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Original question:

DS15BR401: Different output voltages

SN65LVDS250: LVDS250 fault mechanisms specifically driving into a 2.5V powered LVDS input.

Ty Pangburn
Prodigy 50 points
Part Number: SN65LVDS250


Hello,

I am doing a failure mode analysis for this driver that is on an interface card we use. So is it possible for LVDS250 driver to fault in such a way it would continually supply the 3.3V supply voltage to the receiver and would there be any current limiting or voltage limiting if this happened. I am only considering single point failures. In our situation we have an FPGA LVDS 2.5V supplied input buffer and don't want to destroy it if it gets 3.3V to the input. Also, the 100ohm termination on the receiver is internal so that could destroy the resistor and or the input buffer if the current is too high. It looks like in the datasheet the highest the output could be Vcm-max+1/2Vdiff-max ~1.8V. It looks like this driver can supply 27mA when shorted, which is quite a bit more than the older driver we used on a previous version of this interface card we are using.

Thanks,

  • 0
    Prodigy 50 points

    I forgot I was going to ask wouldn't the output have some sort of common mode voltage regulation circuit. In the equivalent circuit in the data sheet suggests it would short directly to the supply voltage.

  • 0 in reply to Ty Pangburn
    TI__Guru**** 180261 points

    Hi,

    Please let me check on the SN65LVDS250 design and get back to you with an answer as soon as possible.

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    TI__Guru**** 180261 points

    Hi,

    The output has a BJT switch, but this BJT switch would not be saturated for the output to be at 3.3V. 

    Also, the output electrical spec has max of 1.375V common mode voltage and 0.454 differential output voltage magnitude, so the max amplitude of the output signal when driving high will be around 1.8V.

    The 27mA short circuit current is when the output shorted by GND, not across the 100k termination resistor. With the max 1.8V, the current across the 100ohm resistor will be 18mA. 

    Do you have SN65LVDS250 samples you can test this out? 

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Prodigy 50 points

    I discussed this with my lead engineer, they were getting at this same point as well that the max voltage would be around 1.8V for this device because it has to meet the max voltage on the LVDS specification as well. I think this diagram is pretty simplified cause in reality there would have to be some sort of common mode regulation as well as maximum swing on the driver internally either by feedback into the Base of the BJT or by regulating the 'VCC' voltage. I don't have a part I can use to test this, since this is on an adapter card we use. I am hoping the 1.8V maximum will be good enough for us at this time.

  • 0 in reply to Ty Pangburn
    TI__Guru**** 180261 points

    Hi,

    The LVDS250 datasheet OUTPUT ELECTRICAL CHARACTERISTICS guarantees that max voltage will be around 1.8V. So I honestly don't see how the output can be shorted to 3.3V.

    I can get you couple LVDS250 samples if you want to test it.

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Prodigy 50 points

    I think you are right. I was thinking if a single component failed on the die (single point of failure only) in such a way that could happen. Hopefully the part is designed to handle such a fault so that could never happen. What I have seen so far in order meet the LVDS specification they would have to. I don't think I need any parts at this time, and it would be impractical to try to fault it. I let you know if I need anything else tho. Thanks!