ESD protection design guide for each external interface of contemporary mobile phones

Before considering the choice of ESD protection components for ESD protection for mobile phones, it is important to understand a key trend that is happening in the electronics industry today. In short, the number of ESD protection circuits included in many chipsets used in today's various applications is decreasing. In other words, the ability of these chipsets to be protected from damage under severe, user-generated ESD events is declining.

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Almost all chipsets currently have on-chip ESD protection. The ESD circuit is placed on the periphery of the chip and adjacent to the I/O pads, which are used to protect the chipset during wafer fabrication and back-end assembly processes. In these environments, ESD can be introduced to the chipset through equipment or plant line workers. Key ESD specifications include Human Body Model (HBM), Charged Device Model (CDM), and Machine Model (MM). The purpose of these test specifications is to ensure that the chipset maintains high manufacturing yields in the manufacturing environment.

Traditionally, chip manufacturers have been trying to maintain the 2,000V level required by HBM. From a cost-benefit perspective, this has proven to be a difficult task. As can be seen from Figure 1, as the manufacturing technology shifts to below 90 nm, the cost of maintaining the ESD protection level at 2,000 V has begun to increase exponentially. Therefore, the new goal now is to reduce the level of ESD protection on the chip, but maintain the same high manufacturing yield level.

The currently accepted critical ESD protection voltage level is approximately 500V. At this level, the cost of the chip is increased more reasonably, and the yield level will not be damaged. This is because typical fabs and assembly plants have policies that limit ESD to 500V or less.

Therefore, even if all chipsets contain some ESD protection circuitry on the die, the goal is simply to ensure high yields. However, this level of ESD protection is not enough to protect the chipset from the serious ESD events that consumers will encounter when actually using the phone. In a consumer environment that cannot be controlled in advance, different ESD protection specifications must be used. This is IEC61000-4-2.

This IEC specification has been used by many application manufacturers (mobile phones, smart phones, MP3 players, etc.) to ensure that their products work reliably and without early failure. This specification has a much higher ESD protection voltage level and is therefore not compatible with HBM. The tests required by the HBM specification are concentrated at 500V. On the other hand, the air discharge method in the IEC requires tests exceeding 15,000V.

This means that there is a very large gap between the level of testing required for the ESD protection capabilities of the chipset and the reliability of the application. This usually means that board-level ESD components such as multi-layer varistors, polymer ESD suppressors and silicon protection arrays must fill this gap. One thing to note is that the ESD protection performance of these technologies is different. Specifically, the on-time and clamp voltage vary widely. This means that for sensitive chipsets, applications that use one of these technologies cannot pass ESD testing, but can use ESD testing when using another technology.

The most common board-level ESD protection devices in the industry currently have the following three types, and their key attributes are as follows.

Multilayer varistor (MLV): This type of zinc oxide based device provides ESD protection and low level surge protection. Their small form factor (down to 0402 and 0201) makes them ideal for portable applications such as cell phones and digital cameras.

Silicon Protection Array (SPA): These discrete and multichannel devices are designed to protect data lines and I/O lines from ESD and low-level transient surges. Its key feature is the very low clamping voltage, which allows them to protect the most sensitive circuits.

Polymer ESD Suppressor (PGB): This is the latest technology designed to produce the smallest parasitic capacitance value (<0. 2pF). This feature allows them to be used in high speed digital and RF circuits without causing any signal degradation.

Since the phone is designed for mass consumers and can be used in any environment, it is very likely that ESD will enter one of the ports or I/O interfaces and cause the chipset to be electrically unstable or completely damaged. Figure 2 is used to help illustrate what ESD protection techniques should be used for different circuits. It shows that all circuits have the potential to provide a way for ESD to enter the phone.

With this information in hand, we can analyze the different circuits in the phone and choose the best solution.

1) Antenna circuit (GSM/CDMA/Bluetooth and FM radio)

These are low voltage circuits and they are also used to carry high frequency signals. GSM/CDMA/GPRS operates at 800MHz to 1900MHz; Bluetooth operates at 2400MHz and FM radio operates at up to approximately 108MHz. In order to maintain the integrity of the RF signal, the key feature of the ESD suppressor is a very low capacitance value. For this reason, it is recommended to use PGB2010402 (PGB). Its capacitance value is only 0. 07pF, it will not attenuate the RF signal.

2) Audio interface (headphones, speakers and microphone)

These circuits carry audio signals between 20 Hz and 30,000 Hz. Since these are low frequency circuits, the capacitance of the ESD protector does not need to be minimal. However, by carefully controlling the capacitance (20pF to 30pF), it is possible to obtain some EMI filtering while performing ESD protection. To meet these characteristics, the recommended solutions include V9MLA0402L (MLV) and SP1003-01DTG (SPA). It should be noted that the SPA product has a lower clamping voltage than the MLV device, so it may be a more appropriate choice if the audio CODEC chipset is very sensitive to ESD.

3) High-speed digital interface (such as USB2.0)

The data transfer rate of the USB data bus has been increased from 12Mbps (USB1.1) to 480Mbps (USB2.0). Since the data transfer rate is so high, it is necessary to minimize the capacitance of the ESD protector to prevent the data signal from producing any distortion. For this reason, it is recommended to use PGB2010402 (PGB) and SP3003-02XTG (SPA) for protection on the D+ and D- lines of the USB bus. Also, it is important to understand the ESD sensitivity of the chipset (digital baseband or USB transceiver). If they are very sensitive to ESD, the SPA product line is a better choice.

4) LCD module / SIM socket / SD and MMC interface

In general, these circuits are not high speed. Its data transmission rate is generally limited to 20 Mbps or lower. Therefore, if the capacitance of the ESD protector is 40pF or lower, there will be no signal integrity problems. Products that meet this requirement are V9MLA0402L (MLV) and SP1001-04XTG (SPA).

It should also be noted that some LCD modules and smart cards are using high speed interfaces up to 200 Mbps. Therefore, as with USB 2.0, it is necessary to minimize the capacitance of the ESD protector. The recommended solutions for these high speed buses are PGB2010402 (PGB) and SP3003-04XTG (SPA).

Finally, for slider and flip phones, flexible circuits that connect the motherboard to the display panel can be susceptible to electromagnetic interference and ESD damage. In these cases, the solution should be able to perform filtering and ESD suppression. The SP6001-04UTG and SP6002-04UTG include just integrated ∏ filters and ESD protection.

3) Keyboard and buttons

These interface circuits are simply DC switches that operate at 5 VDC or less, but they provide a way for ESD to enter the analog baseband chipset. The selection decision can be based on the shape factor of the ESD suppressor. For discrete devices, V5. 5MLA0402 (MLV) and SP1003-04DTG are recommended. If you want to choose a space-saving multi-channel array, it is recommended to use SP1001-04XTG.

4) Charging input port

The charging input provides power for recharging the Li-Ion battery pack, but it can also introduce ESD or lightning surges into the phone. The devices optimized for combating these two types of transient surges are V5. 5MLA0603 (30A, 8 x 20μs) and V5. 5MLA0402 (20A, 8 x 20μs).

5) Wiring guide for ESD suppressor

Here is the last point that needs to be added. If the ESD suppressor is not placed in the correct position on the PCB, the effectiveness of the selected ESD suppressor may be significantly reduced. To ensure that ESD transient surges are immediately shunted and reduced to the lowest possible level, the ESD suppressor should be installed as close as possible to the connector (ESD gateway). It should also be as close as possible to the data or signal lines. This will minimize the trace length and ensure the best possible ESD protection level.

6) The unique advantages of Littelfuse products

Littelfuse offers many unique benefits to the market. As a manufacturer with all three ESD protection technologies, we can provide the best solution for our customers. As mentioned above, each technology has its own characteristics that need to be weighed according to the needs of the circuit. In addition, Littelfuse has an all-round laboratory in Wuxi, China, where component- and application-level ESD testing can be performed. Examples of these capabilities include ESD pulses (IEC61000-4-2), surge pulses, insertion loss, and bit error rate (BERT) tests. We have technicians including field application engineers who can help customers complete the ESD suppressor selection process.

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