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SN65HVD24: Lightning protection design in SN65HVD24

Zhanpeng Li
Intellectual 2130 points
Part Number: SN65HVD24
Other Parts Discussed in Thread: STRIKE

Hi team,

this is brandon. in our before design, SN65HVD24 is linked directly on BUS, which cause the demage when Lighting happened. therefore, we intended to design the Lightning protection to protect our device.

the baisc design solution is shown as below picture:

we may want to know that whether it is ok to set the R at 10ohm with 0402 package?  we also need to make sure that the signal quality should not be influence much with this design.the length of BUS is 250 meter and 84 node. Communication rate is 1Mbps and the Cable characteristic impedance is around 70-120ohm(Ordinary four-core wire).

could you also recommended the value of the resistance connected with 485 and the package size?

BR

Brandon 

  • 0
    TI__Guru 62920 points

    Hi Brandon,

    There are a couple things to consider when evaluating the resistance value.  One is the DC attenuation that it provides due to interaction with the bus resistance.  For example, if the bus is terminated via 120 Ohm resistances at each end (as is typical of RS-485 applications), the differential resistive load of the bus is 60 Ohms.  With 10 Ohms on both the non-inverting and inverting line, you would have an attenuation of (10 Ohm + 10 Ohms) / (10 Ohms + 60 Ohms + 10 Ohms) = 25%.  This can be tolerated in most cases, but may present issues for very long connections which already present a high degree of attenuation.

    The other factor to consider is the low-pass effect created by the interaction of the series resistance and the parasitic capacitive loading of the network.  This effect could slow down the transition times of the differential signal and result in timing distortion (jitter) of the serial data stream.

    In terms of package size, it will depend on the power rating that is needed in order to withstand the expected stress in the environment.  Under normal conditions the current flowing through this resistance would be about 35 mA (assuming again standard dual 120-Ohm termination).  If a short-circuit fault were to occur on one line, the current could be as high as 250 mA if the RS-485 driver is active.  You may also need to consider the transient stresses that could occur during the lightning strike.  If the external clamping circuitry is able to keep the line voltage beneath the absolute maximum rating of the RS-485 transceiver, not much current will flow through the series resistance.  But if the clamping voltage exceeds the transceiver rating, there could be some current flow into the transceiver as its integrated transient protection activates.

    Regards,
    Max