Electronic – TVS protection for CAN bus

The point of ESD protection is to give high voltages an easier path to ground than through the sensitive components on the board. In general, the protection devices will thus be connected to the circuit's ground.

If the case is solidly grounded (either to an earth ground or to another common ground, such as a vehicle chassis), a case may be made for including it in the ESD protection scheme. The idea is to dissipate as much of the power of the discharge as possible in a small amount of time, and the added metal and common ground will help that. However, when introducing an enclosure into a circuit, additional factors come into play:

• Ground loops - if you're not prepared for this possibility, the enclosure should not be considered as a ground
• Connection capacity - your connection to the chassis should allow for high (albeit momentary) currents
• Interference - if the design ends up connecting the ground plane to the enclosure, there is a new point of entry for undesirable signals that may disrupt the operation of the circuit

The end result is that it's much easier to avoid involving the enclosure in any ESD protection schemes. There may be advantages to doing so, but there should be careful consideration involved.

Here is an EDN article on ESD mitigation measures for high-performance serial busses: http://edn.com/design/analog/4314519/Maximizing-EOS-and-ESD-immunity-in-high-performance-serial-buses.

Almost every charger and regulator out there nowadays has an absolute max of 6V. If you have a USB rated TVS and some capacitance on the 5V bus, you probably won't have problems there. You can use a high voltage cap if you want, and put it close to the barrel jack.

For buttons, use a series resistor (as large as you can accept without causing problems with your pullup or pulldown) and a shunt cap. Put the resistor near the button and put the cap near the IC where the signal goes. You probably don't need TVS on the buttons, but it is a good idea to put it in the layout (close to the IC) in case it is needed. A wide variety of 0402 sized ESD diodes are available. If you have a large series resistance between the ESD gun and the IC, the ESD diode will not have to dissipate much energy. The capacitor will help prevent steep dV/dt.

Most importantly, you need to test. You can't solve ESD analytically. You can give it your best shot, but then you need to test to verify.

The two main problems you get from ESD are:

1. IC or transistor inputs damaged (fix with capacitance or ESD diodes or TVS)
2. poorly terminated inputs toggle due to pulse coupling and cause some undesired behavior such as reboot/reset/poweroff.

Rarely does ESD damage anything other than an IC input.

Also make sure you consider case 2. Any reset inputs or interrupt inputs need to be scrutinized. An internal 200k (or whatever) pullup may not be enough. You may want to add either a cap, or a stronger pullup/pulldown.

Finally, the best defense against ESD, when possible, is to keep the gun away from the components by recessing them within a plastic housing. That is mainly a mechanical design issue, though. If the gun tip cannot get near the components, there will be no discharge in the first place.