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TCA8418: position of the resistor VALUE

zhang ping
Genius 3655 points
Part Number: TCA8418
Other Parts Discussed in Thread: STRIKE,

Regarding the ESD protection of this chip, according to the specifications, there is built-in ESD protection. However, even if we apply static air of 10KV, it will still cause damage to TCA8418.

We now have reserved series resistor positions and TVS positions on the circuit.

What size of resistor can be selected for the position of the resistor, without affecting the function of the button matrix? Thank you

  • TI__Mastermind 40853 points

    Hi Zhang,

    zhang ping said:
    Regarding the ESD protection of this chip, according to the specifications, there is built-in ESD protection. However, even if we apply static air of 10KV, it will still cause damage to TCA8418.

    TCA8418 has +/-2kV HBM and +/-1kV CDM. I wouldn't expect this chip to survive a 10kV strike. 

    The TCA8418E has added ESD protection. HBM for the GPIO pins is strengthened to +/-15kV. Not sure if the "E" version of this keypad scanner would help in this case. 

    zhang ping said:
    We now have reserved series resistor positions and TVS positions on the circuit.
    zhang ping said:
    What size of resistor can be selected for the position of the resistor, without affecting the function of the button matrix? Thank you

    The value of the resistor is dependent on a couple factors such as the maximum allowable rise-time for your system and the total parasitic bus cap present on the lines connecting the push button. Series resistance basically acts like a low-pass filter. For some larger series resistance like 100k, I would expect very slow transitions which would mean that if the button was pressed very fast, consecutive transitions might not be recognized due to slow rise-time. 

    Choosing a series resistance can help with limiting the total current through the path. A series resistance such as 100 ohms should be okay, + TVS diode for added external ESD protection from much larger voltages (+/-10kV). 

    Regards.

    Tyler