ESD Protection Fundamentals

What is Electrostatic Discharge (ESD)? | What Damage Does ESD Cause? | ESD Suppression Technology
PESD Principle of Operation | Overvoltage Protection for USB 3.0 | ESD Protection for HDMI Systems
HDMI Reference Layout | PESD Operational Characteristics | ESD Supressor Design Criteria
The Tyco Electronics Advantage

• Charged person - a person can become charged due to walking or other motion. ESD damage can be especially severe if the discharge from the person is via a metallic object, such as a tool.
• A cable rubbing across carpet - if a charged cable is plugged into a conductive contact with any source of charge ESD will occur.
• An electronic device sliding into or out of a bag or tube generates an electrostatic charge as the device's case and/or metal leads make multiple contacts and separations with the surface of the container.

Electrostatic damage to electronic devices can occur at any time, from the factory floor to the end-user’s home. ESD damage is generally classified as either catastrophic or latent failure.

Catastrophic Failure
A catastrophic ESD event may cause a metal melt, junction breakdown, or oxide failure, permanently damaging the circuitry and causing the device to fail. These types of failures are usually identified before the device is shipped, and represent approximately 10% of ESD failures.

Latent Failure
Latent failure is more difficult to identify and ultimately more costly.

• Device may be partially degraded after an ESD event, remaining operable, but with compromised performance characteristics.
• Devices with latent defects may experience intermittent or premature failure.
• Detecting cause of failure may be difficult or hazardous.
• Latent failures increase warranty and replacement costs.

ESD suppression devices attempt to divert a potentially damaging charge away from sensitive circuitry and protect the system from catastrophic failure. A variety of technologies are used for ESD protection. As the figure below illustrates, the PESD device is particularly attractive for high-frequency applications, due to its exceptionally low capacitance.

• Conductive particles are dispersed in a non-conductive matrix.
• The gaps between each particle behave like spark gaps when a high-voltage ESD pulse occurs.
• When the voltage of the pulse reaches the "trigger voltage" these gaps spark over, creating a very low resistance path.
• In normal operation, the leakage current and the capacitance is very low, due to the physical gaps between the conductive particles.

• USB 2.0 & USB 3.0
• HDMI 1.3

Overvoltage Protection Considerations for USB 3.0 Applications
The USB 3.0 protocol was developed to provide higher transfer rates, increased maximum bus power and device current draw, new power management features, and new cables and connectors that are backward-compatible with USB 2.0 devices. The most significant change is that an additional physical bus has been added in parallel with the existing USB 2.0 bus.

 USB 3.0 adds five new pins to the connector to support the new SuperSpeed interface:  USB3_TX (differential pair), GND, and USB3_RX (differential pair), as shown in the figure below.

The SuperSpeed interface of USB 3.0 requires lower capacitance ESD protection than that of USB 2.0. Adding very low capacitance PESD devices is critical to minimize insertion loss to meet eye diagram requirements of USB 3.0. With a typical capacitance of 0.20pF and flat insertion loss to >6GHz, PESD devices are capable of handling numerous ESD transients.

PESD devices provide lower capacitance than traditional MLV (multilayer varistor) or TVS (transient voltage suppression) diode technology, and their low-trigger voltage and low-clamping voltage also helps protect sensitive electronic components. The devices are applicable for ESD protection on USB 2.0’s high-speed D+ and D- signal lines and USB 3.0 SuperSpeed signal lines.

Protection recommendations for USB 2.0 and USB 3.0 are shown below.

Protection Recommendation for USB 2.0

 
Protection Recommendation for USB 3.0

For more information on circuit protection considerations for USB applications download the Application Note titled, Coordinated Circuit Protection Strategies Help Prevent Damage to USB Charger Systems and Portable Electronics.

Timing/Performance Considerations for Adding ESD Protection to HDMI Systems

The HDMI 1.3 standard brings improved color depth and audio output, among other benefits, while doubling the previous HDMI data rate to 3.4Gbps per differential signal pair. Because of the increased risk of cable discharge events and damaging pulses from the operating environment and connected peripherals, aggressive ESD (electrostatic discharge) protection is essential.

When adding ESD protection to HDMI systems it is critical to consider the added impact of additional capacitance and inductance on timing of the chosen device on the high-speed TMDS pairs. When operating at up to 3.4GHz on the TMDS pair, any additional impedance on the line can distort the signal, leading to:

To minimize timing impact on these high speed lines, there are four key technical considerations to be made regarding the ESD protection device.

1. Low capacitance
2. Low insertion loss
3. Stable capacitance vs. frequency
4. A good layout that runs at 3.4GHz, with margin

HDMI’s timing performance is typically measured with eye diagrams – a timing analysis tool used to provide an accurate visual display of timing and level errors. The grey space in the middle of the eye diagram represents the HDMI 1.3 specification.

As the lines encroach on the grey space, the less margin of error there is. The eye width is a good measure of the amount of time the data lines are stable, and if any errors are present. The eye height measures the level, or amplitude, of the signal. Since HDMI’s TMDS pairs are differential signals, it is important to minimize both differential and signal-to-ground capacitance to ensure the rise and fall times of the signals are within specification. Optimally, the capacitance should be as low as possible to give designers as much margin as possible.

The eye diagram performance of Tyco Electronics’ 0.25pF PESD device operating at 3.4GHz is shown below.

Eye diagram of Tyco Electronics’ 0.25pF PESD device operating at 3.4GHz.

This diagram shows that when operating at 3.4GHz, the highest speed prescribed by HDMI 1.3, there is a margin between rise and fall times and signal level. When operating at lower speeds, the eye diagram is ‘cleaner’ and provides additional margin, thus easing design constraints.

As shown in the next figure, silicon solutions have much higher capacitance. Although their eye diagrams are commonly shown at 2.25GHz, or 1.48GHz to show compliance with 1080p 36- and 24-bit color depths, their eye diagrams appear to encroach on the HDMI 1.3 specification, even at these speeds. This can lead to increased board design constraints.

Silicon ESD protection device eye diagram at 1.48GHz

The figure below shows a typical HDMI circuit protection scheme utilizing PESD devices

HDMI Reference Layout Helps Reduce Design and Test Time

Tyco Electronics’ HDMI 1.3 reference layout offers designers a solution that reduces the need to make tradeoffs between size, ESD protection performance, and ease of implementation.

Developed in conjunction with Efficere, Inc., a leader in high-speed interface design, the reference layout meets and exceeds the HDMI 1.3 specifications for operation at 3.4Gbps for Impedance (TDR), Data Eye, and Far End Crosstalk. It has also been validated with full-board performance (including connector, and mating to plug) and has passed with margin at 3.4GHz.

The “cut and paste” layout is backward compatible with HDMI 1.0, 1.1 and 1.2, includes optional overcurrent protection when used in HDMI transmitters such as set top boxes, computers and DVD players.

• HDMI 1.3 compliance test results include TDR and Eye-Diagram
• Designed with 4-layer board and 0603 &  0402 size PESD devices, and nanoSMDC020F-2 Polyswitch™ overcurrent protection devices
• PolySwitch nanoSMD can be easily removed for HDMI receiver applications where overcurrent protection is not required

For more detailed information, download Tyco Electronics’ White Paper titled, HDMI ESD Protection without Sacrificing Performance.

Capacitance vs. frequency in the Raychem Circuit Protection PESD device is flat, and the device maintains a very low capacitance, as shown below.

The following figure illustrates the performance characteristics of a PESD device. In the top portion of the figure, the PESD will turn on when ESD voltage reaches the PESD device’s trigger voltage. The ESD strike will be clamped to the PESD device’s clamping voltage for the duration of the pulse. The bottom portion of the figure shows the current through the PESD device – including shunt current – when the PESD is turned on.

• Operating Voltage (VDC): Defined as DC voltage, under which device is in OFF state and leakage current below certain threshold.
• Leakage Current (IL): Current through device under Operating Voltage VDC
• Trigger Voltage (Vt): Voltage at which the device switches from the OFF to the ON state, during the IEC waveform or the TLP test system.
• Clamping Voltage (Vc): Voltage across device under 8 kV per IEC or measured by TLP test system. Typically measured 30 ns after initiation of the IEC ESD pulse, but 30ns and 60ns are sometimes used for TLP.
• Capacitance (Cp): Capacitance of the device measured at 1 MHz with 0 bias and 1 Vrms signal.

• Data signal ground (GND) and Vbus transients must be suppressed for proper operation.
• Good design practices mandate that data signal ground and chassis ground not be tied together at the board level. Decoupling capacitors between Vbus and chassis ground should be used to minimize EMC issues.
• Having both grounds connected on the board level may allow transients to propagate via the signal ground with respect to chassis ground. This may be especially critical when cables are inserted into a USB connector.
• To optimize ESD suppression, suppressor devices should be installed as close to the source of the ESD transient as possible.

• PESD devices help shunt ESD away from sensitive circuitry and provide exceptionally low capacitance compared to traditional MLV (multi layer varistor) or TVS (transient voltage suppression) diode technology.
• The PESD device's low capacitance, low-voltage clamping levels, and high ESD tolerance are critical performance parameters for high data-rate transmission applications
• The device's form factor meets the board-space and surface mount installation requirements of new portable electronics designs.

Designs for a Smaller World
The electronics industry’s unrelenting demand for smaller, more reliable circuit protection devices continues to drive the miniaturization trend. Tyco Electronics consistently leads the industry with smaller and smaller devices, but the objective is more than size reduction. The real challenge lies in scaling down component size without sacrificing electrical characteristics. Investment in material research and technology has enabled the development of devices that meet existing performance levels within new, smaller, and more convenient packages.