LTC2862A input protection

In the datasheet of the LTC2862A, page 24 there is schematics of a massive input protection I'll just need in our next project. I am used connecting MOV-s shunting the inputs. Now, here they seem to protect the TBUs? Can you shortly describe the idea behind this circuit?

Regards,

Akos

  • Hi Akos,

    I believe they have used this circuit as a simple TVS type protection devices would have too much capacitance! This protection circuit adds only ~8pF of capacitance per line to GND (due to the low capacitance of the GDT and SCRs -bidir thyristors)

    The GDTs offer primary protection and discharge the surge current to ground. The rest of the components are for secondary protection. You're right the MOVs are there to protect TBUs from the fast ESD and EFT transients that exceed the turn on time of the primary protection devices (GDTs). They are for secondary protection. Therefore, the  MOVs clamp the voltage across the TBUs and the TBUs limit the current that pass through the SCRs and the SCRs trigger above 35V to protect the bus pins as we want to maintain the full ±25V common mode range of the receivers.

    Check out page 19 of the datasheet. It describes it pretty well there.

    Regards,

    Naveen

  • Hi Naveen,

    I use a very slow (9600 baud) comm. I need a robust protection because of the harsh electrical environments. Isn't it better in this case to connect the MOVs as shunts in front of the TBUs? A few hundred pF of the MOV doesn't affect such a slow signal  with bandwidth limiting versions of the driver chips considerably. A signal would be visibly deformed by capacitances above 20-30 nF.

    Kind Regards,

    Akos

  • Hi Akos,

    You can absolutely if that's the case. Attaching FAQs from another part that's put together by our designers that may be useful to you. Option 1 is what you're probably looking for. Option 2 is the same circuit given in the datasheet. Option 1 uses MOVs as shunts so yes they can be used as that but it has a slightly more capacitance at the protected pin. You can compare the results that the two circuits achieve and choose whatever suits you more.

    The protection circuit on the datasheet is a low capacitance solution so it would not cause significant impact to the performance of the LTC2862A transceivers at high data rates as you already pointed out.  The high capacitance MOV usually contributes more to the capacitive load on the signal, but on the circuit in the datasheet and Option 2 (which is basically the same), floats on the line and is shunted by the TBU, so it contributes very little to the capacitive load on the signal.

    Regards

    Naveen