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High Speed Digital I/O Line Protectors Circuit

2015-01-14 00:14  
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If the application is a high-speed digital I/O lines or high-frequency RF line, then the protection device should not add capacitance that could distort the signal. PolymericESD Suppressors have ultra-low capacitance (typically around 0.05pF), are fast-acting, and have good voltage clamping capabilities. They cannot be used on power supply lines.A 5.1-V zener diode clamps positive-going transients, and a Schottky rectifier clamps negative-going transients. The Schottky rectifier has problems at both ends of the temperature scale. At 125?°C (257?°F), its leakage current can reach 50 when the input line is at 5 V. This leakage is not a big deal unless the input resistor has a value of 100 kfi or more. More troubling, at temperatures below -40?°C (-40?°F), the Schottky rectifier`s forward voltage rises to about 0.47 V, which is perilously close to the -0.50-V max spec that most HCMOS-type /iP`s inputs can tolerate. The third circuit, Fig. 20-3(c), uses two regular silicon rectifiers. One rectifier is connected in series with the input line, thereby isolating the ` inputs from negative-going voltage spikes. The other rectifier is in series with a 5.1-V zener, which clamps positive-going transients.Single line protection components like ESD suppressors series WE-VE also have to be connected from signal line to GND. For power
line protection it is not necessary to use a low cap ESD suppressor, you might prefer a standard SMD  varistor which can withstand higher surges and higher transient energies.
 

I/O Line Protectors