USBLC6-2SC6 - USB ESD Protection (D4)
TVS diode array for USB Type-C CC line and VBUS ESD protection. Replaces ESDA25L in v1.1 design.
Overview
The USBLC6-2SC6 is a 6-pin TVS diode array specifically designed for USB ESD protection. It provides 2 bidirectional channels for data/CC lines plus a VBUS protection channel, making it ideal for USB Type-C applications.
Why USBLC6-2SC6 over ESDA25L?
| Feature | ESDA25L | USBLC6-2SC6 |
|---|---|---|
| Clamping Voltage | ~44V | ~17V |
| VBUS Protection | No | Yes (Pin 5) |
| Working Voltage | 25V | 5.25V |
| Capacitance | 3pF | 3.5pF |
| Design Target | High-voltage lines | USB signals |
USBLC6-2SC6 has lower clamping voltage, providing better protection for CC lines operating at ~1.7V.
Key Specifications
| Parameter | Value | Notes |
|---|---|---|
| JLCPCB Part Number | C7519 | |
| Manufacturer Part Number | USBLC6-2SC6 | STMicroelectronics |
| Package | SOT-23-6 | 6-pin surface-mount |
| Stock Availability | 354,000+ | Excellent availability |
| Unit Price | ~$0.13 | |
| Working Voltage | 5.25V | USB specification |
| Breakdown Voltage | 6V min | |
| Clamping Voltage | 17V @ 1A | Much lower than ESDA25L |
| Peak Pulse Current | 3A | 8/20µs waveform |
| Capacitance | 3.5pF typ | Low impact on CC signals |
| ESD Rating | 15kV (HBM) | IEC 61000-4-2 |
| Channels | 2 + VBUS | I/O1, I/O2, and VBUS |
Pin Configuration
USBLC6-2SC6 (SOT-23-6)
Top View
┌─────────────────────┐
│ │
I/O1 ──┤ 1 6 ├── I/O1
│ ┌───────┐ │
GND ──┤ 2 │ │ 5 ├── VBUS
│ └───────┘ │
I/O2 ──┤ 3 4 ├── I/O2
│ │
└─────────────────────┘
Pin 1: I/O1 input (CC1 from USB-C)
Pin 2: GND
Pin 3: I/O2 input (CC2 from USB-C)
Pin 4: I/O2 output (CC2 to STUSB4500)
Pin 5: VBUS (optional VBUS protection)
Pin 6: I/O1 output (CC1 to STUSB4500)
Internal Structure
VBUS (Pin 5)
│
┌───────────┼───────────┐
│ │ │
I/O1 ───────┼─────┬─────┼─────┬─────┼─────── I/O1
(Pin 1) │ ─┴─ │ ─┴─ │ (Pin 6)
│ ╲│╱ │ ╲│╱ │
│ │ │ │ │
├────┴──────┼────┴──────┤
│ │ │
I/O2 ───────┼─────┬─────┼─────┬─────┼─────── I/O2
(Pin 3) │ ─┴─ │ ─┴─ │ (Pin 4)
│ ╲│╱ │ ╲│╱ │
│ │ │ │ │
└────┴──────┴────┴──────┘
│
GND (Pin 2)
Application in STUSB4500 Circuit (D4)
USB-C (J1) D4 (USBLC6-2SC6) STUSB4500 (U1)
┌─────────┐ ┌─────────────────┐ ┌─────────────┐
│ │ │ │ │ │
│ CC1 (A5)├────────→│ 1 (I/O1) (I/O1) 6 │───→│ CC1 (pin 2) │
│ │ │ │ └─→│ CC1DB (pin 1)│
│ │ │ 2 (GND) │ │ │
│ CC2 (B5)├────────→│ 3 (I/O2) (I/O2) 4 │───→│ CC2 (pin 4) │
│ │ │ │ └─→│ CC2DB (pin 5)│
│ │ │ 5 (VBUS) │ │ │
│ VBUS ├─────────┼─────────────────┤ │ │
│ │ │ │ │ │
└─────────┘ └─────────────────┘ └─────────────┘
│
GND
Connection List
| D4 Pin | Name | Connected To |
|---|---|---|
| 1 | I/O1 (input) | USB-C CC1 (J1 pin A5) |
| 2 | GND | System ground |
| 3 | I/O2 (input) | USB-C CC2 (J1 pin B5) |
| 4 | I/O2 (output) | STUSB4500 CC2 (pin 4) + CC2DB (pin 5) |
| 5 | VBUS | VBUS_IN |
| 6 | I/O1 (output) | STUSB4500 CC1 (pin 2) + CC1DB (pin 1) |
Why This Protection Matters
| Threat | Source | USBLC6-2SC6 Protection |
|---|---|---|
| ESD | Cable insertion/removal | 15kV HBM rating |
| CC overvoltage | Faulty cable/adapter | 17V clamping (vs 44V for ESDA25L) |
| VBUS transients | Hot-plug events | Pin 5 VBUS clamping |
| Transients | Hot-plug events | Fast response time |
The lower clamping voltage (17V vs 44V) provides much better protection for the STUSB4500's CC pins.
How It Works: Avalanche Breakdown
TVS diodes protect circuits using a phenomenon called avalanche breakdown. Here's how it works:
Normal Operation (V < 5.25V)
I/O1 (CC1) ────●──────────────────●──── I/O1
│ │
▼ (reverse bias) ▼ (reverse bias)
│ │
GND ───────────┴────────●─────────┴──── VBUS
│
(all diodes OFF - high impedance)
- All TVS diodes are reverse biased → no current flows
- Signal passes through unaffected
- Only ~3.5pF capacitance added to the line
ESD Event (High Voltage Spike)
When an ESD spike arrives (e.g., +8kV from human touch):
⚡ Incoming electron (accelerated by ESD voltage)
↓
●→→→→ accelerates in electric field
↓
💥 COLLISION with silicon atom!
╱ ╲
● ● ← 2 electrons now!
↓ ↓
💥 💥
╱ ╲ ╱ ╲
● ● ● ● ← 4 electrons!
↓ ↓ ↓ ↓
... CHAIN REACTION ...
This is impact ionization:
- High voltage creates strong electric field in the diode's depletion region
- Stray electrons accelerate to high kinetic energy
- Electrons collide with silicon crystal lattice
- Each collision knocks loose more electrons (creates electron-hole pairs)
- Exponential multiplication = massive current flows through the TVS
Why Voltage Stays Constant (Clamping)
Current
↑
│ ╱
│ ╱
│ ╱ ← Massive current increase
│ ╱ with tiny voltage change
│ │
│ │ Breakdown!
│ │ (V_BR ≈ 6V)
│________________│___________→ Voltage
0 6V
The avalanche multiplication is extremely sensitive to electric field:
- Small voltage increase → field increases slightly → exponentially more ionization
- Current increases 1000x while voltage increases only ~1V
- Result: Voltage is effectively "clamped" at breakdown voltage
Analogy: Like a pressure relief valve - below threshold the valve is closed, at threshold it opens and pressure stays constant no matter how much more you push.
Energy Absorption
Where does the ESD energy go?
ESD spike energy → Electron kinetic energy → Lattice vibrations → HEAT
(8kV) (accelerated) (phonons) (absorbed)
The TVS diode:
- Clamps voltage at ~17V (limits how high it can go)
- Conducts the current through itself → to GND
- Converts energy to heat in its own silicon body (P = V × I = 17V × 3A = 51W briefly)
The protected IC (STUSB4500) never sees the dangerous voltage - the TVS diode "takes the hit" and dissipates it as heat instead.
Bidirectional Protection
The USBLC6-2SC6 has back-to-back diodes for each channel:
| ESD Polarity | Diode Behavior | Clamping Voltage |
|---|---|---|
| Positive spike (+8kV) | Avalanche breakdown | ~17V |
| Negative spike (-8kV) | Forward conduction | ~-0.7V |
This provides full protection regardless of ESD polarity.
Design Considerations
Placement
- Place D4 between USB-C connector and STUSB4500
- Short traces from pin 2 to GND for effective clamping
- Signal flow: USB-C → D4 → STUSB4500
VBUS Connection (Pin 5)
Pin 5 can be:
- Connected to VBUS_IN (recommended) - provides additional transient protection on the power rail
- Left unconnected - CC protection still works, but no VBUS clamping
Capacitance Impact
- 3.5pF typical capacitance is acceptable for CC communication
- USB PD CC signaling tolerates up to 200pF
- Slightly higher than ESDA25L (3pF) but negligible difference
Alternative Parts
| Part | LCSC | Package | Clamping V | Capacitance | Notes |
|---|---|---|---|---|---|
| USBLC6-2SC6 | C7519 | SOT-23-6 | 17V | 3.5pF | Recommended (ST original) |
| USBLC6-2SC6 (UMW) | C2687116 | SOT-23-6 | 15V | 0.8pF | Budget alternative |
| PRTR5V0U2X | C12333 | SOT-143 | 17V | 1pF | Nexperia, different footprint |