Quick Reference

A cheat sheet of frequently referenced information during design.

🎯 Basic Specifications (At a Glance)

⚡ Power Flow (Architecture)

USB-C        DC-DC        LDO          Output
15V    →   +13.5V   →   +12V    →   +12V/1.5A
  ↓
  ├──  →   +7.5V    →   +5V     →   +5V/1.5A
  ↓
  └──  →   -13.5V   →   -12V    →   -12V/1.0A
      (Inverting Buck-Boost)

🔧 Main ICs and Their Roles

📐 DC-DC Feedback Resistor Values (Voltage Setting)

Formula: Vout = 1.23V × (1 + R_upper/R_lower)

🛡️ Protection Circuit Ratings

Protection Operation Sequence

1. Normal: LED on ✅

2. Overload (110-180% of rating): PTC trip → LED off → Recovery after 30 seconds 🔄

3. Short circuit (200%+ of rating): Fuse blown → Repair required ❌

🔌 Connectors and Packages

💰 Cost Breakdown (Per Board)

※ PCB manufacturing and assembly costs are separate (approx. $15-20/board for 10-piece order)

📊 Component Stock Status (JLCPCB)

🔬 Detailed Performance Specifications

Ripple Noise Target

Efficiency Calculation

🌡️ Thermal Design Estimation

Maximum Heat-Generating Components

※ Approx. 40-50℃ max at 25℃ ambient (within acceptable range)

🛠️ PCB Design Guidelines

Layout Principles

1. Separate high-noise and low-noise sections

• DC-DC section: Left side of board

• LDO section: Right side of board

• Consider GND plane separation

2. Make high-current paths thick and short

• USB input: Minimum 1mm width

• +12V/-12V: Minimum 0.8mm width

• +5V: Minimum 0.5mm width

3. Thermal via placement

• LM2596S (TO-263): 4-6 vias under pad

• LM78xx/79xx: As needed

4. Capacitor placement

• Input capacitors: Close to IC

• Output capacitors: Near load terminals

• Electrolytic capacitors: Mind polarity

Recommended Layer Stack (4-layer board)

📝 Open Items Checklist

• [x] PTC1: 1.1A 16V (1812) - C883148 (BSMD1812-110-16V) ✅

• [x] PTC2: 0.75A 16V (1206) - C883128 (BSMD1206-075-16V) ✅

• [x] PTC3: 0.9A 16V (1812) - C883148 (BSMD1812-110-16V) ※Using 1.1A ✅

• [x] F1: 2A 250V SMD fuse - C5183824 (6125FA2A) ✅

• [x] Stock optimization - All components changed to high-stock parts ✅

  • USB-PD: STUSB4500

  • LDO: L7812/L7805/CJ7912 (3K~272K stock)

• [ ] PCB design (KiCad) - Not started

• [ ] Prototype order - Not implemented

• [ ] Performance testing (ripple/efficiency/thermal) - Not implemented

🎉 All JLCPCB part numbers confirmed and optimized for high-stock parts! PCB design is next.

🔗 Reference Links

JLCPCB

• Parts Library: https://jlcpcb.com/parts

• SMT Assembly: https://jlcpcb.com/smt-assembly

• Design Rules: https://jlcpcb.com/capabilities/pcb-capabilities

Datasheets

• STUSB4500: STMicroelectronics (15V support confirmed)

• LM2596S: Texas Instruments

• L7812/L7805: STMicroelectronics

• CJ7912: CJ (Changjiang Micro-Electronics)

KiCad

• Official website: https://www.kicad.org/

• JLCPCB library: GitHub search "JLCPCB KiCad library"

💡 Frequently Asked Questions (FAQ)

Q: Why use 2-stage DC-DC and LDO?

A: To balance efficiency and noise

• DC-DC only: Good efficiency (85%+) but high ripple (50mVp-p)

• LDO only: Low noise (<1mVp-p) but poor efficiency (50-60%), high heat

• 2-stage: DC-DC for efficiency + LDO for noise reduction = 75%+ efficiency with <1mVp-p ripple ✨

Q: Why use LM2596S inverting buck-boost for -12V?

A: Simpler and more reliable than flyback converters

• Current requirement: -12V rail needs 800mA (charge pumps like ICL7660 only provide ~100mA)

• Inverting buck-boost advantages:

  • Uses same LM2596S IC as other DC-DC stages (reduces BOM complexity)

  • Single-stage conversion (+15V → -13.5V directly)

  • No flyback transformer required (simpler, lower cost)

  • No FB pin voltage violations (previous LM2586 flyback design had this issue)

• Trade-off: Slightly lower efficiency (~75%) vs regular buck, but much simpler than flyback

• Alternative charge pumps insufficient: ICL7660 (100mA), TPS63700 (360mA but 5.5V max input) ❌

Q: Is 2-stage protection with PTC and fuse really necessary?

A: Very important for beginner users

• PTC: Auto-recovery during overload (too many modules) → User can resolve

• Fuse: Final defense during short circuit → Fire prevention

• Cost increase: Only $0.50/board → Worth it for safety ✅

Q: Is 4-layer board necessary? Can't use 2-layer?

A: 2-layer is possible, but 4-layer recommended

• 2-layer: Difficult noise control, complex routing

• 4-layer: GND/Power planes reduce noise, easier routing

• Cost difference: About $5-10/board → Worth it for performance

• Recommendation: 4-layer for prototype, consider 2-layer for production

Q: What should I start with right now?

A: Start from "Step 3" in Project Status and Plan!

1. Search JLCPCB Parts Library for PTC ✅ Done!

2. Add to /notes/parts.md ✅ Done!

3. Install KiCad (15 minutes) ← This is next!

4. Start schematic entry (1-2 hours)

→ All components confirmed! Let's start "PCB Design Preparation"! 🚀