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ISO3088 Transient Immunity and Line Biasing

Shrivardhan BS Rao
Prodigy 120 points
Other Parts Discussed in Thread: ISO3088

Dear Sir,
We are using ISO3088 in our product and would like to have feedback on following items:
Our product will be used in the open area with medium to harsh environment.

1. Transient Protection.
2. Line Biasing.
3. No. of layers in PCB.

1. Transient Protection:
-------------------------    
We are using TVS dides as shown in the attached image.


However, we are not using TBU or GDT in our design.

As per the datasheet of the ISO3088, it is already having CMTI (Common-mode transient immunity) of 25-50kV/uSec.
and ESD protection for up to 12kV.

Question: Do we need to have further protection of TBU and GDT??

2. Line Biasing:
-------------------------    
AS per the data sheet ISO3088 has internal fail-safe biasing.

Question: Do we need to have external pull-up on A line and pull-down on B line??

3. No. of layers in PCB:
-------------------------    
As of now our design is having 2 layer PCB.
This is board does not have any other high clocking device and the RS485 communication maximum speed is of 38kbps.

Question: Do we need to go for 4 layer board with power and ground plane of 2 layer is sufficient??

Please clarify all three questions so that we can go for production at the earliest.

Regards,

Shrivardhan BS

  • 0
    TI__Guru 55795 points
    Hello Shivardhan,

    The amount of protection required completely depends on the kind of application ISO3088 is being used and kind of transients expected. Please do help us with more details on your application and transients expected to be able to comment better.

    Please find below my response to your questions,

    1. For most applications TVS suffices the need, TBU & GDT are generally required for much higher power handling.

    2. Since ISO3088 has internal fail-safe biasing, pull-ups or pull-downs are not required on A/B lines.

    3. Since the datarate is low, 2-layer PCB should be sufficient. I am assuming the bottom layer is a GND layer split (GND1 & GND2) with necessary clearance / creepage.
  • 0 in reply to Koteshwar Rao
    Prodigy 120 points

    Koteshwar Rao,

    Thank you for the reply.. I was on leave, could not reply earlier.

    I have added RS485 section schematic.

    As of now we are using only the TVS diodes for protection.

    The unit is housed in metallic IP68 casing in open environment.

    These units are installed in very harsh conditions like thermal power plant, oil rigs and oil loading and unloading terminals (not petrol stations).

    Our design team is discussing whether to continue with the ISO3088 or switch to the other makes like ADM2485 or LTM2881.

    Do you have any comparison chart or some information which says ISO3088 is better than other two.

    Regards,

    Shrivardhan BS Rao

  • 0 in reply to Shrivardhan BS Rao
    TI__Intellectual 2855 points

    Hi Shirvardhan,

    Schematic looks fine.

    Can you please elaborate on the requirements?

    Are you just looking for an isolated RS-485 solution?

    Regards

    Tejas H

  • 0 in reply to Tejas Hommaradi
    TI__Guru 55795 points

    Hello Shivardhan,

    I just had one comment, the TVS diodes indicated in the schematic look to be unidirectional. I see they are connected in the opposite direction which could pull RS-485 Bus pins to LOW all the time. Or are they bidirectional? Could you please confirm?

    Could you please share us your e-mail ID so that we can send you more information on ISO3088? Thank you.