静電気放電（ESD）現象からキーボードを保護する方法

The Importance of Keyboard ESD Protection

コンピュータの黎明期から、キーボードは人間と機械の間の主要な通信メカニズムでした。長年にわたり、多くのテクノロジーやインターフェースが変化してきましたが、キーボードは形状も機能もほとんど変わっていません。最近、専用のワイヤレスキーボードを購入しました。当初はBluetooth経由でコンピュータと問題なく通信できましたが、一か月ほど経つと、突然動作が止まりました。内部回路を開けてみて驚いたのは、キーに静電気放電（ESD）保護機能がないことでした。電子デバイスのキーパッド、サイドキーや押しボタンは、常に人間と接触するため、ESDに対して脆弱です。キーボード、キーパッドやサイドキーにESD保護デバイスを追加すると、キーボードの故障などの悲惨な状況を回避できます。以下では、ESD全般と、電子デバイスをESDから保護する方法について説明します。

Electrostatic Discharge (ESD) and Its Impact on Keyboard ESD

ESD happens as a result of static electricity discharges. Static electricity is the combination of charged particles on the surface of any device or element. When an element with a higher charge comes in contact with another element with a lower charge, electrons flow from one element to another. An electric field can also induce the sudden flow of electrostatic charge. This transfer of charges may lead to a rapid flow of high current for a short time, and an ESD event occurs, reaching up to thousands of volts. Think of it this way; what if you were hit by lightning?  That is a naturally occurring ESD event.  Every time an ESD event happens, you have the potential to damage the internal circuitry of the keyboard controller, thereby causing it to fail; tiny lightning strikes happen to your equipment. The keyboard/keypad/side keys are contained in a non-conductive case, and touching the contacts directly with a finger is impossible. However, the flow of electrostatic charge can happen via air discharge, even in a non-conductive case. Hence, keyboard/keypad/side key ESD protection is not just important; it is essential to the long-term dependability of the controller circuit.

Recommendations on Protecting a Keyboard from ESD with Circuit Protection Devices

ESD protection can be achieved by placing a transient voltage suppression (TVS) diode on a data line to protect against transient events in less than a nanosecond. Under normal operating conditions, the TVS diode presents a high-impedance path to the protected circuit, making the TVS appear open. During a transient event, the TVS diode offers protection by providing a low-impedance way to divert the transient current from the circuit. You can learn more about what TVS diodes are here. 

Figure 1. ESD protection of a keyboard

Examples of ESD Protection Circuits and Devices for Keyboards and Side Key Protection

Figure 1 shows the ESD protection of a keyboard matrix with TVS diodes. Keyboards are normally powered at an operating voltage of 5V via a USB interface or 2 to 4 AA batteries for wireless communication. Semtech's TVS diode, µClamp®5031ZA, features outstanding ESD protection characteristics. It has an operating voltage of 5V and is rated for a maximum peak pulse current of 7.5A (tp=8/20µs). The typical clamping voltage at a peak current of 4A is 7.5V. The dynamic resistance shown in Figure 2 is only 0.008Ω. µClamp5031ZA provides transient protection as per the specification in IEC 61000-4-2 (ESD) at ±30kV (Air) and ±30kV (Contact). The ESD clamping waveform of +8kV contact is shown in Figure 3. This bidirectional TVS diode is available in a tiny 0201 DFN package (0.6x 0.3 x 0.25 mm).   

Figure 2. TLP characteristic of µClamp5031ZA    

Figure 3. ESD Clamping (+8kV Contact) of µClamp5031ZA

Side Key Protection for Portable Devices

Another part we can mention here for protecting either a keyboard or side keys of a portable device is µClamp0541Z. The side keys may suffer from ESD strikes when they come in contact with a user. Most of the side keys of any portable device are DC switches working at an operating voltage of 5V or less. Semtech's µClamp0541Z features superior ESD protection characteristics and is perfectly suited to safeguard side keys and keypads from ESD events (Figure 4). It has an operating voltage of 5V and is rated for a maximum peak pulse current of 2A (tp=8/20µs). The typical clamping voltage at a peak current of 2A is 15V. µClamp0541Z is capable of withstanding high ESD with voltages ranging from ±17kV (Air) and ±17kV (Contact) as per IEC 61000-4-2 standard. The low dynamic resistance of 0.78Ω and a minimum reverse breakdown voltage of 6V ensure system-level protection against transient events. The TLP characteristic of the part is shown in Figure 5. The bi-directional µClamp0541Z comes in industry-standard ultra-small 0201 (0.6 x 0.3 x 0.25mm) packaging.

Figure 4. ESD protection of side keys

Figure 5. TLP characteristic of µClamp0541Z

Multi-Line ESD Protection for Keyboards and Keypads

So far, we have been discussing single-line protection from ESD. However, the number of keypad/side-key protection lines varies with the applications. Semtech offers options for the protection of multiple lines. At the same time, engineers can select unidirectional or bi-directional TVS diodes per the protection requirements. Ultra-low capacitance is not compulsory to protect the side keys and keyboards. Figure 6 shows the protection of four data lines of a keypad with Semtech's µClamp0504A. µClamp0504A has an operating voltage of 5V and is rated for a maximum peak pulse current of 7A (tp=8/20µs). The typical clamping voltage at a peak current of 7A is 12V. µClamp0504A is available in a SOT-666 package (1.7 x 1.7 x 0.6 mm).    

Figure 6. ESD protection of four lines of a keypad

Ensuring Longevity and Reliability with ESD Protection

A keyboard, keypad, or side keys are still essential parts of our electronic devices in 2023. When you are designing a keyboard for a computer or any other device with keypads and push buttons, you should strongly consider adding protection devices like ESD protection circuits or ESD protection diodes to ensure that they are safe from ESD surges and to create a longer life and a higher perception of quality in your product. It may certainly add components to the BOM, but the users will appreciate a fault-free, glitch-free use of a keyboard for years rather than buying a new one in a short period of time. The last thing any user would want is to experience lost communication to their computer or failure of their new input device. Semtech is a prominent manufacturer of TVS diodes that protects many of the world's most modern electronic devices. Use Semtech's TVS diodes to foolproof your circuit. Learn more about Semtech's Circuit Protection products at the link below.

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