TVS diodes: effective ESD protection for portable electronic devices

ESD protection is of great significance to high-density, miniaturized and complex electronic devices. This article discusses the influence of parameters such as minimum breakdown voltage and breakdown current, maximum reverse leakage current and rated reverse standoff voltage on the circuit and selection criteria when using TVS diodes to prevent ESD, and introduces some typical applications such as video line protection, USB protection and RJ-45 interface in portable consumer electronic devices, set-top boxes, and personal computers.

With the rapid development of mobile products, printers, PCs, DVDs, set-top boxes (STBs), and other products, consumers are demanding more and more advanced performance. Semiconductor components are becoming increasingly miniaturized, high-density, and complex in function. Especially for applications such as fashion consumer electronics and portable products that have strict requirements on the motherboard area, they are easily affected by electrostatic discharge. Some complex semiconductor functional circuits that use deep submicron technology and very fine line width wiring are more sensitive to the impact of circuit transients, which will lead to the above problems becoming more intensified.

ESD protection principle

There are several technologies for circuit protection components. When selecting circuit protection components, if the designer chooses the wrong protection device, it will only provide a wrong safety concept. The selection of circuit protection components should be determined based on the wiring conditions to be protected, the available circuit board space, and the electrical characteristics of the protected circuit. In addition, it is also necessary to understand the characteristics of the protection components. One of the important factors to consider is the clamping voltage of the device. The so-called clamping voltage is the voltage across the transient voltage suppressor (TVS) in the ESD device, which is the strain voltage of the protected IC.

Because the oxide layer in IC circuits manufactured using advanced process technology is relatively thin, the gate oxide layer is more susceptible to damage, which means that a higher clamping voltage will produce a higher strain voltage on the protected IC device and increase the probability of failure.

Many protection components are designed to absorb a large amount of energy, and due to the component structure or design, they also have a high clamping voltage. Since the clamping voltage of varistors is too high, they cannot provide effective ESD protection. In addition, due to the high capacitance of varistors, they cannot provide protection for high-speed data lines. TVS diodes were created to solve this problem and have become a key technology for protecting portable electronic devices.

TVS diodes are solid-state components specifically designed to absorb ESD energy and protect systems from ESD damage. If applied properly, TVS diodes will limit the voltage across the protected device to just above the rated working voltage, but well below the destruction threshold voltage.

TVS related parameters

The best way to deal with the damage of transient pulses to devices is to divert the transient current away from sensitive devices. The TVS diode is connected in parallel with the protected circuit on the circuit board. When the transient voltage exceeds the normal operating voltage of the circuit, the TVS diode will avalanche, providing an ultra-low resistance path for the transient current. As a result, the transient current is diverted through the diode, avoiding the protected device, and the protected circuit is kept at the cut-off voltage until the voltage returns to normal. When the transient pulse ends, the TVS diode automatically returns to the high-resistance state, and the entire circuit enters the normal voltage. After many devices are subjected to multiple shocks, their parameters and performance will degrade, but as long as they work within the specified range, the diode will not be damaged or degraded.

From the above process, it can be seen that when selecting a TVS diode, attention must be paid to the selection of the following parameters:

1. Minimum breakdown voltage VBR and breakdown current IR. VBR is the minimum breakdown voltage of TVS. At 25°C, TVS will not avalanche below this voltage. When the specified 1mA current (IR) flows through TVS, the voltage applied to the two poles of TVS is its minimum breakdown voltage VBR. According to the degree of dispersion of TVS VBR and standard value, VBR can be divided into 5% and 10%. For 5% VBR, VWM=0.85VBR; for 10% VBR, VWM=0.81VBR. In order to meet the IEC61000-4-2 international standard, TVS diodes must be able to handle a minimum ESD shock of 8kV (contact) and 15kV (air). Some semiconductor manufacturers use higher shock resistance standards on their products. For some portable device applications with special requirements, designers can select devices as needed.

2. Maximum reverse leakage current ID and rated reverse shutdown voltage VWM. VWM is the voltage that the diode can withstand in normal state. This voltage should be greater than or equal to the normal working voltage of the protected circuit, otherwise the diode will continuously cut off the circuit voltage; but it needs to be as close as possible to the normal working voltage of the protected circuit, so that the entire circuit will not face overvoltage threats before the TVS works. When this rated reverse shutdown voltage VWM is applied between the two poles of the TVS, it is in the reverse shutdown state, and the current flowing through it should be less than or equal to its maximum reverse leakage current ID.

[page]

3. Maximum clamping voltage VC and maximum peak pulse current IPP. When a pulse peak current IPP with a duration of 20mS flows through the TVS, the maximum peak voltage across it is VC. VC and IPP reflect the surge suppression capability of the TVS. The ratio of VC to VBR is called the clamping factor, which is generally between 1.2 and 1.4. VC is the voltage provided by the diode in the cut-off state, that is, the voltage passing through the TVS in the ESD shock state. It cannot be greater than the tolerable limit voltage of the protected circuit, otherwise the device will be in danger of being damaged.

4. Pppm rated pulse power, which is based on the maximum cut-off voltage and the peak pulse current at this time. For handheld devices, a 500W TVS is generally sufficient. The maximum peak pulse power consumption PM is the maximum peak pulse power consumption value that the TVS can withstand. At a given maximum clamping voltage, the greater the power consumption PM, the greater its surge current tolerance. At a given power consumption PM, the lower the clamping voltage VC, the greater its surge current tolerance. In addition, the peak pulse power consumption is also related to the pulse waveform, duration and ambient temperature. Moreover, the transient pulses that TVS can withstand are non-repetitive, and the pulse repetition frequency (ratio of duration to intermittent time) specified by the device is 0.01%. If repetitive pulses appear in the circuit, the accumulation of pulse power should be considered, which may damage the TVS.

5. Capacitance C. Capacitance C is determined by the cross-section of the TVS avalanche junction and is measured at a specific frequency of 1MHz. The size of C is proportional to the current carrying capacity of the TVS. If C is too large, the signal will be attenuated. Therefore, C is an important parameter for selecting TVS in data interface circuits. For loops with higher data/signal frequencies, the capacitance of the diode will interfere more with the circuit, causing noise or attenuating the signal strength. Therefore, the capacitance range of the selected device needs to be determined based on the characteristics of the loop. For high-frequency loops, the capacitance should generally be as small as possible (such as LCTVS, low-capacitance TVS, capacitance not greater than 3pF), while for loops with low capacitance requirements, the capacitance can be higher than 40pF.

ESD Applications

1. Application of bottom connector

The bottom connector design is widely used in mobile consumer products. Currently, the main application products on the market are mobile phones, PDAs, DSCs (digital cameras), MP3s and other portable products.

Since it is a DC circuit, high capacitance devices can be selected. This port may be impacted by high energy, so devices with integrated TVS and overcurrent protection functions can be selected.

As shown in Figure 1, it is a schematic diagram of the bottom connector protection circuit of a portable product, in which the data line protection ICs are NZQA5V6XV5T1, NZQA6V2XV5T1, NZQA6V8XV5T1, NZQA8V2XV5T1, NZQA5V6AXV5T1, NZQA6V8AXV5T1, MSQA6V1W5T2, SMF05T1 and NSQA6V8AW5T2.

The above products all have 4 single-phase independent line ESD protection, among which the package form of MSQA series, NSQA series and SMF05 is SC-88A, and the package form of NZQA series is SOT-553. Among them, NZQA5V6XV5 is a 5.6V unidirectional TVS protection device; NZQA6V2XV5 is a 6.2V unidirectional TVS protection device; NZQA6V8XV5 is a 6.8V unidirectional TVS protection device; NZQA6V8AXV5 is a 6.8V unidirectional, low-capacitance TVS protection device; NUP4102XV6 is a 14V bidirectional, low-capacitance TVS. These SOT5xx packaged TVS devices are produced for a 260°C reflow temperature process, meet 100% lead-free and electrostatic discharge protection requirements, and reduce circuit board space by 36% and thickness by 40% compared to traditional SC88 packages, making them suitable for portable devices with strict requirements on circuit board space, such as mobile phones, digital cameras, and MP3 players.

2 RJ-45 (10/100M Ethernet network)

The RJ-45 interface is widely used in network connection interface devices, and a typical application is the 10/100M Ethernet network.

As shown in Figure 2, the RJ-45 data line protection mainly uses ON Semiconductor's low-capacitance transient voltage suppression diode - SL05, with an operating voltage of 5V. In fact, the company has a series of SLXX products, from SL05 to SL24, with operating voltages covering 5V, 12V, 15V, and 24V. It complies with IEC 61000-4-2 (ESD) 15kV (air) 8kV (contact) / IEC 61000-4-4 (EFT) 40A (5/50ns) / IEC 61000-4-5 (Lightning) 12A (8/20us) standards. In addition to being used in LAN/WAN equipment, it is also suitable for high-speed data line protection, mobile phone and USB port protection.

3. Protection of video lines

At present, common video output port designs include D-SUB (as shown in Figure 3), DVI (28 lines), SCART (19 lines) and D-TERMINAL (mainly used by Japanese products). Video data lines have high data transmission rates, up to 480Mbps, and some video data transmission rates reach more than 1G, so low-capacitance LCTVS should be selected. It usually connects a low-capacitance diode in series with the TVS diode to reduce the capacitance of the entire line (can be lower than 3pF) to meet the requirements of high-speed loops. ONSEMI Semiconductor's SRDA05-4 has good low-capacitance characteristics and can provide 4-way ESD protection, while its successor SRDA05-6 can provide up to 6-way ESD protection. The SRDA05-X series products are suitable for the protection of all high-speed communication lines.

4. SIM card data line protection

SIM card data line protection has always been the focus of various companies' products, and the device designed specifically for this type of port integrates ESD (TVS)/EMI/RFI protection in one chip, fully demonstrating the unlimited integration solution of chip devices.

When selecting devices for different purposes, it is necessary to avoid making the devices operate near the limits of their design parameters. In addition, devices with fast enough response speed and high enough sensitivity should be selected based on the characteristics of the protected circuit and the characteristics that may be subjected to ESD shocks. This is critical for the effective role of the protection device. In addition, devices that integrate other functions should also be considered first.

Many semiconductor manufacturers provide a variety of TVS diode packages, especially micro packages such as SOT23 and SC-70, as well as flip chips of the same size as chips, which only occupy about 4.8mm2 on the board, but can protect multiple lines at the same time. Many recent new products are more adapted to the high integration and miniaturization requirements of portable devices, integrating EMI/RFI/ESD protection in one device, which can not only effectively reduce space, but also greatly reduce costs, reduce device procurement costs and processing costs, and are the designer's first choice for ports that require these protection functions at the same time (Figure 4).

5. USB protection

Generally, USB ESD protection is divided into two types: upstream (Figure 5) and downstream (Figure 6). For USB 1.1, NUF2101 is mainly used for downstream ESD protection, and STF202 or NUF2221W1T2 is used for upstream ESD protection. These products can not only meet the ESD protection requirements of USB line terminals, but also have good filtering functions.

6. Audio/speaker data line protection

In terms of audio data line protection (Figure 6), since the signal rate of the audio circuit is relatively low, the requirement for device capacitance is not too high, and about 100pF is acceptable. Some mobile phone designs combine the earphone and microphone together, while others use discrete circuits. In the former case, a single-channel TVS can be selected, and in the latter case, if the two circuits are adjacent, a multi-channel TVS array can be selected, and only one device can be used to protect the two circuits.

7. Buttons/Switches

For button and switch circuits, the data rate of these circuits is very low, and there are no special requirements for the capacitance of the device, so ordinary TVS arrays can be used.

Conclusion

The above products are mainly for ESD protection of data lines. Four of the devices listed in Table 1 are filters designed for USB, Ethernet, firewall and other high-speed data applications. These devices meet all USB requirements, including electromagnetic interference filtering and line termination of upstream/downstream ports. These new integrated components can replace the current discrete component solutions that require 12 components. Eight new devices provide electrostatic discharge (ESD) protection with the lowest capacitance in the industry. These new protection devices have passed the 15 kV contact discharge test, which is higher than the 8 kV ESD standard of IEC61000. These devices have a typical capacitance of 1.5 pF, ensuring data line speed and data integrity.

In addition, for portable devices, the increased complexity and precision of various integrated circuits make them more sensitive to ESD, and the previous general circuit design is no longer suitable. The most important thing for a reasonable PCB layout is to avoid self-inductance while using TVS diodes to protect against ESD damage. ESD design is likely to cause parasitic self-inductance in the circuit, which will have a strong voltage impact on the circuit, causing damage beyond the IC's tolerance limit. The self-inductance voltage generated by the load is proportional to the intensity of the power supply change, and the transient characteristics of the ESD impact are prone to induce high-strength self-inductance. The basic principle of reducing parasitic self-inductance is to shorten the shunt loop as much as possible, and all factors must be considered, including the ground loop, the loop between the TVS and the protected circuit, and the path from the interface to the TVS. Therefore, the TVS device should be as close to the interface as possible and as close to the protected circuit as possible, so as to reduce the chance of self-inductance coupling to other adjacent circuits.

The following points should also be noted in circuit board design: avoid running critical signal lines near protection circuits as much as possible, and arrange interfaces on the same side as much as possible; use highly integrated devices, diode arrays can not only greatly save space on the circuit board, but also reduce the impact of parasitic line self-inductance that may be induced by complex circuits; avoid connecting protected circuits in parallel with unprotected circuits, connect interface signal lines and ground lines directly to protection devices, and then enter other parts of the circuit, keep reset, interrupt, and control signals away from input/output ports and the edge of the PCB; the area surrounded by the loop formed by various signal lines and their feeders should be as small as possible, and if necessary, consider changing the position of the signal line or ground line; add grounding points wherever possible.