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  • Updated:
  • Author:
    Takeshi Takatsudo

Quick Reference

A cheat sheet of frequently referenced information during design.

🎯 Basic Specifications (At a Glance)​

ItemSpecification
InputUSB-C PD 15V 3A (Max 45W)
+12V Output1.5A (max)
-12V Output1.0A (max)
+5V Output1.5A (max)
Ripple<1mVp-p
Efficiency75-80%
ProtectionPTC auto-recovery + Fuse backup

⚑ Power Flow (Architecture)​

USB-C        DC-DC        LDO          Output
15V    β†’   +13.5V   β†’   +12V    β†’   +12V/1.5A
  ↓
  β”œβ”€β”€  β†’   +7.5V    β†’   +5V     β†’   +5V/1.5A
  ↓
  └──  β†’   -13.5V   β†’   -12V    β†’   -12V/1.0A
      (Inverting Buck-Boost)

πŸ”§ Main ICs and Their Roles​

ICPart NumberJLCPCB P/NStockRoleQty
USB-PDSTUSB4500C2678061-PD Negotiation (15V)1
DC-DCLM2596S-ADJC34742312,075Buck Converter (U2, U3)2
DC-DCLM2596S-ADJC34742312,075Inverting Buck-Boost (U4)1
+12V LDOL7812CD2T-TRC13456158,795+13.5V β†’ +12V1
+5V LDOL7805ABD2T-TRC86206272,379+7.5V β†’ +5V1
-12V LDOCJ7912C941733,386-13.5V β†’ -12V1

πŸ“ DC-DC Feedback Resistor Values (Voltage Setting)​

Output VoltageR_upperR_lowerActual Output
+13.5V10kΞ©1kΞ©13.53V
+7.5V5.1kΞ©1kΞ©7.50V
-13.5V10kΞ©1kΞ©-13.53V

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

πŸ›‘οΈ Protection Circuit Ratings​

Voltage LinePTC RatingJLCPCB P/NFuse RatingJLCPCB P/NTVS ModelOperation
+12V1.1AC8831482AC5183824SMAJ15AOverload→PTC / Short circuit→Fuse
+5V1.1AC701191.5AC95352SD05Same as above
-12V1.1AC8831481.5AC95352SMAJ15ASame as above

Protection Operation Sequence​

  1. Normal: LED on βœ…
  2. Overload (110-180% of rating): PTC trip β†’ LED off β†’ Recovery after 30 seconds πŸ”„
  3. Short circuit (200%+ of rating): Fuse blown β†’ Repair required ❌

πŸ”Œ Connectors and Packages​

ComponentPackageNotes
STUSB4500QFN-24USB-PD IC
USB-CUSB-TYPE-C-0096-pin (Power only)
LM2596STO-263-5Surface mount, large thermal pad
L7812/L7805TO-220/TO-263-2Heatsink compatible
CJ7912TO-252-2LSurface mount
InductorSMD 13.8x12.8mm100Β΅H 4.5A
Electrolytic Capφ6.3mm / φ10mmSelect by diameter

πŸ’° Cost Breakdown (Per Board)​

StageContentCost
Stage 1USB-PD Power Section$0.43
Stage 2DC-DC Converters$2.09
Stage 3Linear Regulators$0.37
Stage 4Protection Circuits$1.79
TotalComponent Cost$4.68

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

πŸ“Š Component Stock Status (JLCPCB)​

Component CategoryMinimum StockAvailability
Basic Parts Resistors/Capacitors1,000,000+βœ… Very Stable
STUSB4500 (USB-PD)-βœ… Stable
LM2596S (DC-DC)12,075βœ… Stable
L7812/L7805/CJ7912 (LDO)3,386~272,379βœ… Very Stable
Inductor (100Β΅H)2,763βœ… Stable
SS34 (Diode)1,859,655βœ… Very Stable

πŸ”¬ Detailed Performance Specifications​

Ripple Noise Target​

StageExpected RippleCountermeasure
DC-DC Output~50mVp-p470Β΅F electrolytic capacitor
LDO Output<1mVp-pLDO + 470Β΅FΓ—2

Efficiency Calculation​

StageEfficiencyLoss Example
LM2596S85-90%15V→13.5V: 1.5V × 1A = 1.5W
LM7812~90%13.5V→12V: 1.5V × 1A = 1.5W
LM7805~67%7.5V→5V: 2.5V × 0.5A = 1.25W
LM7912~89%-13.5V→-12V: 1.5V × 0.8A = 1.2W
Overall75-80%Max loss ~10W

🌑️ Thermal Design Estimation​

Maximum Heat-Generating Components​

ICMax LossPackageThermal ResistanceTemperature Rise
LM2596S (Γ—3)1.5WTO-263~10℃/W+15℃
LM78051.25WTO-220~5℃/W+6℃
LM78121.5WTO-220~5℃/W+7.5℃
LM79121.2WTO-220~5℃/W+6℃

β€» Approx. 40-50℃ max at 25℃ ambient (within acceptable range)

πŸ› οΈ 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
  1. Make high-current paths thick and short
  • USB input: Minimum 1mm width
  • +12V/-12V: Minimum 0.8mm width
  • +5V: Minimum 0.5mm width
  1. Thermal via placement
  • LM2596S (TO-263): 4-6 vias under pad
  • LM78xx/79xx: As needed
  1. Capacitor placement
  • Input capacitors: Close to IC
  • Output capacitors: Near load terminals
  • Electrolytic capacitors: Mind polarity
LayerPurposeNotes
L1 (Top)Signal + ComponentsSMD component side
L2 (GND)GND PlaneSolid GND
L3 (Power)Power Plane+15V/+12V/+5V/-12V
L4 (Bottom)SignalRouting auxiliary

πŸ“ Open Items Checklist​

  • PTC1: 1.1A 16V (1812) - C883148 (BSMD1812-110-16V) βœ…
  • PTC2: 0.75A 16V (1206) - C883128 (BSMD1206-075-16V) βœ…
  • PTC3: 0.9A 16V (1812) - C883148 (BSMD1812-110-16V) β€»Using 1.1A βœ…
  • F1: 2A 250V SMD fuse - C5183824 (6125FA2A) βœ…
  • Stock optimization - All components changed to high-stock parts βœ…
    • 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

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

πŸ”— Reference Links​

JLCPCB​

Datasheets​

  • STUSB4500: STMicroelectronics (15V support confirmed)
  • LM2596S: Texas Instruments
  • L7812/L7805: STMicroelectronics
  • CJ7912: CJ (Changjiang Micro-Electronics)

KiCad​

πŸ’‘ 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 β†’ -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) β†’ User can resolve
  • Fuse: Final defense during short circuit β†’ Fire prevention
  • Cost increase: Only $0.50/board β†’ Worth it for safety βœ…

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 β†’ 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 βœ… Done!
  2. Add to /notes/parts.md βœ… Done!
  3. Install KiCad (15 minutes) ← This is next!
  4. Start schematic entry (1-2 hours)

β†’ All components confirmed! Let's start "PCB Design Preparation"! πŸš€

Last updated on by Takeshi Takatsudo