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AM26LS32AC: Device behavior when VCC for the receiver IC is turned off and the input pin is pulled up

Isao Takigasaki
Intellectual 2490 points
Part Number: AM26LS32AC

Hi,

My customer is considering about the device behavior when VCC for the receiver IC, AM26LS32AC, is turned off and the input pin is pulled up by output of the driver.  Then, the VCC for the driver is applied.   Is there any problem in the situation?

Customer worry about that any current flow into VCC line of receiver through the input pin of receiver.
I believe that It's no problem based on internal circuit which is described in the datasheet, if there isn't any additional circuit, ESD circuit or something,  in actual device.

Best Regards, Taki

  • 0
    TI__Guru 62910 points
    Hello Taki-san,

    You are likely correct given that the majority of our differential receivers are designed not to become partially powered via the bus voltage, but since this is an older device I would like to look deeper into it to be able to confirm this definitively. I will try to obtain some units to perform this testing and will follow up with you next week on the progress.

    Regards,
    Max
  • 0 in reply to Max Robertson
    TI__Guru 62910 points
    Taki-san,

    I am unfortunately still waiting on the shipment of units - the carrier claims to have attempted delivery a few times without success (which should not be the case since the delivery time was during our normal business hours). I have had this happen before, and it generally takes a few days to resolve. Sorry for the delay - I'll let you know once I am able to start the testing.

    Max
  • 0 in reply to Max Robertson
    TI__Intellectual 2490 points
    Max-san

    Thank you for your effort on this. I will wait for your update.
    Customer will use also AM26LS33 as the reciever instead of AM26LS32. I believe its input circuit is as same as AM26LS32, so that the result of AM26LS32 can cover the case of AM26LS33. Is my understanding correct?

    Best Regards, Taki
  • 0 in reply to Isao Takigasaki
    TI__Guru 62910 points
    Taki-san,

    Yes, the same answer would apply to AM26LS33 or AM26LS32.

    I have contacted the carrier to confirm the address and shipping instructions. They tell me it will be delivered by the end of the day tomorrow. I hope that is the case, but I cannot be certain. Due to the US holiday this week, though, I will most likely have to test the units next week unless they arrive sooner than predicted.

    Regards,
    Max
  • 0 in reply to Max Robertson
    TI__Intellectual 2490 points
    Hi, Max-san

    Thank you for your answer and your effort to arrange the evaluation. I got it.

    Best Regards, Taki
  • 0 in reply to Isao Takigasaki
    TI__Guru 62910 points
    Taki-san,

    Fortunately, I have received the units today in time to take some measurements. There is no notable leakage from the "B" line to VCC, but there is a 100-kOhm leakage path from "A" to VCC. However, voltages that can transfer through this leakage path are clamped to about 650 mV. That is to say, when VCC is measured with a high-impedance voltmeter it will be equal to the voltage at "A" when the input level is below 650 mV but remain at 650 mV as the voltage at "A" increases further. I believe this means that most applications will not encounter any issues, since most circuits that could be using the VCC rail wouldn't turn on at such a low voltage.

    I hope this helps - please let me know if I can provide any further information.

    Best regards,
    Max