ESD Protection: How TVS Diodes Save Your Circuit

Understanding how TVS diodes protect sensitive IC pins from static electricity and voltage spikes.

The Question

When looking at the USB-PD input circuit, there's a small component (D4) connected to the CC lines:

"What is D4 for? Why do the CC lines need special protection?"

Short answer: D4 is an ESD protection device that acts like a pressure relief valve - it dumps dangerous voltage spikes to ground before they can destroy the STUSB4500.

What is ESD?

ESD = ElectroStatic Discharge

It's the sudden flow of electricity when two objects with different charges come into contact.

Real-world examples

You can't feel ESD below ~3,000V, but even 100V can damage sensitive electronics!

The danger to electronics

Human body static: 10,000V typical

IC maximum voltage rating: 5-25V typical

Ratio: 400x more than the IC can handle! 💀

Why CC Lines Are Vulnerable

The USB-C connector has several types of pins:

USB-C Connector (simplified)
┌─────────────────────────────────────┐
│                                     │
│  VBUS ─── Power (has big capacitors to absorb energy)
│                                     │
│  CC1  ─── Signal (thin trace, directly to IC) ⚠️ VULNERABLE
│  CC2  ─── Signal (thin trace, directly to IC) ⚠️ VULNERABLE
│                                     │
│  GND  ─── Ground (already at 0V, safe)
│                                     │
│  D+/D- ── Data (usually have protection)
│                                     │
└─────────────────────────────────────┘

CC lines are the first point of contact when you plug in a cable, and they connect directly to sensitive IC pins with no bulk capacitors to absorb energy.

What is a TVS Diode?

TVS = Transient Voltage Suppressor

It's a special diode designed to clamp voltage spikes by conducting large currents to ground.

Normal diode vs TVS diode

How TVS works

Voltage
   ▲
   │
40V│─ ─ ─ ─ ─ ─ ─ ─ ─┬─────────── Clamping voltage (TVS conducts heavily)
   │                 /
   │                /
27V│─ ─ ─ ─ ─ ─ ─ /─ ─ ─ ─ ─ ─ ─ Breakdown voltage (TVS starts conducting)
   │             /│
   │            / │
   │           /  │
25V│─ ─ ─ ─ ─/─ ─│─ ─ ─ ─ ─ ─ ─ Working voltage (TVS is invisible)
   │        ╱    │
   │       ╱     │
   │      ╱      │
 0V└─────────────┴──────────────▶ Current
         Normal   Spike!
       operation  (ESD event)

Three regions:

1. Below 25V (working): TVS has very high impedance - signal passes through normally

2. 25-27V (breakdown): TVS starts conducting, limiting voltage rise

3. Above 27V (clamping): TVS conducts heavily, dumping current to GND

CC Line ESD Protection

Circuit connection (USBLC6-2SC6, historical — D4 no longer exists)

USB-C Connector        D4 (USBLC6-2SC6)        STUSB4500
      │               ┌──────────────┐              │
 CC1 ─┼───────────────┤ 1 (I/O1)     │              │
      │               │         6    ├──────────────┤ CC1 (pin 2)
      │               │              │              │
      │               │ 2 (GND)──GND │              │
      │               │              │              │
 CC2 ─┼───────────────┤ 3 (I/O2)     │              │
      │               │         4    ├──────────────┤ CC2 (pin 4)
      │               │              │              │
VBUS ─┼───────────────┤ 5 (VBUS)     │              │
      │               └──────────────┘              │

Internal structure

The USBLC6-2SC6 is a dual TVS array with VBUS protection:

     I/O1 (1)              I/O1 (6)
        │                      │
        └────┬────────────┬────┘
             │   VBUS(5)  │
             │     │      │
        ┌────┴─────┼──────┴────┐
        │    ─┬─   │     ─┬─   │
        │   ╲│╱    │    ╲│╱    │
        │    │     │     │     │
        ├────┴─────┼─────┴─────┤
        │          │           │
        └────┬─────┼──────┬────┘
             │     │      │
     I/O2 (3)│    GND(2)  │I/O2 (4)
             │            │

Each channel clamps to GND and VBUS independently

Key specifications (USBLC6-2SC6)

ESD Event Timeline (historical — how the removed D4 responded)

What happened when you touched the USB-C cable with static charge, back when D4 was fitted:

Time 0ns:
├─ Your finger approaches USB-C plug
├─ Static charge: 10,000V
├─ CC line voltage: 0V
└─ D4 state: High impedance (invisible)

Time 1ns:
├─ Spark jumps from finger to CC pin
├─ CC line voltage shoots up rapidly
├─ Heading toward 10,000V!
└─ D4 state: Still high impedance

Time 2ns:
├─ CC line reaches 27V (breakdown voltage)
├─ D4 starts conducting
├─ Current diverts to GND
└─ Voltage rise slows dramatically

Time 5ns:
├─ CC line clamped at ~40V
├─ D4 conducting heavily (amps of current)
├─ All excess energy dumped to GND
└─ STUSB4500 sees only 40V spike (survivable!)

Time 100ns:
├─ ESD event over
├─ CC line returns to normal
├─ D4 returns to high impedance
└─ Circuit continues working normally ✓

Without D4: CC line would reach thousands of volts → STUSB4500 destroyed

With D4: CC line clamped to ~17V → STUSB4500 well protected

Why 25V Working Voltage?

USB-PD CC lines can see various voltages:

The 25V working voltage ensures:

• D4 doesn't interfere with normal CC operation (0-22V)

• D4 does protect against overvoltage (>27V)

Why Low Capacitance Matters

CC lines carry communication signals for USB-PD negotiation:

CC Signal (simplified)
     ┌───┐   ┌───┐   ┌───┐
     │   │   │   │   │   │
─────┘   └───┘   └───┘   └─── Clean signal

With HIGH capacitance TVS:
     ╱───╲   ╱───╲   ╱───╲
    ╱     ╲ ╱     ╲ ╱     ╲
───╱       ╳       ╳       ╲── Rounded, distorted signal ❌

With LOW capacitance TVS (3pF):
     ┌───┐   ┌───┐   ┌───┐
     │   │   │   │   │   │
─────┘   └───┘   └───┘   └─── Clean signal ✓

USB-PD specification allows up to 200pF on CC lines, so 3pF is negligible.

Placement Matters

TVS diodes must be placed as close as possible to the connector:

GOOD placement:
USB-C ──┬── TVS ──────────────── STUSB4500
        │                            │
       GND                          GND

ESD enters here, immediately clamped ✓

BAD placement:
USB-C ──────────────── TVS ──┬── STUSB4500
                             │       │
                            GND     GND

ESD travels long trace before clamping ❌
(trace inductance can cause voltage overshoot)

Other TVS Diodes in the Circuit

The project's other TVS diodes protect different rails. D4 (historical, CC1/ CC2/VBUS, now removed) is included below for contrast with the still-fitted parts:

Each TVS is matched to the voltage of the line it protects. See Board B's Protection Stage for the current fitted parts and their margin analysis.

Key Takeaways

1. ESD is invisible but deadly - you can't feel it below 3,000V, but 100V can kill an IC

2. TVS diodes are voltage-activated switches - invisible during normal operation, conduct during spikes

3. Response time is critical - TVS diodes respond in nanoseconds to catch fast ESD events

4. CC lines are vulnerable - first point of contact, thin traces, directly connected to IC

5. Working voltage must exceed normal operation - 25V TVS for CC lines that can see up to 22V

6. Low capacitance preserves signal quality - 3pF doesn't affect CC communication

7. Placement near the connector - clamp the spike before it travels into the circuit

Common Mistakes to Avoid

❌ Wrong: "TVS diodes are like fuses - they blow and need replacement"

• TVS diodes are not sacrificial - they survive ESD events and keep working

• They can handle thousands of ESD strikes

✅ Correct: "TVS diodes clamp voltage repeatedly without damage"

❌ Wrong: "Any diode can protect against ESD"

• Regular diodes are too slow and can't handle the current

• TVS diodes are specifically designed for fast, high-current events

✅ Correct: "TVS diodes have nanosecond response and high peak current ratings"

❌ Wrong: "Higher voltage TVS = better protection"

• If working voltage is too high, the TVS won't activate in time

• Match the TVS working voltage to slightly above the normal line voltage

✅ Correct: "Choose TVS working voltage just above the maximum normal operating voltage"

See Also

• USBLC6-2SC6 removal background - D4 was deleted entirely in the 2-board split, not relocated

• ESDA25L - Legacy component (replaced by USBLC6-2SC6, then USBLC6-2SC6 itself removed; no current design page)

• SMAJ15A (Protection Stage) - +12V/-12V output rail protection TVS (TVS1/TVS3)

• SD05 (Protection Stage) - 5V rail protection TVS (superseded by SMAJ6.5A — see decision (a))

• USB Type-C Pinout - Understanding CC and other pins