Created:2026/05/07
Updated:2026/05/07
Author:
Takeshi Takatsudo

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

Action	Voltage generated
Walking on carpet	1,500 - 35,000V
Sliding across car seat	7,000 - 25,000V
Picking up plastic bag from desk	1,000 - 20,000V
Normal indoor movement	500 - 5,000V

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

Type	Purpose	Response time	Current handling
Normal diode	Rectification	Moderate	Low-medium
TVS diode	Protection	Nanoseconds	Very high (amps)

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:

Below 25V (working): TVS has very high impedance - signal passes through normally
25-27V (breakdown): TVS starts conducting, limiting voltage rise
Above 27V (clamping): TVS conducts heavily, dumping current to GND

CC Line ESD Protection

Current Design Uses USBLC6-2SC6

The v1.1 design uses USBLC6-2SC6 (D4) instead of ESDA25L for CC line protection. USBLC6-2SC6 provides:

Lower clamping voltage (~17V vs ~44V)
Additional VBUS protection channel
Better suited for USB-C applications

The principles below still apply - only the specific component has changed.

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)

Parameter	Value	Why it matters
Working voltage	5.25V	Optimized for USB signal levels
Breakdown voltage	6V	Starts protecting above this
Clamping voltage	17V @ 1A	Much lower than ESDA25L (44V)
Capacitance	3.5pF	Low enough to not affect CC signaling
ESD rating	15kV (HBM)	Survives typical human static discharge
VBUS channel	Yes (Pin 5)	Additional protection for power rail

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:

Condition	CC voltage	Notes
Normal signaling	0 - 3.3V	Typical operation
VCONN power	5V	Powering active cables
Fault condition	Up to 22V	USB-PD spec allows this

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:

Component	Type	Protects	Working Voltage
D4	USBLC6-2SC6	CC1, CC2, VBUS	5.25V
TVS1	SMAJ15A	VBUS power rail	15V
TVS2	SD05	+5V output rail	5V
TVS3	SMAJ15A	+12V output rail	15V

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

Key Takeaways

ESD is invisible but deadly - you can't feel it below 3,000V, but 100V can kill an IC
TVS diodes are voltage-activated switches - invisible during normal operation, conduct during spikes
Response time is critical - TVS diodes respond in nanoseconds to catch fast ESD events
CC lines are vulnerable - first point of contact, thin traces, directly connected to IC
Working voltage must exceed normal operation - 25V TVS for CC lines that can see up to 22V
Low capacitance preserves signal quality - 3pF doesn't affect CC communication
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"

The Question​

What is ESD?​

Real-world examples​

The danger to electronics​

Why CC Lines Are Vulnerable​

What is a TVS Diode?​

Normal diode vs TVS diode​

How TVS works​

CC Line ESD Protection​

Circuit connection (USBLC6-2SC6)​

Internal structure​

Key specifications (USBLC6-2SC6)​

ESD Event Timeline​

Why 25V Working Voltage?​

Why Low Capacitance Matters​

Placement Matters​

Other TVS Diodes in the Circuit​

Key Takeaways​

Common Mistakes to Avoid​

See Also​