I have two DB37 connectors on my board which ultimately connect to a CPLD. All of these connections/signals are inputs to the device.
To protect from ESD I am using TVS Diodes ESD9C3.3ST5G. I have the board like so:
DB37 -> Diodes -> Pullup resistor -> CPLD.
The 1K pullups are for a different purpose and are not related to ESD protection. My PCB is 4-layers with the following stackup:
The diodes connect to the ground using a via. The trace to the via is thick - thicker than the trace to the CPLD. The ground plane is completely unbroken with the exception of through-hole pads and vias. I assume this protects against at least some mild ESD. But what do I need to do further? This is not a commercial device and will be used internally - however I do need it to be reliable.
- One of the things I thought was to add series resistance (22 Ohms or so) between the diode and the CPLD. However, as all the pins on the CPLD are inputs so they're already high-impedance. The ESD should go towards the ground via the TVS diode. Is my assumption correct?
- I have also read that adding a capacitor in parallel with the diode can help. My signals aren't high speed so this shouldn't distort them much. However, do note that I'll have to 74 of these caps as I have 74 signals. So before I went and added these I wanted to know if this was worth it.
Here's a closeup of the layout:
Finally, one last question - the above described the input side of my board. The output is similar in the sense that I have two further DB37 connectors and a CPLD. In this case, the CPLD's pins are outputs.
The layout is like so: CPLD -> MOSFET -> DB37
In this case, I don't have any diodes. However, as I've read recently, MOSFETs are far more sensitive to ESD than other devices, should I add diodes here as well? The MOSFET's drain is connected to the DB37. This DB37 is then connected to the input-side DB37 described earlier.
If a MOSFET is on, it's drain-to-source resistance would be quite low. And as such, this could prove an attractive path for the ESD pike to go through rather than the TVS diodes on the other end. Am I correct that I should add TVS diodes here as well? If so, oh boy, 72 more diodes!