LTC1685 ESD

Hi!

I use a LTC1685 transceiver pair in an application. To protect the input/output A and B pins of the LTC1685 I use a common mode chocke and TVS diode CDSOT23-SM712. However, the capacitance of the TVS diode is 75pF and the system fail to transmit/receive at 52Mbit/s if I use these TVS diodes. If I remove them the system works fine. Hence two questions are raised:

1. What is the ESD performance of the LTC1685? Can I use is "safe" without any extra TVS diodes?

2. The loading from the TVS diodes should be 75pF. Assume 50R cable the capacitance is about 100pF/m. The length of the cable that loads the LTC1685 should be approximately the length of a rising/falling-edge about 3.5ns, assume propagation speed about half light speed, 1.5E6 m/s, the corresponding length is 3.5ns*1.5E6 = 0.52m. So loading should be 100pF/m * 0.52m = 52pF. A total of 75pF + 50pF = 125pF. Maximum load capacitance is 500pF according to datasheet. According to this coarse estimation, it should not be any problem to use the TVS diodes. Maybe I miss something?

Best regards

Joakim

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  • +1
    •  Analog Employees 
    on Dec 21, 2018 3:47 AM

    Hi Joakim,

    There is ±4kV HBM ESD protection on bus pins of LTC1685, if the part is used in hazardous environment higher ESD protection is needed. As you mentioned "50R cable", is that characterized as 50 Ohm impedance cable? Have you checked the waveform at cable input and output side before and after you added the TVS? Hope you can give more detailed information.

    Regards,

    Neil

Reply
  • +1
    •  Analog Employees 
    on Dec 21, 2018 3:47 AM

    Hi Joakim,

    There is ±4kV HBM ESD protection on bus pins of LTC1685, if the part is used in hazardous environment higher ESD protection is needed. As you mentioned "50R cable", is that characterized as 50 Ohm impedance cable? Have you checked the waveform at cable input and output side before and after you added the TVS? Hope you can give more detailed information.

    Regards,

    Neil

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