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SN65HVD78 external surge protection

Other Parts Discussed in Thread: SN65HVD78


On the datasheet p.22 figure 26 you recommend to use external surge protection.

The MOV1,MOV2 devices recommended are through-hole devices. Customer prefer not to use through hole devices.

Is there any alternative SMD device that can be used instead of this ?

Can the LittleFuse V175SM20 be used instead of the MOV-10D201K ?

Best regards,


  • Hi Nir,

    I compared the absolute max spec in the little fuse part and the Bourns part that is int he datasheet and the parts seem comparable to be. The littlefuse part actually seems like it is a bit more robust that the recommended Bourns parts so I do think there are any issues with this part.

    One thing to note with the MOVs is their high clamping voltage, this is what makes them suitable for things like surge. What other things protection measures is your customer using to help protect against transients?

  • Hi Michael,

    Thanks for your reply.
    We plan to use the same components TI recommended in the DS as appear in the scheme on the right on figure 22 .
    The only exception will be the MOV components which will not be MOV-10D201K.
    I have a question regarding the max capacitance parameter of the MOV – I noticed the Littlefuse has much higher rating – is it problematic for the RS485 interface ? (Actually the BOURNS part also has high capacitance – is it suitable for the RS485 interface ?)

  • HI Michael,

    Would you estimate the higher rating of the Littlefuse could interfere with the RS485 operation as Lior referred above ?



  • Hi Nir / Lior,

    What data rate does the customer want to support? The SN65HVD78 supports data rates up to 50 Mbps which gives us a bit time of 20ns. We can calculate the time constant of of the MOV-10D201k and the V175SM20 using tau = RC. R is 30 ohms, which is conservative for the driver output, and the capacitance or each MOV is 1400pF and 500pF respectively. Tau for the V175SM20 would be 42ns and tau for the MOV-10D201k would be 15ns so neither of them would be a good choice for 50Mbps since the bit time is 20ns. At 50 Mbps we would need to find something in the 100pF range to allow enough time for that capacitance to charge so the bits do not get destroyed. If the customer is planning on a slower data rate than we can recalculate and see where we stand with the two proposed MOVs and the bit time.

  • Hi Michael,

    we want to support 250 Kbps with cable length of 100 meter.

    correct me if i am wrong - Acording to the example you wrote  - i assume we should be ok  with both of the MOV options



  • Hi Lior,

    The bit time at 250kbps is 4us, so with a tau of either 42ns or 15ns will be OK. Of course your customer should perform some system level testing to ensure invulnerability but I expect that there will be no issues since the bit time is so long. Please let me know if you have any more questions?

  • Thank you for the support !