The Tech Blog

To round up the ESD series, when you are going to choose your ESD protection device, you will see a lot of basic parameters that can confuse a designer. I have listed the major ones so that it makes life simpler for you when choosing ESD parts.

Maximum Working Reverse Voltage/Reverse Standoff Voltage: This is the voltage below which the diode barely conducts(Only leakage current). This is where your diode should be in normal operation. Always ensure that this voltage is above the normal working voltage of your line.

Minimum Reverse Breakdown voltage: This is the voltage level at which the diode begins to conduct and break down, typically 10-20% higher than the reverse standoff voltage. Again this value should be higher than the normal operating voltage of your circuit.

Dynamic Resistance: Its the resistance exhibited by a diode(after breakdown) when an ESD strike occurs. It’s the slope of an IV curve which you see in most datasheets. When choosing diodes, this value should be as small as possible so that the graph generated via TLP(Check prev. post) is steeper(To get a constant voltage at the output, when the current increases).

Clamp Voltage: This is the voltage that the ESD diode will show at its output due to dynamic resistance when an ESD pulse occurs. Its voltage the diode stabilises itself after the initial spike. This is very important as this is the voltage that your circuits downstream will see. Your downstream device should be able to handle this voltage for a small instance of time. This is where the problem arises as this value is usually not provided by the downstream device datasheet. You have to usually play a guessing game. Ideally, we try to get a clamp voltage as small as possible(higher than the normal voltage which it protects) when selecting ESD diodes.

Peak Pulse Current: Maximum current an ESD diode can pass before it’s damaged. It varies based on the IEC ESD protection levels a device adheres to.

Hoping this series of posts have covered the most important aspects of ESD protection, giving you a better understanding of how to choose the right ESD protection device for your circuits.

Last week we focussed on one facet of ESD diode selection which was unilateral vs bilateral and focussed on when to use each one of them. Another critical item to discuss is ESD capacitance. Please don’t confuse ESD Capacitance with an “ESD capacitor” which is an entirely different thing.

Every ESD diode is connected in parallel to a line/trace that it tries to protect from ESD strikes. ESD strikes are usually rare and in the normal operation of the device, ESD diodes ideally should not affect the regular functioning of the circuit. This is where the capacitance of the ESD diode matters. In normal operation, the ESD diode in the reverse biased mode, shows a capacitance due to its PN junction. The value of the capacitance has a huge role to play in the lines it protects. If you use a diode with higher capacitance to protect lets a high-speed signal like USB 3.2 Super Speed TX/RX data lanes it will create a problem. These data rates are in 10+Gbps and this pulls the line low and high at a very fast rate. Adding a larger capacitor diode will reduce its rise and fall times which will cause communication errors and you make fail the USB compliance eye diagrams.

Hence when you select an ESD diode make sure to use low-capacitance ones for high-speed data lines. You can use higher capacitance ones for GPIO/Vbus lines as there are slightly cheaper if needed. TI has a good chart(check images) on the website showing you the different capacitance ranges and which you can use for different applications based on speed.