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The following slides are some examples of where ESD diodes would be necessary for ESD protection. This remote control is a good example of a human interface where the electronics within the remote control can be potentially exposed to ESD events through the keypad. There has been a history of air discharge through the small cracks of the keypads that seep through to the PCB and would damage the IC in the remote control unit. The illustration on the right shows a simple topology of the keypad and how each push button sends a signal to the row and column control pins that connect directly to the controller IC. These control lines need to be protected against ESD events that can find a way through the plastic and onto the PCB into the IC. Although the appropriate ESD diode will be application specific depending on the remote controller, as a general case, most remote controllers will operate with a supply voltage of either 3.3 V or 5 V. In other words, the row and column signals going to the IC will be operating in this rail and therefore the ESD diode’s reverse working voltage must be selected to allow the signal to pass without prematurely clamping the signal. As a push button interface, the operating speed is relatively slow, basically as fast as one can push a button. Therefore, the ESD diode’s line capacitance is not critical. Any value over 5 pF would be fine for this type of application. Since these row and column signals tend to be unipolar, meaning that the voltages of the signals are above ground, a unidirectional ESD diode would be preferred. However, a bidirectional ESD diode can be used provided that the forward clamping voltage is fairly low.
