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)

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

What happens when you touch the USB-C cable with static charge:

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

Our circuit has multiple TVS diodes for different protection needs:

Each TVS is matched to the voltage of the line it protects.

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 Documentation - Full component specifications

• ESDA25L Documentation - Legacy component (replaced by USBLC6-2SC6)

• SMAJ15A Documentation - VBUS protection TVS

• SD05 Documentation - 5V rail protection TVS

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