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ISO1228: Sharing Rsurge/Csurge with multiple ISO1228

MAURICIO VIDAL
Intellectual 260 points
Part Number: ISO1228

Tool/software:

Hi everyone:

I'm implementing two ISO1228 on the same board. My doubt is related to the field power side. I'm sorry if this may be trivial, however I want to be sure.

Regarding the pair Rsurge / Csurge, I understand that Rsurge is meant to protect against surge events on AVCC and AVSS in Sink Mode, which is my case. I went with 1K for the default according to the table for Type 1. Csurge is the fixed 4.7uF according to the data-sheet.

However, as I'm using two ISO devices, and this SURGE section is the same for both, can I use a single Rsurge / Csurge pair for the two ISO1228? or should I replicate them as well?

Of course, i understand that each device should have its own pair of Rilim, Cfil, but my doubt is motivated by the statements on the data-sheet than mention that all those components are required for proper functioning of the device.

By the way, Rilim in my case is 0 as I expect IL=2.5mA, and as this resistor along Rsurge are both part of the current limitation, I wanted to be sure I would not affect the circuit just for saving some PCB space and cost by sharing the Surge pair and removing the Rilim footprint.

Thank you!

  • +1
    TI__Mastermind 29185 points

    Hello Mauricio, 

    The two ISO1228s can share the surge components (in the red box). They should protect both devices from surge events to AVCC and AVSS if surge-proof components are used. (from 7.3.1 Surge Protection). Rsurge and Csurge need to be properly rated to handle the impulse since Rilim = 0ohm. 

    Other components cannot be shared. CFIL doubles as a decoupling capacitor, therefore each ISO1228 should have its own CFIL. Also, RTHR protects against surge events on Field Inputs when surge-proof resistors such as MELF resistors are used.

    Best,
    Andrew

  • 0 in reply to Andrew J
    Intellectual 260 points

    Thank you Andrew!

  • 0 in reply to Andrew J
    Intellectual 260 points

    Hi again Andrew, 

    I was thinking about what you mentioned here:

    Andrew J said:
    Rsurge and Csurge need to be properly rated to handle the impulse

    I'm using MELF resistors. However, regarding Csurge, I was planning to use a very similar capacitor than then one used on both ISO18228 evaluation modules (ISO228EVM and ISO1228DFBEVM) which is a C2012X6S1H475K125AC (Ceramic, 4.7 uF, 50 V, +/- 10%, X6S, 0805). Mine is an X7R though. But both references are general purpose multilayer capacitors.

    Should I seek a different kind of capacitor? If so, may I ask if you have any resource where I can learn about it?

    Thank you!

  • +1 in reply to MAURICIO VIDAL
    TI__Mastermind 29185 points

    Hello Mauricio, 

    Yes, you should have a capacitor that is tolerant for the duration of the surge pulse. This document covers the resistor selection (How to Design Isolated Digital Input Modules for Surge Immunity).

    Unfortunately, we are still developing official guidelines for capacitor selection for the ISO12xx families. Therefore, the EVMs are not designed with surge in mind (only for evaluation of the device operation). The current ISO1228DFBEVM has passed 1kV surge with no additional modifications. From my understanding the Csurge should be rated for a 100V+ DC since the surge event is short this rating should be sufficient to pass higher surge pules with surge proof resistors. 

    Best,
    Andrew

  • 0 in reply to Andrew J
    Intellectual 260 points

    Thank you Andrew.
    That's an interesting article.

    Best regards!