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SN65LVDS31: LVDS output / drive for non-100 ohm loads

Todd Reed
Prodigy 110 points
Part Number: SN65LVDS31

Hello,

We have this device driving an unpowered FPGA and we are trying to figure out the implications.

From the datasheet, the drive current is typically 3.4mA across a 100 ohm resistor and short circuit current for one of the output pins (neg or pos) is 4 to 24mA.

In our system, the LVDS output will go through a cable and then forward bias the power clamp diode in the FPGA.  Since there is some load, can we assume that the current will remain around 3.4mA, or could the current go higher?  Is there a current vs load graph for this part that would show this?  How is the 24mA max short current created?

Also, if there is no load will the LVDS output high side be at 3.3V?

Thanks,

Todd

  • 0
    TI__Expert 7730 points

    Hello Todd,

    1) Yes, the output current will remain around 3.4 mA. Most LVDS drivers will have a typical output current of 3.5 mA.

    2) We don't have a current vs load graph for this device

    3) Our short circuit schematic is found below. It states that if we force a voltage (in this case -1V or +3.4V ) to outputs A / B, the current flow is guaranteed to have -24 mA (max).

      

    Regards,

    Josh

  • 0 in reply to Joshua Salinas
    Prodigy 110 points

    Hi Josh,

    Thank you for the response!  However, the schematic picture is not showing up for me.  Can you post it again or attach it or something?

    Thanks,
    Todd

  • 0 in reply to Todd Reed
    TI__Expert 7730 points

    Hello Todd,

    Thanks for the heads up! I have reposted the image below. Are you able to see it now?

    Regards,

    Josh

  • 0 in reply to Joshua Salinas
    Prodigy 110 points

    Yes, I can see it, thanks!

  • 0 in reply to Todd Reed
    Prodigy 110 points

    Hi Josh,

    If there is no load, what would the output voltage be?  The driver is a 3.3V device, but it is driving a 2.5V bank.  The LVDS voltages are compatible, but is there concern that the voltage could go above 2.5V if there is no load?

    Thanks,

    Todd

  • 0 in reply to Todd Reed
    Prodigy 110 points

    Hi Josh,

    One more follow up.  For the 24mA short circuit, I wasn't asking how it was tested, I was trying to ask what happens inside the IC to cause the 24mA, since the nominal current is only 3.5mA.

    Thanks,

    Todd

  • 0 in reply to Todd Reed
    TI__Expert 7730 points

    Hello Todd,

    Let me bring this up with my team to see why we get 24mA for short circuits. I will give an update within a day.

    Regards,

    Josh

  • 0 in reply to Todd Reed
    TI__Expert 7730 points

    Hello Todd,

    First question, if there is no load, what would the output voltage be? I don't have the SN65LVDS31 EVM on hand but I was able to use the DS90LV011 and its EVM. Having Vcc = 3.3V and no input, I measured Out_P = 0V and Out_N = ~ +2.52V. When having 3V at the input, I measured Out_P = ~ +2.52V and Out_N = 0V. 

    The voltage that you see is the output common mode voltage. When adding a 100Ω termination, this brings down the voltage to 1.25V. I would take precaution since having no load can possibly go above 2.5V.

    On your second question, it states that during a short circuit scenario (outputs are shorted to ground) the maximum current flowing from Vcc to ground will be 24mA. This current is not produced by the part itself.

    Regards,

    Josh