Esd Protection For Usb Interfaces

The Threat of ESD to USB Interfaces

USB interfaces are vulnerable to damage from electrostatic discharge (ESD) events. ESD is the sudden flow of electricity between two electrically charged objects, which can deliver thousands of volts to electronic components. This can overload and damage sensitive integrated circuits and chips in USB ports and connected devices.

Common everyday scenarios like walking across a carpet can create hazardous ESD potentials on the human body. A discharge can occur when a user handles a USB cable and connects it to a port. The high transient voltages can couple onto the USB’s data and power lines, posing a risk to interface electronics.

USB Pinouts and Signal Lines Vulnerable to ESD

The USB interface uses four shielded cables that connect the USB host controller to a peripheral device. The cables carry power, ground, and differential data lines:

Table of Contents

VBUS (+5V power)
D− (USB negative differential data)
D+ (USB positive differential data)
GND (Ground)

ESD events coupled onto the USB’s high-speed data lines can damage sensitive integrated transceiver chips and I/O buffers in both the host and peripheral device. The VBUS line is also vulnerable during hot-plugging actions where an ESD charge originates from the human body or the peripheral device itself.

ESD Protection Goals for USB Lines

Effective ESD protection schemes for USB aim to:

Provide primary and secondary protection layers for immunity against ESD strikes up to ±15 kV per the IEC 61000-4-2 standard.
Clamp transient overvoltages to safe levels below the Absolute Maximum Ratings of USB interface chips.
Limit let-through voltages to less than 10 volts on data lines per the USB 2.0 specification.
Avoid introducing excessive leakage currents or capacitances that could impact signal integrity.
Provide robust ESD protection in a small PCB footprint.

Primary ESD Protection Components

TVS Diodes

Transient voltage suppressor (TVS) diodes are discrete components used to absorb ESD transients and clamp voltages. Uni-directional diodes can clamp positive or negative spikes, while bi-directional diodes handle spikes in both polarities. Key characteristics are the breakdown voltage, peak pulse current rating, and clamping voltage behavior.

For USB, low capacitance TVS arrays with a 5V breakdown voltage are recommended to shunt ESD currents. Common USB applications utilize the following TVS diode arrays:

SP0503BA from Littelfuse – Bidirectional 5V array in SOT23 package protecting two data lines.
USBLC6-2SC6 from STMicroelectronics – 6-channel protection for USB2 interfaces.

Transient Voltage Suppressors

Transient voltage suppressor (TVS) devices incorporate large junction field-effect transistors that avalanche at high voltages. TVS chips offer fast responses and provide versatile multichannel protection. Key specifications are peak pulse power dissipation, junction capacitance, breakdown voltages, and clamping behavior.

TVS devices designed to protect data lines for common serial interfaces like USB include:

TPS3H100DBZR from Texas Instruments – 1-channel 3.3V protector.
SDTA024DRH from Nexperia – Bidirectional 2 channel ESD protection rated at ±30 kV per IEC 61000-4-2 with low pinch-off voltage.

Secondary ESD Protection

Series Resistors

Series resistors limit shunt currents flowing into ESD clamping devices during a strike. They also help isolate transients on cables from downstream components. Typical values range from 10-100 ohms for USB data lines.

I/O Clamping

Secondary protection on the IO pins of USB interface chips is needed to absorb residual transients passing through primary devices. Integrated clamps actively bleed currents using internal diode chains coupled to the IC’s supply rails.

Example Schematics for Robust USB ESD Protection

Combining primary TVS diodes and secondary protection gives robust defense for USB interfaces. Below depicts a common scheme:

Here, the SP0503BA dual TVS array clamps transients on the USB 2.0 data lines along with series resistors. All lines couple through capacitors providing AC isolation to the PHY transceiver IC, which has internal pin ESD clamps to VDD and VSS rails. The TVS devices shunt high peak currents preventing I/O overvoltage.

Testing ESD Protection for USB Compliance

Real-world ESD testing per IEC 61000-4-2 involves using a discharge “gun” that generates fast risetime spikes with polarities and levels per human body model (HBM) standards. Repeated gun blasts of +/- 15kV are applied across all USB port pins while monitoring interface chip currents and voltages.

To pass, USB ports must withstand +/-15kV ESD strikes without performance degradation or physical damage. Innovative test techniques like using I-V tracers and near-field probes help characterize protection networks.