# Communication Line Protection

I intend to protect a BUS switches ICs, whose voltage on its pins cannot exceed 5.5V and 3.3V. The current cannot exceed 16mA.

The circuits needs to opperate from 3.3V to 5V. But automotive enviroment will need protection for continuous 14,8V at the input. So, I'm using a voltage-clamp to prevent against overvoltage, a resistor to prevent against overcurrent and a TVS diode to prevent against ESD.

The problem is to choose TVS diode. A TVS with voltage breakdown below 14,8V would burn at continous voltage. So, the voltage breakdown needs to be greather than 14,8V. For a TVS with this value of voltage breakdown, I didn't find one with clamping voltage smaller than 21,5V and I'm afraid that a resistor of of 360/0.5W will not be enough.

For 5V signal-line: (21.5-5)^2 /360 =0.756 W

For 3.3V signal-line: (21.5-3.3)^2 /360 =0.920 W

The question is:

I suppose the ESD events are very short in time, will it be enought to fry the resistor of 360/0.5W ?

The problems to choose greather values for resistance are the interference on communication line and the size of resistors that will ocupy more space on the PCB.

I suppose the ESD events are very short in time, will it be enough to fry the resistor of 360/0.5W ?

IEC 61000-4-2 ESD events will peak around 10 amps after about 1 ns and trail off becoming less than an amp after 100 ns: -

Image from here. I recommend you double check this by reading various on-line articles such as this. However, it's very unlikely that the resistor will suffer any damage given that the peak voltage it sees will be around 21.5 volts for far less than a microsecond.

I doubt that the zener diode will suffer any problem either. It's worth applying high-frequency decoupling capacitors on the Vcc rail to ground close to where the diodes are.

Have you considered that the 360 Ω resistor could probably be a few kohm?

• "... it's very unlikely that the resistor will suffer any damage given that the peak voltage it sees will be around 21.5 volts for far less than a microvolt." I thought that too, but I'm not sure of how to estimate this. About consider a resistor of a few Kohm, I can say that a I considered the smaller value that could protect the circuit. I'm afraid of it interfere in the digital communication Commented Jul 12, 2021 at 20:46
• What data rate or clock rate? Commented Jul 12, 2021 at 21:24
• It can go up to a few units of MHz. Maybe 1 or 2MHz Commented Jul 12, 2021 at 21:28
• What is the input capacitance of your receiver? Commented Jul 13, 2021 at 7:24
• So, you shouldn't pick an input resistor value that is higher than a value that creates a low-pass cut-off frequency of about 10 MHz. That means you could fit a 2k2 resistor. Commented Jul 15, 2021 at 10:25