Dear all,
We think to add varistor between sensor and CINx in order to protect IC from ESD.
Is this way correct? Can I add passive device between sensor and CINx?
Best regards,
Hey Takao,
What you propose could work in this application however, if you have designed the capacitive plate sensor properly there should be little need to protect the chip from ESD. Since you are in automotive and want extremely high reliability, here are some different options though.
- Standard Varistor (your solution):
This will prevent the high voltage spikes of ESD but are typically fairly slow turn on (usec) are and high capacitance. You are able to buy fast varistors (nsec) but the high capacitance will remain, causing excess capacitive loading which will need to be measured and accounted for in your logged data.
- Use a Low Capacitance Varistor:
This type of varistor is traditionally used in the telecommunications industry because it has low capacitive loading to leave the circuit unaffected and have faster turn on times (nsec). The downside is that due to their physical construction they can't handle high inrush current. Most likely not a problem for this application since we're talking about what are essentially sense lines.
- Bidirectional BJT Zener Clamp (Schematic Below):
I saved this circuit for last because it is my personal favorite and go to ESD protection. It consists of two NPN transistors with both collectors and bases tied together so that all four are at the same potential. Then the emitters are tied to the rails, in this case it would be inserted into the circuit the exact same way you would a varistor. The threshold for clamping voltage will be the Emitter-Base Breakdown voltage (Vebo); any voltage below that and the circuit stays in high-z, anything above and conduction starts. This circuit isn't typically used because it relies on avalanche breakdown in the transistor which is an atypical operational mode. However, for clamping and ESD it works a treat. Benefits of this circuit include extremely low capacitive loading and extremely fast turn on . The con is increased part count since a varistor will work with one part and this needs two.
If this answers your question, please hit the green "Verfiy Answer" button below!
Best Regards,
Nick
