Securing ESD protection without sacrificing HDMI performance

The High-Definition Multimedia Interface (HDMI) 1.3 standard doubles the previous HDMI 1.0 - 1.2 data rate to 3.4Gbps per differential signal pair. This increased data rate introduces new challenges in implementing a solid-board design with capacitance low enough to ensure signal integrity. This is of particular importance when implementing a robust Electrostatic Discharge (ESD) protection solution. This Design Note explores the requirements and potential pitfalls of designing ESD protection into HDMI 1.3 systems.

Adding ESD protection to high-definition video systems involves many complex challenges that can increase costs and time-to-market. Often choices are made based on what looks like an easy solution to implement, however, the simplest approach may not provide adequate ESDprotection performance or optimal board footprint. In other cases, what seems like the best ESD-protection solution at first, is later found to require multiple board spins to ensure that adequate timing is met. Providing adequate protection has usually meant making trade-offs between size, ESD-protection performance, and ease of implementation.

The principal cause of complexity in implementing robust ESD protection for HDMI 1.3 systems is operation speed. This design criteria must be considered and defined to provide adequate protection.

 

Defining HDMI

HDMI’s speed is referred to in many ways, making it difficult for designers to select the proper ESD-protection solution. The newest HDMI standard, HDMI 1.3, is commonly referred to as operating at up to 10.2Gbps at 340Mpixels/s. This is an accurate description of the system operating speed, but does not describe the speed of the Transition Minimised Differential Signalling (TMDS), which must be considered in order to select an adequate ESD-protection solution.

When the HDMI 1.3 specification states that the system operates at up to 10.2Gbps at 340Mpixels/s, the key phrase is 'operates at up to'. This effectively means that the interface will change its clock rate depending on the video capabilities of the connected transmitter and receiver. Thus, the higher the resolution or colour depth of both connected devices, the higher the clock rate. HDMI only needs to run fast enough to pass the required amount of pixels to drive the display device. For example, if a high-definition Digital Video Disc (DVD) player and Liquid Crystal Display (LCD) are operating at full 1080P with 48-bit colour depth, more information needs to be processed than if a 480i standard definition DVD is played.

 

 

 

Table 1 shows each resolution and the corresponding number of pixels per line and lines per frame. For each of the colour depths listed in Table 1, there is a corresponding number of encoded bits that need to be transmitted for each pixel’s colour. The amount of data that needs to be sent from the HDMI transmitter to the HDMI receiver can be explained as a relationship of these variables, including the number of frames per second to update the screen. Table 2 and the following formulae describe this relationship:

As shown in Figure 1, three TMDS pairs require a bandwidth capable of transmitting and receiving up to 10.2Gbps. Thus, each of the three TMDS data pairs must be capable of signalling (switching) at:

To illustrate how the link data rate relates to the physical TMDS pairs, Table 3 combines the information from Table 2 with the TMDS pair signalling speed, and the corresponding signalling clock speed.

 

 

Timing and performance considerations for ESD protection

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

• Greater difficulty in meeting the required eye diagrams for rise times and signal levels
• Additional constraints on board design
• Lower system level performance

To minimise timing impact on these high-speed lines, there are four key technical requirements to be met:

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

 

Low capacitance

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 requirement of the HDMI 1.3 specification. As the lines encroach on the grey space, there remains less margin for error. 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 minimise both differential and signal-toground 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 in Figure 2.

 

Fig. 2: Eye diagram of Tyco Electronics’ 0.25pF PESD device operating at 3.4GHz

Fig. 3: Silicon ESD protection device eye diagram at 2.25GHz

 

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 Figure 3, silicon solutions exhibit a much higher capacitance. Although their eye diagrams are commonly shown at 2.25GHz or 1.48GHz to indicate compliance with 1080p 36- and 24-bit colour depths, their eye diagrams encroach on the HDMI 1.3 specification even at these speeds. This can lead to increased boarddesign constraints.

 

 

Low insertion loss

Insertion loss is an important measure of signal attenuation vs. frequency. Higher insertion loss translates to lower bandwidth in the device and system, and imposes additional design constraints in order to meet the levels of the eye diagram.

Figure 4 compares the insertion loss of Tyco Electronics’ PESD device and a common 0.7pF silicon ESD-protection solution. Tyco Electronics’ PESD device shows negligible insertion loss even at 3.4GHz, the highest speed prescribed by HDMI 1.3. Common 0.7pF silicon ESD-protection devices typically exhibit a sharp frequency roll-off, and can impact HDMI TMDS signal levels by more than 3dB at 2.25GHz, the speed of 1080p with 36-bit colour depth. At the full speed of 3.4GHz, silicon ESD protectors can impact signal attenuation by more than 6dB, effectively cutting signal levels by more than half.

 

 

Stable capacitance vs. frequency

An ESD-protection device’s capacitance vs. frequency behaviour can also affect the HDMI port’s performance, as well as imposing design constraints. In highspeed systems, circuits designed for a certain capacitance can behave differently depending on the ESD-protection technology used. This often forces designers to use complicated Software Process Improvement and Capability Determination (SPICE) models and simulations when creating the HDMI circuit-protection scheme. As shown in Figure 5, Tyco Electronics’ PESD device provides stable capacitance vs. frequency up to 3GHz. Its behaviour closely resembles a 0.25pF capacitor, which can greatly simplify design. Since the HDMI TMDS pairs change in frequency depending on the data pattern, video-source resolution and colour depth, knowing that the ESD-protection device’s capacitance is stable over a broad frequency range, gives designers more latitude and flexibility.

It is important to consider the stability of capacitance over a broad frequency range, rather than at a single frequency or within a limited range. For example, silicon ESD protection commonly measures capacitance at 1MHz, but the capacitance at other frequencies is not specified. This may result in a need for complicated modelling to ensure performance is met over all HDMI frequencies.

 

A layout that runs at 3.4GHz with margin

Generally, designers of HDMI-enabled devices face the common challenge of shortening time-to-market. When designing for high-frequency applications, reference designs play a key role in minimising the risk, engineering cost, and re-engineering time. Adding ESD protection to HDMI designs is no exception.

Tyco Electronics has now made available the industry’s first ESD and overcurrent protection reference layout for HDMI 1.3 based on passive devices. This reference layout can help designers focus their time and resources on developing critical and differentiating features, rather than worrying if adding ESD protection will affect HDMI performance.

The reference layout has been tested and meets the HDMI 1.3 requirements at the highest speed of 3.4GHz, with margin. A reference layout that complies with the HDMI specification at 3.4GHz is important not only for designers, but also relaxes design constraints, and can often lower board cost for lower-speed HDMI 1.3 designs. Tyco Electronics’ reference layout key includes:

• Backward compatibility with HDMI 1.0 – 1.2
• Performance validated to meet HDMI 1.3 requirements at 3.4GHz
• Optional overcurrent protection for the +5V rail (applicable to HDMI transmitters)
• Layout design files, PESD spice model, and test results, including Time-Domain Reflection (TDR), eye diagram, and crosstalk measurements.

 

Conclusion

When designing HDMI systems, adding ESD protection need not be a complex and confusing task. Although current generation devices may not require HDMI 1.3 with full 3.4GHz performance, utilising an ESD-protection solution that can pass at 3.4GHz minimises design headaches, increases system margin and facilitates next-generation designs by removing the need for a full redesign of the ESD protection scheme. Tyco Electronics’ ESD and overcurrent protection reference layout complies with the HDMI 1.3 specification at 3.4GHz, provides IEC 61000-4-2 ESD protection, optimises board space, and helps minimise design risk.

 

Email info@my-ftm.com quoting response number 18 to request the Tyco Electronics Reference Design.

 

www.circuitprotection.com/esd/