed to negotiate USB-PD. Root cause: pin 18 (VBUS_VS_DISCH) unconnected, pin 22 (VSYS) shorted to VREG_2V7. Bodge wires applied for analysis.
✅ v2 schematic fixes applied — R14 (470 Ω, pin 18 pull), R15/R16 (4.7 k I2C pull-ups), J2 (pogo pads for NVM programming) added. VSYS connected to GND.
✅ v2 PCB layout complete — All components placed and routed. DRC clean as of 2026-04-18.
⏳ Generate manufacturing files — Gerbers, drill files, BOM, CPL for JLCPCB reorder ← This is next!

Hardware Acquired

NUCLEO-F072RB (STM32 Nucleo board, used as USB-to-I2C bridge for STUSB4500 NVM programming) — purchased
4P 2.54 mm pogo pin clip (AliExpress item 1005006108783889, mates with J2 pogo pads) — purchased
PCBA v1 physical boards — N boards retained for bodge reference (see JLCPCB order history)

What's Next?

Generate Gerber/BOM/CPL files and place PCBA v2 order at JLCPCB

✅ Schematic and PCB layout done — v2 schematic and layout with all debug fixes applied
✅ DRC clean — Zero errors as of 2026-04-18
📐 Next Action: Generate manufacturing files from KiCad

Gerber files → Drill files → BOM (JLCPCB format) → CPL (component placement)

📋 Next Steps (Priority Order)

🔴 Priority: High - Do Immediately

Step 1: Search for Unconfirmed Parts ✅ Complete!

Parts found:

✅ PTC Resettable Fuses × 3 types

PTC1 (1.1A 16V, 1812): C883148 - Stock: 11,029
PTC2 (0.75A 16V, 1206): C883128 - Stock: 51,532
PTC3 (1.1A 16V, 1812): C883148 - Stock: 11,029 ※0.9A part unavailable

✅ SMD Fuse (2A 250V)

C5183824 (6125FA2A, 2410 package) - Stock: 744

Step 2: Finalize BOM ✅ Complete!

✅ Part numbers added to /notes/parts.md
✅ Final cost calculated: $4.68/board
✅ Reflected in /doc/do../components/bom.md
✅ All parts optimized to high-stock items (CH224D, L7812/7805, CJ7912)

🟡 Priority: Medium - Do Next

Step 3: Prepare PCB Design ✅ Complete!

KiCad Project Setup:

Create new KiCad project
Enter circuit in schematic editor
Add JLCPCB footprint library
Assign footprints to all parts

Required footprints:

/footprints/CH224D.png - Already available
/footprints/USB-TYPE-C-009.png - Already available
Other standard footprints use KiCad standard library

Step 4: PCB Board Design ✅ Complete!

Layout Policy:

4-Layer Board Structure:

Layer 1 (Top): Signals + component placement
Layer 2 (Inner): GND plane
Layer 3 (Inner): Power plane (+15V, +12V, etc.)
Layer 4 (Bottom): Signals

Power Layout:

Place USB-PD → DC-DC → LDO in sequence
Physically separate high-noise (DC-DC) and low-noise (LDO) sections
Make high-current paths thick and short

Thermal Design:

LM2596S (TO-263) → Place thermal vias
LM78xx/79xx (TO-220) → Reserve heatsink area
Consider electrolytic capacitor heat dissipation

JLCPCB Design Rules:

Minimum trace width: 6mil (0.15mm)
Minimum clearance: 6mil
Via diameter: 0.3mm (hole 0.2mm)

🟢 Priority: Low - Pre-Prototype Preparation

Step 5: Generate Manufacturing Files (Time: 1 hour) ← We are here!

Generate Gerber files
Generate Drill files
Generate BOM file (JLCPCB format)
Generate CPL file (component placement data)

Step 6: Get JLCPCB Quote (Time: 30 minutes)

Quote contents:

PCB manufacturing: 5 or 10 boards
SMT assembly: both sides or single side
Parts procurement cost
Shipping

Estimated Cost (for 10 boards):

PCB manufacturing: ~$30
SMT assembly: ~$50-100
Parts cost: ~$50 (for 10 boards)
Shipping: ~$20
**Total: $150-200** (10 boards =$ 15-20 per board)

Step 7: Order Prototype (Time: 15 min order + 2 weeks manufacturing)

Recommended Initial Order:

Quantity: 5-10 boards
SMT assembly: All parts installed
Delivery: DHL (2-3 weeks)

PCBA v1 Failure Findings

The first prototype PCBA (v1) failed during testing. The STUSB4500 USB-PD controller did not negotiate power delivery. Root cause analysis identified three issues:

Issues Found

Pin 18 (VBUS_VS_DISCH) not connected - This pin was left as NC but must be connected to VBUS_IN via a 470ohm series resistor for VBUS voltage sensing. This is a critical connection required by the datasheet.
Pin 22 (VSYS) floating - After cutting the incorrect VSYS-to-VREG_2V7 trace, VSYS was left floating. The datasheet requires VSYS to be connected to GND when not used.
Pin 22 (VSYS) shorted to Pin 23 (VREG_2V7) - A routing error in the original schematic connected these pins together, overloading the internal 2.7V regulator. Fixed by cutting the trace on the PCBA.

Required Schematic Fixes Before Next Order

[x] Add R14 (470ohm) from VBUS_IN to VBUS_VS_DISCH (pin 18)
[x] Connect VSYS (pin 22) to GND
[x] Verify VREG_2V7 (pin 23) decoupling is correct (C30 only)
[x] Run DRC and review all STUSB4500 connections

See the full PCBA v1 Debug Report for detailed analysis, bodge wire instructions, and reference design comparison.

🤔 Design Concerns and Considerations

Issues Resolved in Current Design

✅ Noise countermeasure: DC-DC + LDO 2-stage design expected to achieve <1mVp-p
✅ Beginner-friendly: PTC auto-reset for automatic recovery from overload
✅ Cost: Parts cost under $5 using many Basic Parts
✅ Procurement stability: All parts have abundant JLCPCB stock

Items Not Yet Verified (Confirm with Prototype)

⚠️ Thermal design: Is LM2596S heat dissipation sufficient?

Maximum loss: Each 1.5V × 1A = 1.5W
TO-263 package should handle it but needs actual measurement

⚠️ Ripple noise: Can actual measurement achieve <1mVp-p?

Design should be fine but needs measurement

⚠️ Efficiency: Can actual measurement achieve 75-80%?

LM2596S: 85-90%
LDO loss: 10-15%
Calculated overall efficiency: 75-80%

⚠️ EMI/EMC: DC-DC switching noise impact?

Countermeasures with input/output filters but needs measurement

📝 Design Philosophy Review

Why This Design?

DC-DC + LDO 2-Stage Method

Reason: Balance efficiency and noise
DC-DC only: Efficient but high ripple
LDO only: Low noise but poor efficiency (high heat)
2-stage: Best of both worlds ✨

USB-C PD 15V Input

Reason: Can use generic chargers
No AC adapter needed → Easy to carry
Any PD-compatible charger works
15V voltage optimal for generating ±12V

All Parts from JLCPCB

Reason: Stable supply, low cost, automated assembly
Many Basic Parts → No extra fees
Abundant stock → Long-term procurement possible
SMT automated assembly → No hand soldering needed

PTC Auto-Reset Protection

Reason: Safe for beginners
Module overload → Notice when LED goes out
Auto-reset after 30 seconds → No repair needed
Fuse for short circuits → Safety ensured

🎯 Project Goals

Final Goal

"Manufacture beginner-friendly modular synth power supply with JLCPCB for under $20/board"

Achievement Criteria

[ ] Ripple noise <1mVp-p (measured)
[ ] Efficiency 75-80% (measured)
[ ] Output voltage accuracy ±1% (measured)
[ ] Overload protection operation confirmed (LED off → auto-reset)
[ ] Short circuit protection operation confirmed (fuse blown)
[ ] Manufacturing cost under $20/board (when ordering 10 boards)

Secondary Goals

📖 Comprehensive English documentation → Contribute to international Maker community
🔧 Open-source KiCad project
📝 Write build article (Blog/Medium)
🎓 Share JLCPCB SMT utilization know-how

💡 What You Can Do Now

After reading this document, you can immediately start:

5 minutes: Search for parts ✅ Complete!
30 minutes: Update BOM ✅ Complete!
1 hour: Install KiCad and create new project ✅ Complete!
1 day: Enter complete circuit in schematic editor ✅ Complete!
1 hour: Generate Gerber/BOM/CPL files in KiCad and place PCBA v2 order ← Start here!

PCBA v2 schematic + layout done and DRC clean! Generate manufacturing files and reorder! 🚀