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DS90LV047A: Unused channels

Hideki Hayashi1
Expert 8390 points
Part Number: DS90LV047A
Other Parts Discussed in Thread: DS90LV048A,

Hello, experts,

I have inquiries about DS90LV047A / DS90LV048A.

Each datasheets describe "The DS90LV048A is a quad receiver device, and if an application requires only 1,2, or 3 receivers, the unused channel(s) inputs must be left OPEN." at 8.3.1 LVDS Fail-Safe.

How about the case of the DS90LV047A unused channel(s) inputs/outputs?

Are there any problem when the driver/receiver unused channels inputs/outputs are left to connect to Vcc(=3.3V)?

The reason of asking is that the customer who use these ICs is planning to replace ROHM IC with TI IC as P2P, and ROHM IC is recommended to be connected to Vcc.

I have seen 

https://e2e.ti.com/support/interface/f/138/t/502244

https://e2e.ti.com/support/interface/f/138/t/736989

, but I don't understand well. 

Best regards,

Hideki

  • 0
    TI__Mastermind 43795 points

    Hi Hideki,

    It is okay to pull up the DS90LV047A CMOS inputs to 3.3V when they are not in use.

    The LVDS outputs should not be pulled up to 3.3V, they can remain floating since the input is pulled to 3.3V.

    Regards,

    Lee

  • 0 in reply to Lee Sledjeski
    TI__Expert 7395 points

    HI Lee-san,

    Thank you for your answer.

    How about DS90LV048?

    DS90LV048 can also pull up  to Vcc?

    If No, Could you please let me know about your concern when it pulled up to Vcc? 

    Thanks

    Muk

  • 0 in reply to Yusuke Mukuno
    TI__Mastermind 43795 points

    Hi Muk-san,

    The DS90LV048A inputs are LVDS.  For these unused differential inputs I would recommend to tie the "+" input to 3.3V and the "-" input to GND.

    This will ensure a stable output and not consume any extra power.

    Regards,

    Lee

  • 0 in reply to Lee Sledjeski
    TI__Expert 7395 points

    Lee-san

    Thank you for your answer!

    Regarding DS90LV048A

    Customer is already designed LVDS function with competitor device base. Customer mentioned, competitor recommend both "+" and "-" inputs of DS90LV048A connecting to Vcc. 

    They plan to design change for our device. If "-" side connect to Vcc, what concerns do you have? 

    Regarding DS90LV047A

    As you mentioned,  DS90LV047A CMOS inputs can pull up. Is it meaning they can pull up to Vcc with any resistor?

    Now, customer is designed to connect to Vcc directly. (from competitor recommendation) Is it any problem?

    If they need to use resistor, could you please let me know the value of resistor?

    Muk

  • 0 in reply to Yusuke Mukuno
    TI__Mastermind 43795 points

    Hi Muk-san,

    I would not recommend to connect both "+" and "-" inputs on the DS90LV048A to Vcc as a first choice.  It is okay to design the circuit with both tied to Vcc.  The inputs do support open/short/terminated failsafe so the output will be in a high state.  Shorting the inputs together minimizes the margin in the failsafe circuit so if I have a choice I recommend to pull the inputs apart just to be sure.

    For the DS90LV047A CMOS inputs, they can be connected directly to Vcc.  There is no need to put a resistor in series.

    Regards,

    Lee

  • 0 in reply to Lee Sledjeski
    TI__Mastermind 43795 points

    Hi Muk-san,

    Shorting to Vcc may be an issue.  See section 8.3 of the datasheet.  Shorted failsafe is only applicable with no external bias voltage.

    Shorted Inputs. If a fault condition occurs that shorts the receiver inputs together, thus resulting in a 0-V differential input voltage, the receiver output remains in a HIGH state. Shorted input fail-safe is not supported across the common-mode range of the device (GND to 2.4 V). It is only supported with inputs shorted and no external common-mode voltage applied.

    Regards,

    Lee