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RS422 TVS diode junction capacitance

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ku637

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Hi,

For a design implementation using DS34C87T and DS34C86T, i need unidirectional TVS diodes on the output and input lines for the protection of the device.I'm interested to check the feasibility of using a TVS diode, SMBJ6.5A which has a junction capacitance which @ 7.6V typ(Vbr) is approx 5000pF .Can this capacitance add any problems like stress to device or degradation in terms of driver/receiver signal quality if im using a data rate say typical 115,2k?

Any suggestion will be really helpful.

Thanks
 

Hi,

5nF is too big for a RS422 line. Expect reflections.

Independent of the used baudrate...the receiver is fast..it will transmit up to some MBaud.
There are overvoltage protection circuits with much less capacitance.
I recommend to read some application notes regarding transient voltage suppression.
Especially read about the clamping voltage.

Klaus
 

I didn't yet observe problems with SMBJ6.8 diodes protected RS-485 at 115 kBaud.
 

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