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Need advice for general ESD protections on a 2 layer PCB, the ESD lead to MAX7219 controlled LEDs go dark

Shawn Sun
Prodigy 30 points

Hi all,

I have a 2 layer PCB which have quite a lot of buttons, ALPS EC11 rotary encoders, SPST toggle switches on it, the MCU scan the inputs and send commands to a Max7219 8 digit LED driver, my current board have not applied any ESD solutions so when I touch the switch or encoder's handle it may sometimes fail the LED ( all the 8 digit LED display go dark ), I need to press a "reset" button for the MCU to make the LED works again.

This is the simplified schema and part of the PCB layout :

As you can see the rotary encoder is a SMD type, and it's near the

For this particular case, where should I put the ESD solutions on the PCB?

  a) just after the each rotary encoders and switches. This approach need quite a lot of ESD protectors, right?

  b) just before the MCU keyboard scan pins: Columns and Rows

  c) between the MCU and the Max7219 inputs

Any recommendations are great appreciated .

Thanks and regards

Shawn

  • 0
    TI__Genius 12190 points

    Hi Shawn,

    You will want to place any ESD solutions (TVS) as close to the ESD source as possible. Based on the need to reset, it sounds like you are experiencing a ground shift. Because this is a 2 layer board it does not have a ground plane. This can make it difficult to prevent ground shifts during an ESD event, regardless of where the TVS is placed. What type of power supply does this have? If it plugs in then you have a ground you would want to set the TVS as close to as possible (lowest impedance to ground). If this is battery powered, you may have a difficult time preventing ground shifts while handling the device.

    You could investigate this by using an o'scope to measure the voltage drop between the button/toggle switch/rotary encoder input pins of the MCU and its ground pin (using as short of a ground lead on the o'scope probe as possible) to look for voltage fluctuations while trying to recreate the problem. The same could be done for the input pins on the LED driver. If there is noise found the issue might be solvable with bypass caps on the signal pins adjacent to the IO pin of the IC.

    Of course, having a 4 layer board with one or two solid ground planes is a very good idea, too.

    Regards,

  • 0 in reply to Guy Yater
    Prodigy 30 points

    Hi Guy

    thanks so much for your detailed information, I'm trying to use some TVS array but you mentioned that there lacks of a dedicated ground layer, so if I pour  ground zone as much as possible on the back of the PCB will that help ? By reading SLAA530 - "MSP430 System-Level ESD Considerations" I can see that one example layout used gound zone to improve the ESD immune.


    Thanks

    Shawn

  • 0 in reply to Shawn Sun
    TI__Genius 12190 points
    Hi Shawn,

    A ground pour for your logic ground should help with stabilizing the voltages. ESD can come in very low levels too, like a few hundred volts instead of the 8 kV we usually think about. These low ESD events, while not necessarily causing hard failures, can wreak havoc on a vulnerable logic ground by causing ground shifts and all the related problems associated with that.

    Regards,