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ISO7710: Power-up delay

BEN AMOR Hamdi32
Prodigy 30 points
Part Number: ISO7710

Hi, 

Am designing a board where i need to isolate some digital inputs. This design suppose to replace a part of an old design (a new daughter board for an existing mother board).

The inputs to this board are logic signal out of micro controller and the customer request that those outputs need to be electrically isolated from the daughter board. The problem is there is no ground signal, the old design just use a 5V rail and the i/o of the micro-controller as an open-drain with pull-up. This situation will be perfect for an opto-coupler but this open-drain is not capable of 20mA

So what am planing to do is to use the ISO7710 as isolator and power up the ISO7710 from this 5V and the this logic open-drain (VCC1 will be wired to 5V, GND and IN will be linked to the OD input, VCC2 is always powered up). i will use the model with default output high so when VCC1 is off it will have the same effect of the pull-up resistor when OD is off.

Signal speed is not high. But didn't find any information about power-up delay, since basically the device will be switching on and off. instead of toggling IN logic level.

Thanks.

Hamdi.

  • 0
    Guru 244230 points

    What is the signal speed? How much current is available?

    Wouldn't an optocoupler be much simpler? Why do you want to replace it with a digital isolator (are you assuming that all optocouplers need 20 mA)?

  • 0 in reply to Clemens Ladisch
    Prodigy 30 points

    Hi,

    Maximum 5Mhz.

    totally agree an opto-coupler be much simpler, however the motherboard use a 80C51 directly connected to this daugther board. 80C51 can deliver 15mA max on a single output and something around that from the whole port. I have to use two pins that can be enabled in the same time, so this make it 7.5mA per pin. Opto-Coupler can be triggered with courant around 5mA but i think it's a bit risky. I had before some boards where opto-coupler current was too close to the minimum they stop switching after the system heat-up a bit.

    Hamdi

  • 0 in reply to BEN AMOR Hamdi32
    Guru 244230 points
    Optocouplers like the TLP2361 have a much lower guaranteed threshold current. And there are digital isolators (but not from TI) with LED-compatible inputs (e.g. Si8719, IL610).
  • 0 in reply to Clemens Ladisch
    Prodigy 30 points
    Cool, down to 2mA, this should be good for this design. Thanks a lot.
  • 0 in reply to BEN AMOR Hamdi32
    TI__Intellectual 2855 points
    Hi Hamdi,

    TI Digital Isolators can efficiently function even with open drain configured Micro-controller I/O. With TI Capacitive Isolation technology, you get more reliable solution and greater benefits over opto isolation solution.
    Please share your schematic and we can provide application specific suggestions.

    Meanwhile I have few questions,
    1) What is 5V supply referenced to? what does "no ground signal" mean?
    2) How many signal lines needs to be isolated?
    3) Can you please share the block diagram of open-drain logic system that you are planning to design using ISO7710.

    Looking forward for your reply.

    Thanks,

    Regards
    Tejas