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DS90LV804 Maximium Rating differential Input - Protection of device?

Stefan Seiwert
Prodigy 20 points

Other Parts Discussed in Thread: DS90LV804

Will the device be damaged, when input is driven by external LVDS Signal during its Supply Voltage VDD is off? How to protect device without affecting performance of the link? Protection needed? (Situation: Device used in Camera Link Framegrabber as Input Device, Camera is powered on, Framegrabber is powered off)

  • 0
    TI__Guru* 97656 points
    Moving to high speed interface forum for best support.
  • 0 in reply to Emrys Maier
    TI__Expert 3175 points

     

    The LVDS input pin is protected by a ESD diode from pin to VDD, and diode from GND to pin.

    Under power off condition, and if there is input voltage applied to the LVDS input, it can forward bias the protection diode and present current flow from input pin to VDD.

    One option to avoid this undesirable situation is to use AC coupling instead of DC coupling.

    Regards,

    TK Chin

  • 0 in reply to T.K. Chin
    Prodigy 20 points
    1. AC Coupling can show problems with signals with DC component like Camera Control CC1 to CC4 from Framegrapper to camera.
    2. In Datasheet of DS90LV804 is a footnote at "LVDS Input voltage" Maximum Rating: "V ID max < 2.4V". What does it mean?
    3. I heard from two other engineers that it happens very often that one part of an (arbitrary) LVDS Link is powered off. The link should not be damaged in this case. In real life , no damage was observed by them so far. Is it really a problem in real life?
    4. Does a "normal" LVDS output provide enough current to damage the input of the DS90LV804 ?
    5. Does exist an other LVDS Levelshifter device, which can withstand a normal LVDS input current at its input during power off?
  • 0 in reply to Stefan Seiwert
    TI__Expert 3050 points
    Hi Stefan,

    We can look at a few sections of the datasheet to resolve this question.

    The Absolute Maximum table says that theLVDS input voltage should be between withing -0.3V to (VDD + 0.3V).

    So if VDD = 0V, for your power off condition, then the voltage on the inputs should be no greater than 0.3V.

    To answer your question about current, you can figure out the current of LVDS lines by dividing out the 100 ohm termination resistor. LVDS drivers push out current across a 100 ohm resistor to create the output VOD. So if the output VOD is 500 mV, the output current is 5mA.

    The other area of the datasheet to examine is the LVDS input DC Specifications. Here the paramters VTH, VTL and VID help define what levels are acceptable for normal device usage. The differential thresholds are +/- 100mV. The acceptable VID range is 100 mV to 2400mV.

    So it is possible that if the VOD from your source driving the DS90LV804 is less than 300mV and greater than 100mV that you should have any major concern. Note that we should be only concerned about the voltage measured at the input of the DS90LV804 and not at the output of your source.


    Mike Wolfe
    DPS APPS / SVA